Hugh Franklin from Aeroseal UK explains how the airtightness of a building can be improved using a water-based, low VOC atomised polymer which is delivered when the building is pressurised. This simpler approach speeds up the process of air sealing and gives real-time feedback on the airtightness score.
Interview with Hugh Franklin
Hugh Franklin is the Head of Operations at Aeroseal UK, where he leads the development and deployment of the AeroBarrier airtightness system. His career began in archaeology, spending years de‑constructing historic buildings and extracting lessons about passive cooling, solar shading and other age-old strategies. Those insights sparked a shift toward eco‑construction, where he focused on scaling sustainable building practices before narrowing his expertise to airtightness over the past eight years. Today, Hugh combines his deep appreciation for traditional building wisdom with the latest technology to help builders achieve high performance homes.
The ‘mist’ sealant automatically hones in on the leaks
Airtightness is a key characteristic of a high performance home, but airtightness strategies can be labour‑intensive using boards, membranes, tapes, liquid paints and so on. Hugh describes how a mist of low‑VOC polymer sealant, delivered under pressure, automatically seeks out leaks and creates a uniform seal. This reduces material waste, shortens installation time, and delivers measurable improvements in the building’s air‑leakage score.
“We create a fog, a mist of a water-based low-VOC sealant… the sealant is drawn towards those leaks by the escaping air.”
It is possible to dial‑in the airtightness you want
AeroBarrier is introduced into a building during a standard blower‑door test. When the building is pressurised the mist travels through any gap or crack, collides with itself and solidifies around the leak. Because the product is an acrylic polymer that is 60–80 % water, only a few litres are needed for a typical 200 m² home. The sealant does not expand; it accumulates only where airflow exists, allowing the airtightness score to be dialled down to the exact target.
The building is carefully controlled during misting
Everyone steps outside while the mist is applied and it doesn’t take long. Workers inside wear appropriate respiratory protection, but the low‑VOC formulation produces minimal fumes. After sealing, the space can be re‑entered within 30–60 minutes once the mist has dried, making the process practical for busy construction sites.
“We control the space during the install, much like if you’re doing an airtightness test.”
Long‑term durability is proven in the lab and the field
AeroBarrier has undergone lab testing up to 5 000 Pascals (hurricane‑level pressure) and shows no degradation after 50 years of simulated ageing. Field studies report only a 1–2 % loss of airtightness over 25 years, confirming that the polymer remains flexible and effective long after installation.
Automation works best when paired with manual detailing
The system can seal gaps up to 10 mm. Larger openings generate a visible “white halo” during misting, signalling that manual patching is required. This hybrid approach lets builders concentrate on the biggest defects while AeroBarrier handles the multitude of smaller, hard‑to‑detect leaks.
“AeroBarrier will try and seal that, but it will start to form a white halo around a hole that’s quite large… you can walk around the building… and find those gaps and then fill them manually.”
Real‑time feedback turns airtightness into a visible metric
A key obstacle in airtightness is the lack of instant performance data. AeroBarrier integrates with the blower‑door test, displaying the current ACH (air changes per hour) score on a screen as the mist seals each leak. This immediate feedback lets site teams see which details are performing well and which need attention, fostering ongoing learning and improvement.
There is an optimal moment to apply the sealant
For new builds, the best time to apply the mist is post‑first‑fix, after windows, glazing and major services are installed but before interior finishes. In retrofits, the building must be sufficiently sealed to become a “pressurisable box”; otherwise, the mist will escape and the process loses efficiency. Coordinating with the contractor to schedule the test at the right stage is essential for success.
“We want the building to look as much like a porcupine as possible. Post first fix is really when we want to come in.”
A draught-free home will deliver savings
AeroBarrier is priced at roughly £12.50 per square metre of floor area. For a 200 m² home, material costs are about £2 500, plus labour. Hugh notes that the return on investment typically appears within three to five years through reduced heating and cooling demand, especially when moving from a standard Building Regulations home to the Passivhaus standard.
Achieving airtightness this way requires a slightly different plan
Hugh says that their best clients keep an open mind about the approach to airtightness. It’s much better to have a talk upfront about how to get the most out of the system. For example, setting clear tolerances early on will avoid unnecessary work (e.g., keep penetrations ≤ 5 mm for high‑performance builds). Builders, plumbers, electricians and ventilation suppliers need to understand these limits and coordinate their work to avoid unintentionally creating new leaks. Brief on‑site training and visual guides help keep everyone aligned.
Once achieved, airtightness must be preserved
If a great airtightness result is achieved at first fix this is not a cue to rest on your laurels. Throughout the project it’s important that everyone who is working on site understands the airtightness line. Typical mistakes might include cutting through an airtight membrane without making good afterwards. Hugh shares a couple of examples. When electricians are installing recessed lighting they might not quite have enough room so they cut through the membrane. Also when it comes to adding services, like cables, pipes and ducting, a similar problem can occur. Because the airtight layer is often invisible, any breach can dramatically reduce the airtightness of a property. Scheduling blower‑door tests at key points and encouraging a culture that reports issues without blame can help maintain a good airtightness score.
Scaling airtightness for mass‑market housing
Hugh believes that making airtightness cheap, fast and easy to verify is the key to bringing low‑energy homes to the broader market. By combining automated sealing with clear performance metrics, builders can deliver consistent results without needing specialist airtightness gurus on every site. The goal is to demystify airtightness, turning it from “black magic” into a routine part of construction.
Es Tresidder from Highland Passive explains what an air-to-air mini split heat pump is, why he believes it’s a good choice for heating a high performance home and the various pros and cons of going down this route. He also reflects on one of the toughest stages of his retrofit when they took the roof off the existing dwelling and then faced torrential rain.
Interview with Es Tresidder
Es Tresidder is a certified Passivhaus Designer with over a decade of experience in low energy buildings. His consultancy Highland Passive helps self-builders and retrofitters create better homes, that are healthy, comfortable and energy efficient. He’s also been working on various projects for John Gilbert Architects over the years, and he’s a regular at House Planning Help as part of our ‘ask the expert‘ team in The Hub.
Es has been on the podcast before, explaining how he upskilled in DIY during his retrofit project. In this session he’s reflecting on the whole process, including a key challenge he faced with water damage, and he’s going to go into detail about how he heats his home with an air-to-air mini split heat pump.
Es is a regular in The Hub where he helps answer questions on building physics and retrofit
Vehicle-to-load allows you to use an electric car’s battery to power external devices
This podcast was recorded during a power cut and was only possible because Es has an electric car capable of vehicle-to-load, which means that you can plug an adapter in to where you normally plug it in to charge, and then plug a three pin, normal domestic plug into that, and run that into your house. He explains that you can’t power your whole house, but you can take three kilowatts from it, which he was using with three or four extension cables strewn around the house, to power the fridge freezer, the dishwasher, the Wi-Fi, to charge computers and even to make a loaf of bread overnight!
“…logically, you know that electricity is very, very useful in life, but life just becomes much harder as soon as you don’t have electricity.”
When Es put the services into the house, he created a service void, which is effectively a pipe that goes through the external wall with an airtight membrane on one side and a windtight membrane on the other. This means he can add new cables into the house and make them airtight easily. He did this when adding solar PV to the roof and now he’s done it again, putting in a permanent extension cable that can connect to the car, ready for the next power cut.
Home battery systems with ‘island mode’ help you operate independently from the grid during a power cut
Es explains that domestic inverters shut down when the grid fails, preventing the house from drawing power from solar PV panels during an outage. However, if you have a home battery system you can pay extra so that you can island from the electricity grid, which would happen automatically during a power cut. However, domestic batteries are only around seven to 12 kilowatt hours, so if it was January or February, with little solar power generation, you’d quite quickly go through your battery’s power, even in a Passivhaus. A car battery is much bigger, around 77 kilowatt hours, which would last several days during mid-winter, running heating as well as everything else. Another advantage is that if the power cut lasted much longer, he could just drive into town to a public rapid charger and top the car back up.
Getting the MVHR back on was a priority because it immediately improves the health and comfort of the home
Es says the MVHR has a really small energy requirement and is beneficial in lots of ways, so it was the first thing they plugged in. Until that point they had been keeping windows open for ventilation and, with an outdoor temperature of 12-13 degrees, the temperature inside had dropped down to around 17 degrees.
The existing dwelling before any alterations
The completed project
Photography by Alex Trowski, commissioned for a case study in Passive House Plus Issue 50
The retrofit, extension and remodel has made the whole house work better as a home
Es’s house was a 1975 timber-frame build of one and a half storeys, where the first floor rooms felt quite pokey. So the strategy was to take the whole first floor off and rebuild it as a one and three-quarter storey house. This may not sound like a big increase, but Es says it gave a lot of extra floor space on the top floor, with enough for an extra bedroom, and an increase in size for the other bedrooms. The floor plan just worked much better.
The upper floor is a new build Passivhaus, with an I joist timber frame sitting on top of the existing structure, which was quite complicated to do. Downstairs is a retrofit of a 100mm timber frame house, where the plasterboard was taken off and insulation put between the studs. He then added another 40 mm of wood fibre insulation board, an airtightness membrane, and then outside of that is white rendered blockwork. There’s a cavity between the two. As a Passivhaus Designer, Es did all the energy side of things and a lot of the retrofit work himself. The new build was carried out by a local contractor.
The project involved removing the pitched roof and extending upwards
Keeping a flat roof dry is incredibly difficult
This project required the first floor and roof to be removed, which is slightly unusual. When they were looking at the options, one way to keep everything dry was to put up a massive tent over the whole structure, but this would set them back £30,000. The builders, however, were confident that they could keep the water out because they had done this many times before. There was also the fact they would be doing this in May which is statistically the driest time of the year in Fort William. Unfortunately the weather did not play ball and it was an unusually wet month. Es also points out that although the builders take off lots of roofs they probably don’t remove the whole structure leaving effectively a flat roof to keep dry.
So it just became impossible to protect the structure from the torrential rain. Es had already insulated the ground floor and achieved good airtightness but the rain soaked the floorboards and insulation, forcing him to strip them out and replace everything.
“You don’t want to install any insulation until the building is watertight.”
A lot of insulation got waterlogged and had to be replaced
Sequencing the work differently would have avoided damaging the insulation
If doing the project again Es would not necessarily pay for the giant waterproof tent but he would sequence things differently. The work downstairs had been done first to ensure structural stability for the new first floor and roof. However once rain had compromised the work, they had to go ahead without the extra racking boards on the walls anyway or the racking strength from the floor. He would therefore consult a structural engineer to look at any structural elements that could be done without involving the insulation.
The unit inside the house
An air‑to‑air mini split heat pump is a cost effective way to heat a low energy home
For Es, a heat pump is an obvious choice over direct electric resistance heating. However, instead of a conventional air‑to‑water heat pump system, Es chose an air‑to‑air mini‑split system. He explains that this is like an air conditioner, which usually pumps heat from inside to outside a building, but can also be used to pump heat from outside to inside a building. They have a similar efficiency to an air-to-water heat pump, but are much cheaper to install, because there’s no need for radiators or a wet system. The mini split has an outdoor unit and an indoor unit, with refrigerant lines between the two. The unit and installation cost was around £1,500 compared to £12 000 for the air‑to‑water system. Es is clear that this system is great for a Passivhaus but wouldn’t work for a normal new build house as it only delivers heat into one room and relies on heat spreading to other rooms.
The unit outside the house
Reversible operation provides useful cooling in heatwaves
Mini‑splits can run in reverse to deliver modest cooling, which has proved handy for Es during a heatwave which coincided with the midge season (when they don’t really want to open any windows) and a large group of friends staying over and thus raising the indoor temperature.
Sizing the mini split above peak load ensures comfort in cold snaps
The house’s peak heating demand is around 1.3 kW and by selecting a 2.7 kW mini‑split Es has provided a comfortable margin, which allows the system to cope with occasional freezing cold days without needing supplementary electric heaters. In cold weather he will run the system overnight when electricity is cheaper, so increasing the temperature in the main parts of the house, whilst the bedrooms stay cooler. Once the doors are open the heat can spread through the house. He admits that having his office in his bedroom means occasionally it can be cooler than he would like and so a small electric heater would probably be a good addition.
Photography by Alex Trowski, commissioned for a case study in Passive House Plus Issue 50
Locate an air-to-air mini split system in a large open area
Es explains that the mini split system needs to be in a space big enough to deliver a lot of heat into it, before warming that space to above the set point temperature. A small room would reach that temperature quickly then the system would shut off and so not provide enough heat for the rest of the house. The indoor fan also produces a low hum comparable to a refrigerator, louder than the house’s MVHR, so you need to place it where the noise won’t matter too much. Es stresses that whilst the MVHR supplies fresh air, and recovers heat it only moves a small amount of warm air between rooms. Keeping interior doors open lets the warm air from the mini‑split circulate more effectively.
Grants favour air-to-water systems, but air-to-air systems may still be cheaper
Even though Scottish renewable‑heat grants are available for air‑to‑water heat pumps and not for air‑to‑air units, Es feels the air‑to‑air solution may still be cheaper. The grant system also involves a lot of paperwork and restricts you to using MCS certified contractors, who are not necessarily the best. For a Passivhaus going down the air-to-air route also means you keep things simple.
A standalone hot water pump works alongside the air-to-air system
Whilst air-to-water heat pumps can also be used to supply hot water, an air-to-air mini split system is just warming the air in the house. So Es has installed a heat pump water heater, which is a 200 litre insulated hot water tank with a small heat pump on top. They are normally ducted to the outside to bring fresh air into the heat pump, the heat pump takes some heat out of that outdoor air, and then it ducts the exhaust air back out. Es has set his up to take heat from the house, which gives some free cooling in the summer and a much faster reheat time in the winter. It’s also easier from an airtightness point of view as it is not ducted to the outside at all. In the winter, the water heat pump is slightly cooling the house, so the mini split has to work a bit harder, but overall, it is still very efficient.
Living in the house has been amazing
The house before Es started had quite serious mould and damp problems. They were always a bit chilly in winter and couldn’t dry things because there was no reliable ventilation, and the house wasn’t warm enough. Now things are completely different.
“It’s hard to remember how it was before, because it’s just so nice now.”
The house is warm and comfortable all the time in every room. Es believes it’s almost more impressive in hot weather than it is in cold weather, when you expect it to be warm. Es says that the first summer they moved in, they had two separate weeks of more than 28 degrees as peak temperatures in the afternoon. And in those conditions, the house was at 23 degrees, and it felt like walking into an air-conditioned space. It has really improved the quality of life. The air is clean and just walking outside the house, Es immediately notices if the neighbours oil boiler is running or there’s a coal or wood fire on locally. He thinks his sense of smell has improved, because he’s used to clean air and the house smells fresh all the time. The best thing isn’t the bills, but it’s the most quantifiable thing. He’s reduced his heating bill by at least 80% while also massively increasing the average temperature of the house.
“After one full year of monitoring we have spent £175 on heating and hot water for a family of five, which is remarkable.”
Andy Mitchell, Managing Director of 21 Degrees, explains what factors influence human comfort and how we have tried to make our homes more comfortable through the years. Embracing building physics has been a turning point and while we have the knowledge and skills to create healthy and comfortable homes today, it’s still not the default approach. So Andy outlines the options available to get yourself into a better home.
Interview with Andy Mitchell
Andy Mitchell is the Managing Director of 21 Degrees. Engineering runs in the blood as he comes from a family of engineers and he has always been fascinated by houses and buildings, even considering architecture as a possible career path at one stage. Studying product design at A-Level he then became interested in company structure and business. His first significant role in sustainability was at a company called Natural Building Technologies in the early 2000s, where there was a focus on healthy bio-based materials. Today he heads up 21 Degrees, a company which supplies key technology to homeowners and architects enabling them to build more efficiently, and generate and store energy.
In this podcast Andy explores the concept of comfort and explains why we can do much better when it comes to creating a comfortable and healthy home in which to thrive.
We can define a comfortable temperature
While we might think that we are all different and therefore prefer to live at different temperatures, that is not true. We can define a comfortable temperature range because of our physiology.
“Any human being, no matter where they are on the planet is most comfortable between 20°C to 22°C. The reason is simple. If the temperature is higher, your body has to work harder to keep you cool; and if it’s colder, your body has to work harder to keep warm.”
The body doesn’t cope well under 15°C
Below 15°C humans have to work hard to keep warm. At 9°C or less, it’s a matter of survival.
How active we are, alters what we perceive as a comfortable temperature
If we’re engaging in a lot of activity, such as cleaning our homes, we are generating heat and keeping ourselves warm, so may desire a cooler temperature. Alternatively, if we are working at a desk and sedentary then the reverse is true and we may want it to be warmer. Generally speaking the impact of this is plus or minus 2°C.
Putting on clothes is another dynamic
You only need to look to the Inuit in parts of Greenland to understand what an impact clothing can have. They could survive in subzero temperatures. The downside is that their clothing was bulky and would limit the activities they could perform. Any clothing will go some way to making us feel warmer because it’s trapping the body’s heat and slowing it down from moving away from the body. It would be the same effect sleeping under a duvet. The duvet is insulating your body heat.
Relative humidity levels impact our health and comfort
Humidity, or the level of moisture that’s in the air relative to the temperature, has an impact on comfort. The warmer the air, the more moisture the air can hold. And generally, humans are most comfortable at 50%, plus or minus 5%. It’s quite hard to fix humidity levels but between 40-60% is seen as a healthy range. If the levels exceed 70% regularly then these conditions can lead to damp and mould, which are damaging to human health. Below 40% is not good either and could cause respiratory issues.
Lowering the temperature to save money will create an unhealthy environment
Andy explains that it may be a false economy to turn down the thermostat in winter to save money or reduce fossil fuel usage. This is because as you drop the temperature the relative humidity goes up.
“If you had a temperature of 22°C and the relative humidity was 50%, and you dial the temperature down to 18°C, you’ll actually increase the relative humidity to 64%… You’ve made yourself slightly more uncomfortable because you’ve increased the relative humidity.”
With poor ventilation, which is common in many British homes, the relative humidity may have been closer to 55% anyway.
“If we’re closer to 55% relative humidity that really starts to get interesting, because if you dial down from 22°C to 18°C, you’re at 73% relative humidity and that starts affecting your health. For example, dust mites proliferate above 70%, eczema and asthma are a direct consequence of allergic reactions to dust mite poop. You’ve also got issues with mould growth.”
Radiant heat also plays its part in comfort
In addition to temperature and humidity, radiant heat plays a significant role in how comfortable we feel. Andy shares a couple of examples of when you might have experienced this.
If you’re taking a stroll on a sunny day and a cloud goes in front of the sun, you might notice you suddenly feel a bit cold. The air temperature hasn’t changed. What’s happened is the radiant heat that’s been hitting your body has dropped because it’s been obscured by the cloud.
Radiant heat is also absorbed and emitted by surfaces in our homes such as furnishings, walls, windows, etc. So the temperature of those surfaces will have a significant impact on your comfort levels.
Our homes have got better over time!
Over the centuries we have turned to fires to keep us warm. In Victorian times there were open fires, which were extremely inefficient because most of the heat went straight up the chimney. However, the rooms felt more comfortable because of the radiant heat being absorbed and then given off from walls and furnishings, etc.
“The big, winged armchairs that we perhaps thought were to stop granddad falling out of his chair when he fell asleep were actually to keep the radiant heat in… It captured that heat that was being radiated out from the fireplace in the room.”
With the advent of cheap energy, such as access to North Sea gas in the UK, we could pipe fuel straight into our houses. Burning this in boilers, heating water and then piping it around radiators was a big leap forwards. Although fossil fuels are energy dense and have been an easy source of fuel, their emissions have contributed to global warming and we have nearly exhausted the supply in a few decades.
Indoor temperatures have been increasing
Back in the 1970s, the average temperature in a British house in winter was 12°C – 13°C. These days, it’s probably closer to 18°C – 19°C. Andy says that figure could be impacted by a number of people who are in genuine fuel poverty. In other words, they find it unaffordable to heat their homes. One reason for this is that the UK has a large amount of old and leaky properties.
Leaky homes are just about tolerable in a mild climate
Andy says that the UK has the most inefficient housing stock in Europe. While Germany is on a latitude further South and might face temperatures of -10°C, a British winter is milder and more akin with Italy. Therefore with a few token measures, such as double glazing, and cheap energy, we have got by.
“Sadly, I think we focus more on basic survival and then toleration, rather than what makes for a great home? What really does give us that extra level above survival, so that we can thrive? Again, to me, this is very much what comfort brings… It means you can be more productive in terms of your work. You’ve got higher levels of energy, because you’re not using it to keep yourself warm or cooler.”
Sensing a ‘muggy’ environment indicates pollutants have built up
As we live in our homes the indoor air is changing. We exhale CO2 and generate moisture from our activities. Relative humidity builds up and there may even be VOCs given off from furniture, paints, etc. Our instinct is to open a window, which Andy says will solve the problem.
“By opening the window, you purge the air to the outside, because generally warm air will move to the outside, particularly if there’s a bit of cross ventilation, and as a result, arguably, the temperature will be dropping a bit. But if you don’t leave that window open for too long you’ll have a complete air change, and the air that’s come into that room, because the walls and the fabric are warm, to some extent, that air will warm up. And what was outside air, which was probably around about 70% relative humidity, would have dropped down to around about 50% relative humidity, and suddenly you feel much better again!”
The problem is this purge is needed regularly and is hard to manage. More often than not there will be a drop in internal temperature.
In winter another source of discomfort is draughts, where you’re sensing changes in air temperature. For example, if you stand near windows that are single or double glazed, they are colder than the internal air temperature so you will feel the heat moving away from your body.
You may also experience stratification of the air and this heat differential causes air movement, which in turn can make you feel uncomfortable. For example, your feet can feel colder than your torso. You may not even be aware of it.
We have got to grips with the physics of buildings and know how to build better homes
In the past we have simply put our buildings together without worrying too much about how effective they are, focusing more on aesthetics. However, there is an approach that works and has been around for over thirty years. Dr Wolfgang Feist and the team at the Passive House Institute decided to look at things from the first principles and get a holistic picture. This led to the development of the Passivhaus standard, which is a formula that anyone can follow to make a building that works. Core principles include insulation, airtightness, high performance components such as windows and doors, reducing or eliminating thermal bridges, introducing mechanical ventilation with heat recovery, and orientating the building towards the sun.
Andy points out some of the benefits. For example, in a Passivhaus the temperature of the surfaces will be plus or minus 3°C of the air temperature. It will be draught-free and the heat loss will have been reduced significantly. The introduction of mechanical ventilation with heat recovery also means there’s constant fresh air coming into the building, but the heat from the outgoing stale air is retained. And this is a way of controlling the relative humidity, too.
So building or buying a certified Passivhaus is one way of ensuring you have a comfortable and healthy home.
Buying a new home doesn’t necessarily mean it’ll be efficient or healthy
Although the Passivhaus standard was developed decades ago, the learnings and methodology have not been incorporated into the building regulations. Therefore there is still a performance gap in most new buildings, where the design intent is never reached in the completed building.
We are all responsible for the status quo
Not only do we as homeowners tend to accept the status quo rather than demanding higher standards, but architects, designers and contractors struggle to adopt new strategies. So sometimes the drawings fail to give enough detail, particularly around things like airtightness or thermal bridging, which is just where you’ve got surfaces or materials joining together, and you can get additional heat loss that you wouldn’t do through the principal wall or roof or floor areas. It can be the contractors on site, who don’t fully understand the goals of the project and might cut through a membrane not realising the impact of doing this.
Then there’s the building standards themselves. The reality is the building standards stop at the point of design. Currently there is no ‘as-built’ check of the performance.
“So the performance standard stops at the design stage because you demonstrate through something called SAP, Standard Assessment Procedure, what is the heat loss from this building based on the building fabric. It’s pushed through a calculation. Effectively, it says yes this fabric will pass, and that’s where the testing ends. There is nobody from Building Control actually measuring the heat loss of the building once it’s occupied.”
The choices are getting better
Andy believes that building to a performance standard above and beyond building regulations is becoming more common. We’ve mentioned the Passivhaus standard but there’s also the retrofit standard EnerPHit. The AECB New Build standard is another option, which is slightly easier to reach, but has the same rigorous building physics approach.
Building your own home allows you to get what you want
When people are building or commissioning their own homes they almost always strive for higher standards. And doing a self-build project is not quite as onerous as it might seem, with more and more help available. Andy says we build about 5% of our homes in the UK this way, or 15,000 homes a year, which is equivalent to the output of the fourth largest volume house-builder in the UK. This is a good start but to put it into perspective Germany deliver around 50% of their homes as self-build. So maybe it’s not surprising that these standards have taken time to be adopted.
It is possible to improve the performance of an existing property
If building new or buying from the open market is not proving successful, then there is another option. Existing homes can be retrofitted to a performance standard. It is often harder, and some homes will be more challenging than others. For example, how the building is orientated is something that is hard to change. There are also significant costs for a deep retrofit.
“The more complex a building is in its form, often the more challenging it is to reduce the heating requirement, the heat load. You can still go a long way, though, to improve airtightness and therefore comfort levels but you might not achieve the same low energy results that other more simpler forms of building can.”
On a phased retrofit, improve the ventilation first
Doing a retrofit in one hit often makes life easier, but not everybody is in a position to do this. So another option is to do a phased retrofit. The most important part is to have a comprehensive whole house plan at the beginning.
Andy says that a good first step is to improve the ventilation of a property because that will give you cleaner, fresher air, with more controlled levels of relative humidity. While it may cost a bit more in heating initially until you have reduced your heat loss, you can immediately benefit and also reduce the risk of damp and mould building up in places where you have improved the fabric but have not improved the ventilation yet.
The average family home produces up to as much as 15 litres of moisture a day that goes into the air. This evaporates from our bodies when we’re sleeping or walking around the house. Moisture is also generated when we’re cooking, taking a shower or doing the washing, and it has to go somewhere.
So after the ventilation has been addressed, the next phase might be looking at improving the airtightness and replacing the windows.
Adding insulation changes the physics of the building
When you apply products to your building, you are changing the physics of the building. So if you apply a piece of insulation to a part of your property, you are changing what was there before, and if it hasn’t been well thought through you could end up with unintended consequences. Andy gives an example of fitting closed cell insulation internally which is principally a bad idea because you are likely to trap moisture in the wall. This interstitial condensation could ultimately lead to the building fabric being damaged or breaking down, or potentially creating mould pockets, which then can affect the quality of the air in the room.
Mechanical ventilation must be designed well
There are various ducted ventilation systems on the market but they have to be designed well to be effective.
“The ducting design, for example, in MVHR is far more important than the actual unit that you plug into the end of it. A decent fan and heat exchanger is important, but you can have the best MVHR unit on the market and you plug it into poor ducting design, and you’ll just have a poor system. Whereas you could have an average MVHR unit and plug it into some effectively designed ducting, and you’d have actually a very good system.”
MVHR can be useful if the outdoor air is not clean
If the external air quality is poor, such as on a busy road, or if there’s a lot of pollen that is having a particularly bad effect on a hayfever sufferer then a fully vented, ducted system often takes the air through filters. So you can filter particulates from outside and stop them coming into the house.
Making a home more healthy and comfortable shouldn’t be about payback
When people go on holiday they don’t look for the payback on their holiday! People are also prepared to spend a lot of money on a kitchen. However, when you tackle a big project that will improve health and save money on bills, they suddenly believe it should pay for itself.
Treat homes as homes, not as assets
It would be good if we moved away from viewing houses as assets, and actually looked at them as a way to help us live effectively.
Andy is tackling his own retrofit
We are all on a journey to creating better homes for ourselves and our families. And that includes Andy! He is making improvements to his Victorian home with a phased approach.
“You do a measure to your own property, and you still find yourself being quite impressed! So we live in a Victorian property, and half of the floor of the property is a cellar. The other half is solid floor, and we insulated in the joists of effectively the cellar part, and that made a significant improvement. And then the following year, we had an airtightness membrane installed underneath the joist and the insulation, and that made as significant, if not more significant an improvement in both the thermal performance of that room.”
Andy believes that most of us become complacent with our homes when we actually have the opportunity to enhance our lives.
“As human beings, we have this incredible gift of adaptation. The problem is it’s also a curse, and we just tolerate an awful lot of rubbish. If our awareness was increased and we could begin to learn what is possible, particularly in terms of how we can live within our built environment I think the change for our welfare, both as individuals and society in general, would be significantly impactful.”
Toby Cambray talks about the risks associated with moisture in buildings, and the modelling techniques – like WUFI® – that can be used to assess those risks.
Interview with Toby Cambray
Toby Cambray co-founded Greengauge building & engineering consultancy in 2010, specialising in sustainable and low energy design. He is a certified Passivhaus Consultant and has completed a PhD on moisture in buildings.
Condensation on windows is a common sight in British homes
In buildings, moisture is water in any form
When we use the term ‘moisture’ it can refer to liquid water, water vapour in the air, or even ice. People might be most familiar with water vapour, and how humidity changes with temperature.
It’s also important to think about materials – especially historic building materials – and how they interact with liquid water. And when you add insulation to buildings, moisture is something that really needs attention paid to it.
Moisture movement in buildings is about equilibrium
In the laws of physics, all things try to minimise their energy. For example, if you have an area of high pressure next to an area of low pressure, the molecules try to ‘even themselves out’ and minimise the total net energy. In a different context, that’s what is happening with moisture. In an area of high vapour pressure with lots of water, the water will try to distribute itself as evenly as possible.
The potential risks from moisture in buildings are varied
Organisms that cause mould or timber decay can thrive where moisture is present. Moisture also leads to problems like corrosion, especially of steel. In heritage buildings, it can lead or contribute to deterioration in masonry, through processes like freeze/thaw damage, or salt transport and crystallisation.
Damp and mould inside a historic building
The two main moisture strategies are moisture open or moisture closed
Putting it very simply, you can either manage moisture or block moisture. Most modern buildings are moisture closed, or moisture blocking. Features like damp proof courses and vapour control layers aim to stop moisture getting to where we don’t want it.
In traditional buildings, when these modern products obviously weren’t available, builders worked with materials in a way that managed moisture – often by absorbing it and holding it, before allowing it to dissipate safely. Porous materials like stone and brick can generally store and transport both liquid water and moisture vapour.
There is still a lot to be understood about moisture in buildings
Toby has been carrying out moisture modelling work for over a decade. Any simulation has shortcomings, so he sees it as the responsibility of practitioners to understand as much as possible about the physics of moisture so they can then understand when models are most useful. Toby acknowledges that he still has lots to learn about the physical processes that calculation models are trying to simulate.
Dew points are not the best way to assess condensation within building structures
While we want to avoid condensation within the structures of our buildings, dew points are not always a great way to understand potential moisture risks. This is especially true in masonry materials and traditional constructions. A dew point is the temperature at which condensation occurs for air of a given humidity.
Simpler calculation methods that rely on dew points, like the Glaser method, don’t reflect the full range of real-world conditions. They don’t take account of rain on the outside of the building, or moisture transport and storage in liquid form. Their limited scope might be okay for some situations, but they are not sufficient for making sensible judgements about many structures, including historic buildings.
For example, you could have a structure that never suffers from condensation, but still gets a serious problem like mould. The calculation method might tell you about the condensation risk, but it isn’t sophisticated enough to calculate that humidity is high enough for mould to form.
WUFI software is a more complete way to simulate moisture movement
It’s not perfect, but it is a more complete approach to modelling moisture movement in buildings and modelling how water moves around within porous materials.
In the software, the user builds up the element to be modelled. They then select a climate file with data about sun, wind, temperature, humidity and rain. Then the internal conditions are inputted, some other parameters are set, and the simulation can be run. And the simulation predicts the temperature and moisture content at any point within the element’s structure. Using the temperature and humidity profiles that it outputs, together with some post-processing tools, it’s possible to assess whether problems like mould could occur.
Temperature and relative humidity data over time
There are still uncertainties in WUFI calculations
As with any software or calculation, nonsense in leads to nonsense out. The WUFI software has a database of several hundred materials. Some are modern products with known performance.
It also includes porous materials. Because these materials have unique combinations of liquid transport properties and vapour permeability, there can be enormous variation even within a single category of materials. For an existing building, some assessment and measurement of the material is needed to then make an informed selection from the database.
Other uncertainties include internal conditions. For example, you might not know if you should be accounting for a densely occupied house with poor ventilation. Toby explains that the advantage of modelling is that you can play around with some of these variations and explore the different issues to understand their effect.
An example of how moisture moves within a wall build-up that has wood fibre insulation
The simulation is quick to run, but interpreting the output is key
Toby says his team likes to spend a couple of days on a project, normally. This gives time to go to site, collect material specimens and analyse them. Once the data is available to enter into the software, the actual simulation can take anything from five minutes to a few hours to run, depending on complexity.
The real value is in the interpretation of the results, and providing a professional opinion and risk assessment. In complex situations, things are much more nuanced than a simple pass or fail. It can often mean going back to a design and altering it to reduce risk further. Toby’s work really comes into its own in more unusual situations with higher risk constructions, perhaps featuring an unusual combination of materials.
There are lots of good sources for help and advice
For simpler enquiries, Toby and the team at Greengauge will often direct people to a variety of reputable suppliers who they have built relationships with over time. Examples include Ecological Building Systems, Back to Earth, Natural Building Technologies (now part of SOPREMA), and Best of Lime.
For WUFI calculations, there are no specific qualifications to look for when engaging someone. There are a small number of training courses, but a lot of it comes down to experience: reading academic papers for background, and looking at buildings on site to understand the difference between simulation and reality.
The most common use of WUFI is with internal insulation
A sophisticated modelling tool like WUFI is most suited to applications that have a lot of variables. That’s particularly the case in retrofit, where we have no control over the existing construction and we need to be careful about what we do to the structure to avoid introducing moisture problems.
In new build construction, we tend to have a good idea of what works, especially if following prescriptive solutions. In that case, there’s no need for complex calculations. BS 5250, the code of practice for moisture in buildings, has guidance about when different calculation methods ought to be used.
An analysis of water content over time
A consistent layer of thin internal insulation can help to manage moisture
Using hygroscopic, capillary active and vapour open materials helps to maintain a wall’s ability to dry to the inside. Generally, the thicker the layer of insulation applied to the inside, the harder it is for that drying to happen. Most people are now using 40mm, 60mm, or possibly 80mm of wood fibre insulation, and that seems to work okay in most places. A continuous layer of 40mm of insulation, with thermal bridges resolved, is much more effective than a thicker layer of insulation that’s hit and miss.
Good practice when doing a simulation is to include moisture loads or moisture stresses. This is something WUFI allows the user to do which simpler calculation methods can’t. It’s possible to allow for a certain amount of rain coming in through a crack, or a certain amount of warm air moving out through the airtightness layer.
Mike Whitfield shares his experience of running a local construction company that specialises in Passivhaus. He explains how the business is structured, and what tasks are tackled on a daily and weekly basis.
Interview with Mike Whitfield
Mike Whitfield, the owner of Mike Whitfield Construction, began his career in his early twenties, working in America, Canada, and Australia. He had always wanted to be a carpenter and learnt about timber frame building whilst working abroad. Returning to the UK in the 1990s he got involved in oak frame construction for a while before rekindling his interest in energy-efficient buildings. The AECB was a great source of information to keep at the vanguard of green building, and so Mike was well placed to take on Passivhaus building when it came on the scene.
Once Mike Whitfield began to focus on building high performance homes it naturally led to more of this type of work. So from 2010 onwards nearly all his projects were to Passivhaus or EnerPHit. As these projects require a greater attention to detail and high quality workmanship, over time Mike Whitfield Construction has built up a reputation of being able to deliver.
In fact, as they are seen as the go-to Passivhaus builders in Herefordshire, Mike even believes they may have missed out on jobs!
“If you get a reputation for being too busy, you can miss out on jobs. People say I didn’t bother asking you, because you’re always busy.”
A core team of people is key to his way of working
Mike explains that he has a core team of people employed by his company. This is different to the typical British builder model, which uses self-employed subcontractors on ‘zero hours’ contracts.
Being part of a regular team makes it possible to develop skills and knowledge, which is essential to a Passivhaus project.
One of the advantages of being a well-established company is that Mike has built up a network of additional collaborators, from architects and groundworkers to electricians and consultants. It all helps the operation run smoothly.
A local builder can often end up wearing many hats
Mike is now 67 and trying to step back and hand over the reins to others.
This has been an interesting process because he’s been able to contrast where the business is today, as it moves to become employee owned, versus how he started as a sole trader.
In the early days, as is common with many small builders, the paperwork gets done in evenings or over weekends (which means working long hours each week).
As a more established company of 11 people today, Mike has been clear that there needs to be more than one person stepping up so the load can be shared in a more sustainable way.
There’s now a slightly different structure in place. For example they’ve got two site foremen, with one taking on more responsibility for liaising with the clients, and the other one able to concentrate on the construction. An office manager is also in place to handle the paperwork and order materials.
Maintaining momentum requires good communication and organisation
Mike’s work has changed recently now the foremen are taking on more responsibility. However, even with the new structure the same key tasks need to be completed.
There’s a briefing on site at the beginning of the day so that each team member knows what needs to be achieved and can get to it. There’ll be various aspects for the foreman to oversee, such as the quality of work, the progress, and receiving deliveries.
There’ll be meetings with the client or consultants, always looking ahead to the next steps.
And in the office there’ll be various ordering and invoicing.
“It’s just a question of keeping the materials flowing to it, making sure our office manager knows what’s going on and our office management system is working well. In my previous life I was on site at 7.30am to get people going, then in and out of the site hut organising things. I’d be lucky to do an hour or two of the practical work before leaving at 3.30 to go back to do a couple of hours work in the office.”
Mike prefers a ‘collaborative procurement’ process
Drumming up new business can be a process that takes years. It might start with a phone call from a potential client, who is looking for a plot of land. Mike would normally refer them to the architect he works with most frequently. It might go quiet for a couple of years, and then finally the discussions about the design begin.
Costing a new scheme draws on data from previous projects
Mike has detailed records for the outputs and costs for each stage of a build, so he can input prices at the design stage. He calls it a collaborative procurement, rather than doing a tender or some other method. A lot of architects in the Passivhaus world now accept that it can be good to find a builder you like and trust, and just work up the price with them.
“Working from a basic sketch we can have a good go at pricing the build using our previous experience and our pricing programme. If we know the client’s budget, we can work with that and assess what is possible for the money. Mutual trust is important.”
The team must always be working on a project
One of the trickier elements of being a builder is keeping a steady flow of work for the whole team.
Mike describes the ideal scenario being where their groundworkers have started the next project, whilst his team is on the main project, and maybe one or two people are second-fixing the last project.
Of course life doesn’t unfold perfectly and sometimes they have to do things differently.
Mike shares an example from last year when he had two jobs running in parallel. Luckily, they were only a few miles apart from each other, which helped.
A builder will often have a site on standby for quieter times
When there are gaps in the schedule Mike can move the team onto one of his own projects. He says last year they had three jobs, totalling £1.5 million, in the pipeline but couldn’t start any of them. So having a plot as a fallback option means they can divert resources to do part or all of the build. Again, in a perfect situation the team would be completing the whole project in one go, but they could just do the groundworks and pause, or they could get the shell watertight and then bring things to a halt.
Building to Passivhaus standard demands more from a business
In Mike’s experience, Passivhaus building requires more attention to detail and greater supervision, and the profit margins are tighter.
Although there is a lot of job satisfaction in delivering high quality buildings there is no doubt it is much harder than simply complying with building regulations. For a conventional build it is relatively easy to find a good site supervisor, use a quantity surveyor for pricing or give drawings to a builder’s merchant for them to work out the quantities you need. Mike has always had to do his own pricing and recognises it is harder.
Want to see how a timber frame project comes together? Check out our Buckinghamshire Passivhaus case study in The Hub
Track record is the best way to gauge a builder can deliver a Passivhaus
Mike thinks that clients who commit to building to Passivhaus standard have thought a lot about what they want, and therefore the service needs to reflect this. In the early days people would realise part way through construction that they’d taken on the wrong builder and they weren’t going to get a Passivhaus at the end.
It’s slightly easier now because a builder should be able to show you a portfolio of certified Passivhaus projects. If they’ve only completed one or perhaps just do ‘Passivhaus principles’ then this may be the point to ask a few more questions.
Pricing up retrofit projects is challenging
Although there is demand for retrofit projects Mike says they are certainly trickier. They’ve tackled 4 or 5 so far, and it took until the fourth one for them to make any money. There is an argument for doing them on a ‘cost plus’ basis as you don’t know what you’re going to find before you start.
However Mike generally prefers to work on a fixed cost basis, with everything agreed at the start and then its up to him to bring it in for that price. It can mean tight margins if prices spiral after you’ve quoted, but on the whole it works well.
It’s easier for builders to upskill today
As Mike gradually steps back from the business he reflects on his career in a positive light.
Some aspects of running a building company are tough, especially on your own, but he has always loved the craft of the work.
To others who want to follow in his footsteps and build to Passivhaus standard, Mike says that the good news is that there is a lot more information and training available compared to when he started out. It will demand more from you and your team, but if you’re up for a challenge and committed to this way of working it can be extremely satisfying.
“It is hard to find the right people, but if you can, and you’re all working together, it is so much easier.”
His advice to young people starting a construction company now is to find men and women who share the same goals as you, so you can form a team to share the workload and support each other.
Paul Testa from HEM Architects explains the difference between renovation and retrofit, using his own home as an example of a deep retrofit.
Interview with Paul Testa
Paul Testa is a director of HEM Architects in Sheffield. He’s been on the podcast before highlighting a few hidden costs you might experience on a house build project. Today he’s explaining some common terminology when it comes to altering homes: renovation, retrofit, remodel, extension and eco renovation.
While these words might seem self-explanatory they can get lumped together and we perhaps miss some of the nuances. So Paul is going to explain each term as well as share a few details from his own retrofit.
Renovation is about bringing your house back up to standard
A renovation is work that brings a house back up to a good standard. Not only will it be about making it look nice but it will deal with any disrepair, and perhaps replace the electrics and plumbing. New windows might be installed. It may also include putting in new kitchens and bathrooms, and then plastering and repainting.
Paul gives an example of buying a Victorian terrace that needs lots of work. You are renovating the house to bring it up to date and make it suitable for modern lifestyles.
Remodelling is rethinking how the spaces work
Remodelling is when you consider how the spaces work together at a more fundamental level. It’s not just about what the home looks like, but how the spaces are connected, maybe bringing multiple rooms into an open plan layout. Upstairs it may be about making better use of bathroom and bedroom spaces. It will likely involve moving internal doorways and removing walls, and whilst it could include external changes to doorways and windows this is less common.
Extensions are about creating more space
Extensions or additions are about adding more space. Sometimes a client may think they need more space but a remodel would actually solve the problem. Often extensions are needed as families grow or needs change. Paul says that it’s quite common for people to want a larger kitchen, dining space that faces onto the garden, which is very different from how houses were laid out in the past.
“We live very differently now, we congregate in our kitchens, and often remodelling can deal with that, but sometimes you do need to gain more space in that area to fully enjoy those benefits.”
Retrofit deals with performance
Retrofit is a holistic approach that improves the building fabric, making a space more comfortable, healthy and energy efficient. As part of this upgrade moisture will need to be managed and a good ventilation strategy is required.
Clients may have different drivers for carrying out this work, such as wanting consistent energy bills, so they are not so affected by energy price jumps, or increased comfort because they can’t get the house warm enough in the winter, or they may be motivated by reducing their carbon emissions.
With the right budget a retrofit can deal with all of these pain points but often work might have to be phased and the immediate solution will address the priority.
Doing work on your home locks in the performance for the coming years
While a renovation project might create a beautiful home it may not improve the performance at all and once the work is complete the homeowner is unlikely to want to make any significant changes for the foreseeable future. So this might lock in poor performance for a number of years.
The benefits of retrofit are ‘felt’ rather than seen
When a retrofit is complete it may look much the same as if renovation work had been carried out. The difference will be in how the building feels and performs.
The investment is going into modelling and understanding the initial home, developing detailed designs and then installing additional insulation and introducing an airtightness layer. High performance windows and doors will be added and ventilation will have to be addressed, often using mechanical ventilation with heat recovery (MVHR).
Paul retrofitted a 1970s semi-detached home
In 2013 Paul and his family bought a 1970s semi-detached house, which had a two storey side extension that was poorly laid out and a small sun room to the rear. The sun room was boiling in the summer and a cold store in the winter, and the main house was also freezing. This property was ripe for an intervention.
The harsh winter of 2013 was the driver to do something about it but it wasn’t until 2018 that work began. These processes take a long time and architects, much like builders, are always last in the queue.
Once the plans were formulated the main structural changes saw the original extension and old garage demolished. Although there would be a new single storey rear extension and a two storey side extension, overall there would only be 15m2 of extra space (making the floor area 136m2) when compared to the existing dwelling.
Downstairs the suspended timber floor was removed and replaced with a concrete raft. This had between 20-70cm of insulation beneath it and provided both the new floor finish and also the structure for the extension to sit on (so there was no need for separate footings).
The extension is timber frame, with 140mm deep studs, insulated with mineral wool in between and then wood fibre on the outside of the studs and rafters.
The original house was insulated internally with wood fibre insulation and careful consideration was given to the thickness of this so that moisture is managed well and the risks of interstitial condensation are minimised.
If this condensation appeared inside the construction it would damage the fabric over time and potentially produce mould spores that will end up in the air.
The roof of the original house has also been well insulated with mineral wool and wood fibre.
When it comes to the windows, Paul used a mixture or inward and outward opening triple glazed units. The reason for this was because it depended whether the insulation was internal as in the main house or external as in the extensions.
Mechanical ventilation with heat recovery (MVHR) provides constant fresh air and Paul describes this as a game-changer.
“The house always feels fresh, we don’t have any condensation anywhere, and we’re not losing that heat. I think the MVHR also helps with heat distribution, as we don’t have many heat emitters in the house. In 2018/19, when we were doing this work heat pumps were not as readily available as now, so we chose a boiler with a hot water tank. It’s the smallest system boiler we could buy, so that it can modulate down low enough for our 2 kW heat demand or heat load. This provides the hot water and runs a radiator in the living room and towel radiators in the bathroom and shower rooms.”
Paul says that in the autumn and spring they don’t have the heating on at all. The house might be at 19C initially but by the time everyone is up and moving all the incidental heat gains (from activities such as showering and cooking) warm it up to 21C or 22C.
They also have a couple of portable panel heaters (30W each) that can be moved around the house as required.
Unfortunately Paul’s house did not reach the EnerPHit standard as the final airtightness test came in slightly above the required levels at 1.08 air changes per hour at 50 Pascals. Interestingly during the build process they were on track, with readings of around 0.7 air changes per hour at 50 Pascals.
This goes to show how important it is to get the team to protect the airtightness layer once it’s in place.
The space had to work well for the family and perform brilliantly
As this conversation has been around the nuances between renovation, retrofit, remodelling and extensions, we asked Paul what his priorities were as he considered his funds.
Our budget was fantastic for a renovation and remodelling project, but once you included the retrofit elements it was much more constrained. I wasn’t really willing to compromise on performance… I was also aware that whatever we were doing to the house we were likely to be investing more than the house was really worth. So it wasn’t about getting a monetary return but getting a return in terms of comfort and health.
Although the work did put the family under financial strain at the time they don’t have a huge mortgage and now they’ve got some control over their heating bills.
They also didn’t complete everything at the time and left some DIY jobs to tick off over the years.
If you’re one of our Hub members, Paul also has a retrofit case study of Sycamore Hall.
Quality assurance is a big part of performance
If you can’t afford or don’t want to do a high performance retrofit you have to realise that there will be an impact on performance.
Paul says that if you’re adding space and extending you will need to meet building regulations which are better than they once were. The challenge is in making sure there is good workmanship on site so insulation is well installed and continuous. Moisture must also be considered and good ventilation is important.
Damp, mould and condensation are likely if you ‘just’ renovate
Just renovating an existing dwelling will not change the performance. So if there were problems with discomfort then they will still be there.
Paul has been to houses with brand new bathrooms and observed black mould around window frames. This is a sign that the moisture balance, ventilation and surface temperatures haven’t been considered.
Stripping back a building can help identify and fix problems
Quite often damp patches can stay hidden and you might not even realise you have a problem until something fails.
So major work is an opportunity to check that the home really is in good health.
National Retrofit Hub is providing a central Knowledge Hub to support all retrofitters
In terms of useful resources Paul suggests looking at the National Retrofit Hub, which aims to collaborate with those already carrying out retrofit projects, empower the next generation and facilitate retrofits in the UK.
It’s a nonprofit organisation that brings together all those involved in the retrofit sector to share their expertise and work together to enable a National Retrofit Strategy to be delivered.
It’s also teaming up with various organisations to reach architects and builders as well as those who might be considering a DIY approach.
Their goal is to see retrofit carried out at scale.
We have to start valuing health and comfort
The dial is moving slowly but we are starting to value good air quality and comfort in our homes. A few years ago internet speed was a more important factor!
Also, historically, energy has been cheap and it has been simpler to keep heating our leaky homes. However, we’ve already experienced sudden energy price hikes and this has forced people to become more aware of energy performance. Although this may not be translating into people investing large amounts to save money on heating bills it does mean they are paying more attention to energy performance and EPC certificates when buying houses.
Key challenges to consider for retrofit projects
Before you embark on a retrofit project the first thing is to make your home retrofit ready. Deal with leaky gutters, a roof in poor repair or bad pointing to make sure your home is in a good state first.
The second challenge is who to turn to for advice. You need to be sure that the advice is independent and solution agnostic. In other words you need to be careful the advice is not part of a sales pitch.
Paul mentions a few organisations and community interest companies around the UK:
Finally Paul underlines the importance of working with professionals who understand the moisture risks.
The newer the building that you’re working with, the less of a risk that is, but for example, with an old stone barn conversion you need to be aware of moisture risks. We’re now starting to see some retrofit assessors and retrofit designers who are specifically qualified in heritage buildings as well.
Neighbours Wookey and Rob Leedham explain why they were motivated to retrofit their homes, how they built up knowledge and why engaging with others on a similar path has been extremely useful.
Interview with Wookey and Rob Leedham
Neighbours Wookey and Rob Leedham share their experiences on their individual retrofit journeys. With technical minds and a passion for problem solving, this pair of DIY retrofitters have undertaken a vast range of tasks, from installing external wall insulation (EWI) to building a ground source heat pump! In the process they have saved a lot of money but we look at why they were well suited to this DIY approach and the value that they have taken from networking with fellow retrofitters (through green building organisations such as the AECB).
Wookey’s retrofit
Wookey’s journey began when he and his wife bought a house in Cambridge that initially looked presentable and tidy. However, it was draughty and not particularly comfortable and soon they decided to retrofit it. Having hired builders in the past and not been completely satisfied with the standard of work, Wookey decided to undergo the renovations himself.
Wookey’s house before retrofittingWookey’s house after retrofitting
The existing dwelling is a 7 x 8 metre square, 1960s detached house with a separate garage. During their renovations they began by insulating the loft and replacing the front door. They then continued on to wall insulation, both internal and external in different parts of the house. This was followed by replacing the old and inefficient boiler, which is now also about to be upgraded to an air source heat pump and eventually replacing all of the windows. Furthermore, Wookey chose to build an extension that connected the house with the garage and added a sun room.
Extension beforeExtension designExtension After
As the existing house is masonry cavity wall construction Wookey decided to keep things simple and construct the extension in the same way. They did hire a builder to put up the superstructure but Wookey undertook all the internal work, fitted the underfloor heating, tested the airtightness and even learnt how to fit an air source heat pump.
This DIY approach has been time consuming, with it coming up to a decade of renovations. However, it has also allowed them to save a remarkable amount of money.
“I learned quite a lot from having done it mediocrely once [renovating his previous property] and going, right, this time we’re doing things properly.”
DIY allows you to control the quality of the work
DIY projects can be stigmatised as below a professional standard, but Wookey explains that DIY has enabled him to get a much better finished product. After some bad experiences with builders cutting corners, Wookey found that doing the job himself allowed him to get the standard he wanted and all the details correct.
In Wookey’s experience, the nature of the building industry means that the right way is considered to be the one that gets the job done in the fastest time.
“The real problem with tradespeople is that everything’s time based. How many hours is this going to take? So they won’t necessarily thread the plumbing or the wiring in the right place, they’ll do whatever they can get at in half an hour, rather than doing it properly which is literally going to take 10 times as long. I think that’s the main advantage of doing things yourself is you get those little details right.”
Having a long-term plan for your retrofit is essential
It is commonly recognised that an over-arching retrofit plan is very important to ensure a successful project, mainly because it will help to avoid having to retrace steps or undo your work. DIY retrofits can take a long time and if you choose to live at home during that time, it will likely happen bit by bit or one room at a time. For example, replacing your windows without considering how it links up with insulation may be a missed opportunity.
Addressing a thermal bridge at the front doorSolar PV
Modelling your home in PHPP is invaluable
Wookey also decided to do the energy modelling for his home himself. Initially he was reluctant to use PHPP but discovered that it was fairly straightforward and offered a lot of benefits. It is a great tool for retrofitters as it helps you plan your retrofit, predict the potential performance of your house and identify what is needed in order to reach a given outcome. Sarah Lewis from the Passivhaus Trust helped us break down the topic of PHPP, providing an in-depth explanation of the benefits and sharing her own experience on a retrofit journey.
“I put PHPP off for a long time, because it’s a pain in the arse and it took a month from a standing start, even though I kind of understand the principles. But you know, it’s just if you’ve not done it before, it’s hard. But then you realise, actually, we could get to EnerPHit or at least pretty close on this house.”
You will need a certain set of skills for a DIY retrofit
Wookey’s background is in software engineering and he has always been interested in fixing his own cars. This meant that shifting his mindset to building physics wasn’t too difficult. Wookey advises to anyone starting out on this journey to spend the time learning the processes and discovering the how and why. Wookey admits that it can be a bit difficult to find this information online, but the Denby Dale Passivhaus study is a great place to start as they provide clear examples of where techniques have worked. This is also why retrofit and self build communities such as Cambridge Carbon Footprint, Cosy Homes Oxfordshire and The Hub here at House Planning Help are amazing resources that allow you to ask questions and learn from experts or others who have already been through the process.
“I’ve got a fair amount of practical skill. I’ve always fixed my own cars and that was where it started… and electronics. Buildings are quite simple in comparison to cars and electronics! It’s often fairly basic stuff. It’s mostly a matter of understanding building physics.”
There is a shortage of green builders in the UK
Wookey explained that in the Cambridge area and many other parts of the UK there is a real shortage of tradespeople who are experienced in sustainable construction. Hopefully this will change over time as demand grows and the industry comes to understand the need for attention to detail. In the meantime this might be another reason why a DIY approach is preferable.
External Wall Insulation (EWI)Eaves insulation
Some areas are worth paying the professionals
Tiling and plastering were two areas where Wookey decided to bring in a professional because they both require a level of practiced skill in order to achieve a good quality outcome.
Wookey also paid £1500 for some drawings for his extension, but did end up regretting this as they designed a standard box that didn’t consider energy efficient principles. As a result, Wookey ended up drawing his own plans and only used the plot measurements from the initial drawings to aid his own design. This offered him the freedom to create exactly what he wanted, however there is an added pressure to this and if things are calculated wrong then that is your responsibility.
“Generally, my assumption is that I can do everything, and it turns out that’s true for everything… apart from plastering, that’s the hardest thing in the world.”
DIY retrofits can be all consuming, it is important to maintain a balance
A DIY retrofit can be a long and time consuming process. For Wookey, the challenges and puzzles are enjoyable, but they can become all consuming if you are not careful. Wookey and his wife decided early on that at least one weekend a month they would do something other than DIY to ensure that they maintained a good balance. Wookey also highlighted that it was easier to give so much attention to their retrofit as they don’t have children and the commitments that come with family life.
Guy Hargreaves employed a contractor to carry out his retrofit. Watch our in-depth video case study in The Hub.
Many people are interested in retrofit but not the DIY approach
Since undergoing his own retrofit Wookey has encouraged family, friends and colleagues to consider taking on their own projects. While they understand the potential benefits a retrofit can deliver they do not always warm to the DIY approach! So, you can of course bring in a contractor to do the work.
DIY retrofits are a great option for people who have a passion for doing the manual work, enjoy learning and have plenty of time available.
“A surprisingly large fraction of the people I meet, think, yes, that is a good idea and I should definitely find out about it, a very small fraction of them want to do it as DIY or take ownership, they don’t really want to learn about the whole thing. They just want somebody to do it and there’s a terrible shortage. There aren’t enough people around here at the moment. I know several people who’ve been quite keen, and have tried to get quotes and struggled or been told that we can come and get to you in two and a half years time.”
You will begin to notice the benefits
Wookey explained that as well as the obvious benefits of an energy-efficient house being cost effective and warmer, they also began to notice small things that improve their daily life. For example, when you achieve a good level of airtightness and have triple glazed windows it becomes more peaceful. Also dust mites disappear when the MVHR is running and keeping the home within an optimum humidity range.
“When you go and rent somewhere to stay, you go to these cold and draughty rented cottages. And you think it’s not like my nice house at home. So you get quite picky about buildings once you’ve tried living in one that’s actually good.”
Rob Leedham’s retrofit
Wookey’s neighbour Rob Leedham has also been gradually improving his home. Rob and his family have lived at this house for 23 years and although he had no grand plan at the beginning, he has been consistently adding one green measure after another.
Rob has undertaken the whole of his retrofit as a DIY journey, including installing 150mm of External Wall Insulation (EWI) on all external walls and insulating the footings of the house. Rob is also passionate about solar power and energy generation. 12 years ago they installed solar PV panels on the roof and have more recently hooked it up to an inverter that will be able to accommodate both additional solar panels and batteries for energy storage.
Community access to airtightness testing kits will help make the retrofitting process more affordable and more accessible
One of the trickiest parts of retrofit is reducing the air leakage of a property, which calls for a good airtightness strategy. Having the airtightness layers all in place is not enough – the airtightness needs to be checked and then further leaks can be identified and addressed.
Rob showed Ben two fans that can be used for testing. One is a professional blower door kit and the other is a homemade version. These can loaned to other retrofitters in the area. A standard airtightness test is time limited and will cost around £300. Rob’s hope is to get a few for their group in Cambridge so that retrofitters can take their time to achieve the best airtightness results possible and not feel like they need to rush on.
Using their airtightness kit, here are the current findings for both Rob and Wookey:
Wookey had aimed for below 1 air change per hour (at 50 Pa) and is currently measuring at 1.52.
Rob had aimed to get his below 3 air changes per hour and is currently measuring a 2.1.
Rob’s DIY blower door kit!
Rob built his own ground source heat pump
Rob is also technically minded and opted to build his own ground source heat pump from scratch.
“The thing about ground source heat pumps is that they are actually very simple. There’s a compressor or an expander, and a pump and two heat exchangers. That’s basically it. So it’s not that hard to build your own. Some parts of it are inside and some parts outside. And so the electronics is inside and the header tank. And there’s a little display there that says it’s got a coefficient of performance of 4.4 at the moment. So inside, we’ve got that part that calculates the efficiency. And there’s a little electric meter, a standard electric meter for measuring how much electricity is used. It’s been going for about two years and a couple of months, and it’s up to four megawatt hours of electricity in to provide all of the heating and hot water for those two and a bit years.”
The key to heat pumps working efficiently is a well insulated house that doesn’t need much heat.
Rob does not recommend this route however unless you really enjoy the process and recognises that the sensible choice is to buy one and have it installed by a professional!
Stuart Dyer’s retrofit
Stuart Dyer is another member of the Cambridge retrofit community and has been retrofitting a 1930’s detached property. Stuart took a different approach and moved out for a year whilst they carried out an extensive retrofit. The process included external wall insulation, triple glazing, fitting mechanical ventilation with heat recovery (MVHR) and installing an air source heat pump. Stuart highlights that one of their biggest challenges was get their airtightness to a good standard and they worked quite hard on this area. They also put in underfloor heating, which works well with the air source heat pump, and insulated quite a lot under the floors.
Stuart likes being in Cambridge because there is quite an active retrofit scene, consisting of monthly meet-ups of the Association for Environment Conscious Building (AECB) (that Wookey organises). This offers a great chance to meet with people in the pub, discuss with those who have already done a lot of work, the good things to do and the pitfalls to avoid. Transition Energy Group is also quite active in terms of new technologies, new interests, and there is a lot of good advice there. Cambridge Carbon Footprint have lots of great case studies and also runs an Open Eco Homes each year where you can visit other people’s houses, whether they are new build Passivhaus or up to EnerPHit standard.
Search for ‘green building’ meet-ups in your area
In every town or city there will be opportunities to network with like-minded individuals and professionals by tapping into local green building groups. These are often as much about inspiration as sharing knowledge but it’s particularly useful for those on a DIY journey who may encounter challenges and be looking for advice or contacts.
Sarah Lewis, the Research and Policy Director at the UK Passivhaus Trust and author of PHPP Illustrated, explains what the Passive House Planning Package (PHPP) is and how it is used during the design process.
Interview with Sarah Lewis
Sarah Lewis is the author of PHPP Illustrated and an architect with a wealth of experience when it comes to Passivhaus building. In 2004, whilst at university, Sarah wrote her thesis on the Passivhaus standard at a time when there were no Passivhauses in the UK. Her first job was working for a firm that were beginning to look at building to Passivhaus standard and it was here that she became a consultant on London’s first Passivhaus. From then on, Sarah has worked on an array of Passivhaus projects and now with her current role as the Research and Policy Director at the UK Passivhaus Trust, it is safe to say her knowledge of Passivhaus and the Passive House Planning Package (PHPP) is extensive. In this chat, Sarah guides us through the PHPP, offering tips and tricks that will optimise the user experience and shares a bit about her own retrofit project.
Carrowbreck Meadow by Hanson Barron Smith
PHPP is a part of the quality assurance process
Passivhaus is a whole building approach to construction that aims to deliver healthy, comfortable and energy-efficient buildings. It focuses on high quality construction and through an exacting quality assurance process, provides clear and measured targets. Although there are different targets for new build and retrofit, the process remains the same; it’s about understanding and embracing building physics.
“It’s a really good standard to look at if you just want to have good quality, low energy buildings.”
The Passivhaus Trust has useful resources for both professionals and self-builders
Targeting the Passivhaus standard can seem daunting if you are not familiar with the process, but the Passivhaus Trust has lots of excellent resources to help you:
The Benefits Guide highlights the advantages of Passivhaus – from reducing our environmental impact, to reducing energy usage/bills and just generally providing a comfortable living environment.
The How to Build a Passivhaus Guide is aimed at those who have already decided to embark on a Passivhaus project. The guide includes top tips from experts in the construction industry and helps you to avoid the mistakes that are often made on first projects.
The Passivhaus Designer Course teaches construction professionals to deliver projects to the Passivhaus standard. It includes training in PHPP as well as considerations and methodologies for meeting the standard on site.
PHPP helps you make better predictions about the energy performance of a building
The Passive House Planning Package (PHPP) is a modelling tool that is used in the design process of a Passivhaus. What makes it work so well is that it considers a wide range of variable characteristics which affect heat loss, energy use and internal comfort. This means that the predicted energy use is much, much closer to the real-world performance.
PHPP is an ‘energy balance’ tool
PHPP is not a dynamic modelling tool. Dynamic models might use data points at 30-minute intervals, for example, but it doesn’t mean it would be more accurate than a simplified model. Dynamic modelling’s strength is when you are moving into unknown territory and don’t know whether a simplified model will be accurate.
So PHPP is powerful because it has a simple way to do the energy calculations. It’s based on the average monthly data which allows us to get annual energy demand. All this is quick. The data entry points are reduced to the essentials.
Sarah’s book: PHPP Illustrated is a comprehensive guide to PHPP that aims to break down the steps within the PHPP and provide clarity through her range of diagrams and flowcharts.
In every building there are heat gains and heat losses
All buildings will have heat gains and heat losses at various points throughout the year. In a Passivhaus, these losses are less noticeable because of the higher quality fabric but they will still occur at a lower rate through the walls, the roof, the windows, the ventilation system, etc. It is therefore essential to balance these losses with your heat gains. In a Passivhaus, there are three main categories of heat gain:
Internal heat gains – This is the energy that is created when people move around the house or use equipment, such as an oven.
Free heat gains – this is the solar gain, where sunlight passes through windows and creates a warming effect.
Supplementary heat gain – Once you have calculated the internal heat gains and the free heat gains, whatever is still needed to balance the heat loss becomes the heating demand for the building.
“The number that you often hear in Passivhaus being quoted 15 kilowatt hours per metre squared per annum. So that’s your space heating demand for the building. There’s also an alternative metric, which is the peak heat load, which is 10 watts per metre squared. But the PHPP is constantly reworking that balance.”
It can be useful to visualise the heat loss and heat gain as a seesaw that needs to be balanced. The exact figures and components will be different for every building, so the solutions will need to be specific to your design.
“If you have a bungalow with a huge roof, you’re going to have a lot more losses through the roof than if you had a tower block, which has a very small roof, but much more walls. So you can see how each building will end up with its own bespoke set of losses, and therefore its own bespoke set of gains as well.”
Lots of different ways to use the PHPP
The PHPP is a giant Excel book with many different sheets that you work your way through inputting the specific details of your project. The PHPP has built in assumptions that are based on general data and can help to provide an estimate without having to input into every single section. However, Sarah did highlight that these assumptions are often on the conservative side, and as you begin to input more of your own data, you will see your PHPP improve little by little.
“For example, there’s a default assumption for how much the windows are shaded until you actually have your site set. And then you can work out how much those windows are actually shaded and put that information in.”
In the early stages of your planning using the PHPP you will want to provide information on the fabric of the house: how big the envelope of the building is, the orientation of that building and the size of the roof, walls and floor. From there you can play around adjusting elements, for example adjusting the orientation and seeing the impact that that has on the energy performance.
There are also a wide range of plugins that can supplement the PHPP and help to analyse certain aspects of your design:
Design PH – A sketch up plugin that allows you to create a fast model which can be exported into the PHPP.
Bim2PH – Allows you to export your BIM model into PHPP.
PassivLink – Lets you export your Revit project data directly into PHPP.
Avoid design by ExCel spreadsheet!
Although PHPP is an excellent tool for predicting and testing the performance of a building, it should not become a replacement for actively making your own design choices.
“The PHPP, as an architect, allows you to go: okay, what if my building was orientated slightly differently? How does that affect the energy balance? Or if I have a big window here? Do I have a summer comfort issue? Or is that really good for my thermal performance of the building? What we don’t want to do, is encourage design by Excel spreadsheet. People should be using their design intelligence, first and foremost, they need to make those intelligent design choices.”
Form factor, glazing ratios, complexity and orientation will all have a massive impact on a project and these can be optimised early in the design process.
“A common mistake is to use a model and then just tweak the entries. For example, window sizes, just to get the desired number to that 15. So while yes, of course, you could just increase your free gains, solar gains, in this case, by making your window bigger and bigger and bigger. You need to be smart and think about summer comfort, embodied carbon usability, and other things.”
As more Passivhaus projects have been delivered around the UK the key learning around window design is to focus on excellent daylighting, rather than maximum solar gains (which can lead to summer comfort issues).
PHPP gives us a ‘whole picture’ of space heating, energy use and delivery
The bulk of the PHPP looks at the form factor, airtightness, insulation, windows, ventilation system and orientation of the building. All of this information combined will provide you with the space heating demand or cooling demand of the building. Then within this, the PHPP goes further than the general UK standards and metrics by looking at both regulated and unregulated energy. In the UK this is referred to as the EUI (Energy Use Intensity) which aims to look at the whole picture of energy use. This takes into account all the heating and cooling systems, the pumps, the lighting, the electrical equipment/plug loads, all the cooking that’s happening in the house and all the hot water being used. All of these combined make up the EUI, which provides the overall energy performance of house.
PHPP includes where the energy comes from
PHPP is not only concerned with the energy that is used in the building, but also the energy that is created and delivered to the building. This includes all the storage losses and distribution losses of transporting a kilowatt of energy from a wind turbine or solar panels, into your building (Primary Energy Renewable). All of these combined create a very comprehensive picture of how energy flows around the building, including where it is generated and where it is lost.
The PHPP looks at components in more detail than standard UK metrics
Take the example of windows. Manufacturers will provide the U value of their windows which measures how effective the overall product is as an insulator. This measurement is not enough for the PHPP because windows are made from a range of different materials. In order to get an accurate understanding of the window’s overall performance, it needs to know the performance of each material used in the window. Manufacturers who sell Passivhaus rated windows will often have this data available, even for products which aren’t certified, but it can sometimes to be difficult to get the necessary data from suppliers who are not used to working on Passivhaus projects. Heat pumps can also be difficult because you need to input a variety of test points both externally and internally, it has been challenging to get the enough information for the PHPP to accept it. However, Sarah believes this is changing and that the more Passivhauses that are built in the UK, the more accessible this information will become.
PHPP can be used for a quick check
People who are experienced with PHPP can make a mock-up in a couple of hours, especially if they are also familiar with the project. One way to upskill is on the Passivhaus Designer Course where you will learn to use the PHPP tool over a couple of days. Because the system is based in Excel, which is a familiar software for most of us, once you have learnt how to input data, it isn’t that difficult to use. It is very common for designers to tailor the process and create their own base PHPP, which has all the values of the construction systems they usually work with already pre-programmed.
“It makes it a bit quicker the next time around, maybe your favourite Windows already input, or the air source heat pump you like working with. All these things you can put into the PHPP and then use for future projects”
Your PHPP can provide a lot of useful information that you can take away
Because there are a lot of people who use the PHPP, there have been number of different plugins developed to help you get the most out of the software and analyse the data it provides.
The Passivhaus Trust Summer Comfort plug in assesses your PHPP and creates a summary of anything that could be a point of risk, for example, a large temperature swing over the course of a single day. The plug in would then ask for a justification of why this is happening and if there is an adequate control strategy. By highlighting these areas, it helps to minimise an oversight in design that could effect the performance of the building.
The DeltaQ plug in analyses your data and determines whether your building will comply with the RIBA 2030 challenge and the LETI climate emergency design guide targets. This therefore aligns the building performance with UK metrics rather than just the Passivhaus specific ones.
There is also a section of the PHPP which can analyse the data to calculate an estimate of the running costs for your Passivhaus.
PHPP is a core principle of Passivhaus design
Completing the PHPP is an essential step in creating a Passivhaus.
Sarah says that there is a common misconception that Passivhaus principles only refer to the fabric characteristics of the building such as super insulation, triple-glazed windows, MVHR and so forth. However, these alone do not create a Passivhaus. The quality assurance comes from the certification process and PHPP is a big part of this.
There must also be attention to detail on site, otherwise we may not close the performance gap as much as we expect to.
PHPP can be used by anyone!
You don’t necessarily need to work with an architecture firm that is experienced in Passivhaus design. The PHPP should be easy enough for any architect to use and its always great to get more consultants to come on board.
“I would certainly say that if you’ve got an architecture team or consultant team who haven’t worked with it before, it’s something where the Passivhaus certifiers can bring a lot of value. So we always recommend that the certifiers are appointed somewhere around RIBA stage two or maybe the beginning of RIBA stage three once you really understand what the building is going to be so the certifiers have something concrete to price. At that point, they can come on board, they can do design checks on the PHPP to really highlight if there’s been any early errors or omissions and then they can do construction assistance… And then obviously, they’re doing the certification at the end as well. So they can really help if a project team are new to Passivhaus.”
However, Sarah also highlights that the PHPP isn’t just exclusive to architects and that if you are keen to give it a go yourself, the manual and wealth of other supporting documents can help you along in that process.
“I think anyone can do it. The very first PHPP I ever did, I had to just use the manual and fill it out because there was no English courses at that point. I was self taught in that sense. I would say it took me a bit longer to fill out. You have to be quite experienced to be able to do in a couple of hours. And I think the main thing is orientating yourself. There’s a lot of spreadsheets to work through. But the manual comes with a flow diagram. And you can follow through the sheet by sheet and you can fill out everything. But it is definitely helpful to have gone on a course before trying to delve in. I didn’t have that luxury when I did my first PHPP, which is why I ended up writing the book.”
The PHPP has had a few changes over the years
In 2016, the formula for how the Primary Energy was calculated was changed, which also allowed the Passivhaus Institute to bring in different classifications: Classic, Plus and Premium. These classifications are based on both the usage and generation of primary energy.
“So rather than just standard Passivhaus, it brought in Plus and Premium, which were ways to acknowledge projects which are generating energy, so renewables as well as being very energy efficient.”
Primary energy hadn’t been a massive focus in UK Passivhaus practices and this change to the PHPP helped formalise a shift in mentality towards renewable energy generation and promoted including these aspects in the designs. Before, it could have been considered acceptable to fit a gas boiler into a Passivhaus, but this change shifted the intention of Passivhaus to also work towards a future grid supplied primarily by renewable energy. Apart from this, the majority of other changes have only been minor tweaks or improvements to the interface, to make it more user friendly.
Sarah’s Retrofit Project
In 2018, Sarah bought an ex-local authority, 1950s terraced house in East Lothian, Scotland. She has since been renovating the house with the aim of ultimately getting it to reach EnerPHit standard.
“I would probably say, we had exchanged and had the keys for about a day before I got into the house, measured everything and started inputting it into PHPP. The main reason for that is I wanted to see what the heat loss from the building was, so that I could plan what we’re going to do to improve the building. So that’s one of the ways that you can use PHPP in a retrofit, you model the existing building and then you can see where all the heat loss is, and you can see where you should focus your money to save that energy.”
Sarah’s retrofit has not quite reached EnerPHit standard yet as it does not meet the airtightness requirement. This is one of the largest challenges in a retrofit project and Sarah says that it is the roof of her project that is not currently up to spec.
“There’s some air leakage in the roof, which is just really difficult. We didn’t replace the roof so it was just really difficult for us to address all of that. I think when the roof comes up for its replacement, so it’s a 1950s house, but might have another 20 years in it, at that point, I hope we can get the airtightness down.”
For Sarah, retrofits can really challenge you to think creatively with your design because there are so many constraints for you to work within. A challenge arose in her project when it came to insulate the floor, as to add insulation to the floor would reduce the clearance height of the room. Sarah worked around this by taking the ceiling right back to gain some extra headroom, which in turn exposed the beams, adding a lovely design feature to the room. This shows that with a creative mindset, constraints can be turned into intentional design choices that add character and detail to a home.
“You can plan out a retrofit and you can do that over a few years or you could do over a few decades”
Create a ‘long term’ retrofit plan for your home
So long as we have a comprehensive, well thought through retrofit plan it doesn’t all have to be completed all at once. PHPP could be used to help us break down a retrofit into steps.
“The EnerPHit retrofit plan encourages you to design all of that upfront, and it almost sits as a kind of retrofit passport with your building, which can be passed on to future owners if you move or if it’s an asset, which is owned by like a local authority, or a council, or housing association or someone, they can also sit with that asset as people in the company change. And then people always know what’s the next plan for any particular building.”
This procedure could be revolutionary in the UK and PHPP will be a really useful tool to analyse the existing housing stock and work out the best way to get as many houses as possible to higher standards.
“Going for a full EnerPHit retrofit is probably not the right solution for all buildings. We’ve got a huge job in the UK to retrofit our existing stock and we need to take the easy buildings and go as far as we can. And then we need to recognise that some buildings will never get to go that far and planning out a lesser retrofit design for those projects is also really important and can also be done in a really robust tool like the PHPP.”
Jonathan Dixon talks about incorporating home automation into a Passivhaus retrofit of a 1960s detached house, built using cross wall construction.
Interview with Jonathan Dixon
Jonathan Dixon is a software engineer with a background in mobile app development, medical imaging and artificial intelligence. He now provides consultancy to others looking to take advantage of home automation.
Learning lessons from different climates
In the mid-2000s, Jonathan lived in a typical South London terrace. The biggest challenge in the UK was keeping the house warm enough in winter while managing the humidity and the potential for damp.
He then moved to California for six years, renting a classic wood-framed Californian bungalow. It had no damp issues, but was draughty. Cold nights would be freezing, and Jonathan also became very aware of the risks of overheating.
Having spent time skiing in the Alps, he also saw that better quality construction was possible. Jonathan didn’t know much about it, but would often joke that his ideal home would be in California, designed by a Swiss person.
Applying software development workflows to construction
Having relocated back to the UK, Jonathan discovered a colleague was working on a Passivhaus retrofit. It was a complex project due to conservation area restrictions. This served as Jonathan’s introduction to EnerPHit and he began to research more about it.
What excited him most was the way the project team worked. Client, architect, contractor and other key trades all used a project tracking app to communicate, manage any changes, and sign off work through sharing photos. Everything in the app then became part of the Passivhaus documentation. The workflows reminded Jonathan of how software development would be managed, and felt like something he could get involved with.
Finding a home with the right build quality was almost impossible
The main criteria for finding the right house was location. Jonathan knew it wouldn’t be possible to buy something that had the level of build quality he wanted, so he expected to do work on whatever he bought.
Original house (south elevation)
Original house (north elevation)
He went in understanding the deep retrofit mentality. If you’re going to do ‘X’ then you might as well do ‘Y’ at the same time. And then you might as well go the whole way and do ‘Z’ too. That meant he was willing to buy something that needed quite a lot of work.
A priority for Jonathan was to install MVHR, as he saw that as an answer to keeping the home warm while managing the humidity.
Home automation wasn’t the driver
Jonathan didn’t buy the property with home automation in mind, but decided to incorporate it while doing everything else. The colleague who showed Jonathan his retrofit project gave him a spare Loxone mini server to play with, and Jonathan spent about a year getting an idea of what he could do with it. The whole house needed rewiring anyway, so he adopted a forward-thinking approach of how to incorporate it.
The Loxone server is like the brains running the house
It’s also very small – about 150mm wide – and could even sit inside a consumer unit. As a programmable logic controller, it monitors what’s happening in the house and executes commands programmed into it. Those commands are whatever you might want them to be, such as changing the temperature or lighting. And it works reliably, without ever going offline to do system updates like a typical computer would do.
For Jonathan, the key to automation is that it’s based on software, so it’s easy to change. You can alter the conditions in the house without having to rewire things, or replumb things. It’s as easy as opening a laptop and changing a few bits of programming.
A lot of technology can be connected!
The start of work on the house coincided with the first year of the pandemic, so Jonathan spent a lot of time in lockdown experimenting with the technology. Understanding the mini server was relatively easy, but then it was a case of figuring out the communication protocols that everything else would use to ‘talk’ to each other.
Lights and switches were one thing, but there were also blinds, shading, automated windows, carbon dioxide monitors, and so on. That required a lot of exploring.
In the end, Jonathan stayed within the Blockchain system because it reduced his own learning curve, but he made sure the wiring could be switched to a different protocol in future if needed.
Loxone’s growth provided reassurance
Jonathan felt a natural tension between choosing a proprietary system, where you don’t know if the manufacturer will be around for the long term, and using a combination of different technologies, where compatibility can’t be guaranteed as different components update at different times.
He looked at alternative systems, but went with Loxone because they have invested in their platform and expanded to building control on larger projects. As a result, Jonathan is happier that his system is much more robust now.
The house’s cross wall construction presented an opportunity
In the 1960s, cross wall construction was an experimental build-type. It means the two gable walls are loadbearing, while the front and rear walls are non-structural ‘infill’ curtain walls.
Jonathan spoke to nine different architectural practices, looking for local knowledge and Passivhaus expertise, but none really picked up on the construction method. Just as he was about to appoint someone, he came across A D Practice. The cross wall construction, and the retrofit potential it offered, was the first thing they noticed.
The non-structural walls could be demolished and rebuilt to a much higher standard, rather than trying to insulate them and get all the junction detailing right.
‘Buy in’ was essential for all team members
Because EnerPHit projects are relatively rare, it’s hard to find people with relevant experience. Jonathan’s main requirement was that the architect and contractor be serious about Passivhaus being part of their future. As a beginner himself, he didn’t want to pull people along on his own journey.
The foundations had to be dug twice
Building Control changed their minds and were unwilling to sign off the original floor design, despite the ground being fine. Rather than backfilling what had been dug out, they decided to double the amount of insulation in the extension floor. It cost the same as backfilling and turned this frustrating situation into a positive.
A software development mindset was beneficial
Jonathan was very open to change as the project progressed. He puts this down to the iterative development process, and looking for opportunities for improvement. You can’t design something and expect it to turn out perfectly as conceived.
Caring about the construction, especially the invisible elements, also played a part. Knowing that the airtightness couldn’t be compromised, that the right levels of insulation were important, and so on, meant caring about the process as much as the end result.
Help with the lighting design paid dividends
Jonathan began to put in the automation infrastructure, and was varying it as he went along. But he realised trying to ‘wing it’ was only getting him so far, as he hadn’t thought much about what lighting would actually work with the automation.
At the same time, he avoided going over the top with settings. With the Loxone system you could effectively have disco lighting if you wanted! Whereas the real benefit is in settings like night lighting. For example, if you have a guest staying, you can program low level lighting to help them find the bathroom easily during the night.
Energy management is possible with home automation
However, Jonathan thinks the real gains would be seen across an estate of many properties. He has experimented with things like running the heat pump according to times of high solar production, and he has other ideas, but in a single home the savings are marginal.
Home automation helps to balance different Passivhaus features
In a typical house, you have the heating on until it is warm enough, turn it off, and then turn it on again when it gets cold. In a Passivhaus, you have multiple things that could end up working in conflict, and which need balancing: winter heating, summer shading to guard against overheating, MVHR bypass, and so on.
Multiple systems running together can be inconvenient, and even waste energy. Integrating them all through automation means they all just do what they need and work towards a single goal.
There’s not much point trying to manage the temperature in individual rooms, because Passivhaus delivers such stable temperatures. But Jonathan found that a room used occasionally as a guest room could be too warm, so he had to invent a way to allow guests to set a slightly cooler temperature when needed. That was the only zoning he incorporated.
Simply logging and monitoring data can be helpful
Through his consultancy work, Jonathan has actually convinced several friends to build to Passivhaus standard where they weren’t originally planning to do so!
For other properties, data logging and monitoring helps to identify particular things causing energy bills to be high. It can go to the level of monitoring individual circuits and seeing where energy is going. With the data gathered, you can then use automation to manage different areas of the house, keeping some cooler when they’re not being used. Motion sensors or predictive models can provide heating when it’s needed.
Alan Budden explains how a phased retrofit might work and what preparation is required. He also shares how he’s retrofitted his own home, which includes trialling an innovative roof solution.
In this session he reflects on his own retrofit which is getting towards the final stage.
Alan’s practice Eco Design Consultants is well versed with deep retrofit and were the architecture firm leading the Kinver project.
Get access to our in-depth video case study of the Kinver retrofit in The Hub
If you have the funds, tackle a retrofit in one go
Nobody ‘chooses’ to do a staged retrofit because it’s a better option to do all the work in one hit.
Not only will it be cheaper overall (for example tradespeople might have to make a few more visits during a phased retrofit) but the disruption will also be minimised.
And from a planning perspective it will be simpler too.
There’s lots of junctions you have to consider in terms of how it’s going to be weathered and work during that stage, as it could be six months, it could be 10 years between stages. That’s one of the issues. So you end up detailing things, then for the final construction detailing it back.
Retrofitting in stages is a way to progress with the money you have today
Recently Alan has noticed a lot more enquiries from young families who are keen to upgrade their homes.
Unfortunately they lack the funds for a comprehensive retrofit so Eco Design Consultants recommend tackling it in stages.
The most important aspect is to have a whole house plan before you start.
A staged retrofit could allow you to stay put during building work
Depending on the size of the house one benefit of a phased approach is that you might be able to move around the house while the work is being done, thus save money on moving out and renting accommodation.
Retrofit at an appropriate stage in the maintenance cycle
All houses require maintenance and upgrading so there will naturally come a point where you’ll be spending money on your home anyway.
The house still had single-glazed windows
The single-glazed windows were not fit for purpose and had been something Alan had wanted to change ever since he bought the property. While he could have upgraded to double-glazing he bided his time so that it would be a switch straight to triple-glazing. His cladding was also tired and needed replacing so doing all these things together can contribute to the project costs.
The front of the house (before the retrofit)The back of the house (before the retrofit)
Think of a deep retrofit as creating a house for life
A deep retrofit is significant work so it’s unlikely that you’d just want to sell the house straightaway afterwards. So you should be trying to make the house work for you now and into the future.
Think carefully about how you want to live in the house. Could you also add a bit of value (by increasing floor area, if needed)?
Can the house shape be simplified?
Alan wanted to do the design work himself
Despite having an architecture practice at his disposal, Alan thought the ethical approach was to do the design work and modelling himself. Consequently progress was slow, being confined to evenings and weekends.
Modelling the house in PHPP is a key step
Alan says that after deciding how you want to live in the house and optimising the layout, the next step is to begin modelling it in the Passive House Planning Package. Setting an energy target for the project, such as the Passivhaus retrofit standard EnerPHit, helps inform decisions about insulation, windows, etc.
The shape (or form factor) of the house was improved
Where the first floor was slightly smaller than the ground floor Alan decided to square up the building. This increased the floor area, allowing for a new dressing room and increasing the size of the en suite. but also simplified the shape of the building.
Alan’s knowledge meant he could optimise his approach
When selecting materials Alan was always looking at a balance between cost, performance and embodied carbon.
I looked at some interesting things with the external wall upstairs. Timber frame work projects off the wall with GRP brackets with just ordinary cavity wall insulation inside. So I’ve looked at the DriTherm 32, the DriTherm 36 and a couple of others, so it’s different types of mineral wool. And as it was less… the thermal conductivity was worse, the density was less, and the embodied carbon was less and the cost was less. So trying to optimise how thick the insulation was, and which ones I used was quite interesting. And depending on what sort of brackets I could get as well for the timber frame. So it was quite a complex lot of options.
Think how you can stretch funds
Alan planned to do parts of the work himself to save money and therefore, hopefully, take the project further.
He was also planning to test a new roof system which would be manufactured off site and placed onto the existing trusses, but it had a few teething problems. Knowing there would be a delay in the programme there was nothing to be gained by hiring builders to get through the work quickly.
The builder was happy to step in as required
Alan describes how he and his family put on the membrane (from the scaffolding). The builders then came and put the brackets on and the timber framework. And then it was back to Alan and family to fill it with insulation. And so on.
After taking the roof off the utility room and building upwards, they stripped the tiles off the rest of the building and made a few adjustments in preparation for the new roof cassettes.
A stick-on airtightness membrane, Siga Majpell 500, was installed.
A multi-purpose roof was manufactured off site
Alan has always been interested in how retrofit might be tackled at scale and as such he’s been part of a group developing an innovative roof panel system.
With funding from Innovate UK the project has progressed and he has become the guinea pig for the first full installation.
The idea was to develop a structural roof panel that includes PV and solar thermal, using that to heat the house.
Off-site manufacture comes with benefits
Using a crane the panels were lifted into place in just one day.
This is a big health and safety benefit because it reduces the amount of time that people are working at height.
Alan comments that making the roof panels have more than one purpose should bring about some economies of scale.
Unfortunately some of the technology underneath the panels that was being used to cool them didn’t work as planned, so they are looking at an alternative and hoping to lift off two of the panels off and replace them with new ones. Then they can get them plumbed in and see how much energy they are reaping from the roof and how that’s helping to improve the performance of the PV.
Project delays meant Alan was without a roof for 6 months!
With development delays and then the pandemic Alan was caught in a tricky situation. He had taken off his old roof, added a membrane and then discovered the roof panels were not ready!
The manufacturers of the membrane normally suggest that it can be exposed to the elements for up to a month.
So Alan had a challenge on his hands, particularly when a big storm came along.
My roof was left exposed for just over six, including the winter, which is not the best of times, which meant that I was getting condensation forming on the inside because the insulation was planned to be put on the outside and I had some insulation left in the loft to keep us a little bit warm in the winter. So I was having to mop it down, which shows you really, the amount of water I was taking off was quite considerable. At the worst times, I almost had a bucket full of water. I was at the end of it with my mop! And it does show you how much moisture you’re putting out as a family. And if that’s going to condense somewhere that could cause all sorts of problems. Luckily I could see it, I could deal with it. And now obviously that’s gone away.
The membrane did survive and seems to be doing its job.
Get the MVHR online as soon as the house becomes airtight
In a new build the MVHR might be commissioned towards the end of the process but in a staged retrofit it needs to go on sooner, otherwise moisture and CO₂ will build up.
The only downside of that is you will probably go through more filters as they get clogged up with the dust and the dirt from both the outside and the inside. Alan has been switching off the unit altogether if there are particularly dirty or dusty jobs going on.
The insulation will go internally on the ground floor
Alan wanted to keep some of the brickwork visible externally to match other houses in the estate, which means an internal insulation strategy is required downstairs.
The windows downstairs will open outwards to maximise insulation on the frames
All the windows upstairs have been changed because they went within the external insulation so they’re opening inwards. On the ground floor we’re doing internal insulation and those windows can open outward so we can maximise the insulation onto the frames on the insides. So there’s a number of windows downstairs still to be done. I did, we did take a few opportunities to insulate some of the windows on the ground floor, for instance outside our snug and the downstairs toilet because there’s some external cladding there with some vertical battens instead of horizontal just to emphasise… a bit of architecture! And also squeeze in a bit more external insulation.
There’s added motivation as they near the end of the project
The final stage of the project will be to insulate internally on the ground floor and replace the windows.
One of the things that’s happened because of the extended programme is that we’ve been able to keep going. We weren’t necessarily expecting to do the ground floor stuff straightaway but it’s carrying on now! And because upstairs is so much warmer than downstairs, it’s really given us an emphasis to say we need to get this done. You realise the difference. Generally upstairs has been about three to four degrees warmer than downstairs. And that shows all the Passivhaus physics that air moves when there’s three to four degrees difference.
The builders will come back to replace the windows
With a house that’s getting warmer as the project progresses, the builders will be coming back to install the windows.
This will get the changeover done quickly.
Then it’s back to Alan and family to tackle the internal insulation room by room.
So we’ll put some studwork up, insulate it, membrane it, re-plasterboard and finish it off. And also on the ground floor, what was going to be phase three that I think might happen at the same time is the floor insulation. The plan is to dig up the screed, put down a phenolic insulation and the board on top.
A preliminary air test was conducted before the roof went on
Having installed all the airtightness membranes as a family, Alan wanted an airtightness test before the roof panels went on.
There were a few reasons for this. While they are always interested in an overall score, it’s more about identifying key leakage hotspots that can then be fixed.
As with the Kinver project, once the roof is on there will be little hope in fixing leaks in this area.
That’s why attention to detail is important. The number on this first test was 1.7 air changes per hour at 50 Pascals.
Biggest risk with retrofit in stages is having to redo work
Alan says one takeaway from the project is the importance of a well thought out initial plan.
One thing I’m not enjoying quite so much is in our utility room and kitchen, I had already insulated some of the wall there with 50mm of polystyrene, mainly because at that stage, I was worried about the interstitial condensation. So I was limiting it to what I could do. But because now I’m insulating some on the garage side as well, I can increase the thickness of insulation internally. So I’m having to take out some insulation to put different stuff back in, which is a little bit frustrating to be honest. So get the whole plan done, make sure that works, and then do bits towards it, because that way you’re not going to be replacing those bits.
