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.
Marion Baeli, author of “Residential Retrofit,” offers hands-on experience of designing and retrofitting high performance homes. As well as drawing on her personal retrofit she shares the lessons learnt from revisiting properties that took part in the Retrofit for the Future competition over a decade ago.
Interview with Marion Baeli
Marion Baeli is Principal, Sustainability Transformation at 10 Design and author of “Residential Retrofit,” covering the evolution of retrofitting over the past decade. Originally a French architect, Marion came to the UK in 2002 and later took a master’s degree in sustainable architecture and design. This brought a fundamental shift in her approach so that she now embraces the Passivhaus methodology, and considers all aspects of embodied carbon and material circularity in her work.
She was fortunate to be part of the Retrofit for the Future competition and has been able to revisit several of the properties, ten years later to assess their performance. She also draws on the experience of her personal retrofit project to highlight the differences between good and bad retrofits.
Retrofit needs a holistic, system‑level approach
Marion stresses that many retrofit failures stem from treating the building as a collection of isolated fixes. She points out that focusing solely on insulation, without addressing moisture control, ventilation, airtightness, and thermal bridges, inevitably creates new problems. A coordinated design that balances all performance factors is essential for long‑term durability. This mindset is now embedded in standards such as PAS 2035, which require a comprehensive retrofit plan before work begins.
“You can’t really do a retrofit by looking at just one thing… you need a holistic approach.”
Start with thorough planning and expert co-ordination
For Marion the most reliable retrofits begin with careful upfront planning, bringing together architects, engineers, contractors, and a retrofit coordinator to ensure that each intervention is contributing to the overall performance target. Marion recounts that the original “Retrofit for the Future” case studies succeeded because the teams invested heavily in design time, avoiding costly re‑work later. Skipping this step often leads to unintended consequences and poor performance.
Long‑term performance holds up when quality is built in
When revisiting a number of the original case studies that featured in the book and carrying out building performance evaluation, Marion reports that airtightness, insulation integrity, and thermal bridge mitigation have largely remained intact. No major failures such as tape peeling or board collapse were observed, confirming that a well‑executed retrofit can be long-lasting. The key takeaway is that investing time and expertise at the start pays dividends in sustained performance and lower operating costs.
Watch our in-depth video case study of the Kinver retrofit in The Hub.
Regular maintenance of the building fabric is essential
Even the best‑designed retrofit can be undermined by neglected maintenance (which is obviously nothing to do with retrofit). Marion cites a case where a leaky gutter caused brickwork to spoil and moss growth which could have threatened the structural longevity. Simple upkeep, cleaning gutters, repairing cracks, and checking flashing helps preserve the gains achieved through retrofit.
Window and door seals degrade over time
One common observation of the post occupancy evaluation was that gaskets around windows and doors have begun to lose elasticity, potentially starting to compromise airtightness. Although these can’t simply be replaced at the moment Marion believes we should all be raising this concern with our window manufacturers. She explains that on bigger projects and social housing they develop relationships with these suppliers and so units might be removed, repaired and replaced. This circular approach is needed for one-off homes too.
Financing retrofit needs to look beyond ROI
Marion argues that retrofit is not about return on investment. She likens it to buying and maintaining cars; you don’t expect a return on your investment when you buy a car and you maintain your car to ensure its safety and longevity. In many cases, the key benefits of retrofit are reduced energy bills, improved comfort and health, along with future‑proofing against climate change. Creative financing, phased implementation, and leveraging grants can make the upfront cost manageable.
Marion bought a mid terrace home with a plan to retrofit it
The plan and timelime of the step-by-step retrofit
Step‑by‑step retrofitting can be a good way to spread out the costs
Rather than tackling the whole house at once, Marion describes the phased strategy that she adopted. This involved making changes to the internal layout first as well as installing ground floor insulation and the ducting for the MVHR. Later she was able to address the external façade and windows, making sure that the house maintained its integrity with the other 1960’s houses on the estate. This allowed her to spread the costs over several years, while still moving into the house after the first phase. Planning each stage to dovetail with the next ensures that interim works do not compromise later performance targets.
“We’ve spent a lot of time planning the ductwork route – that’s the most difficult part of a retrofit.”
Significant construction work is better in one hit
Marion contrasts two approaches to construction disruption; to fully vacate the property for intensive works versus a staged, live‑in alternative. Her own project required a temporary relocation for six months, but she notes that many retrofits can be completed incrementally, allowing occupants to remain on site, perhaps moving around the house as the work is carried out. Choosing the right strategy depends on the building’s condition, the scope of work, and your personal tolerance for inconvenience!
A retrofit is often the time to look for other opportunities to make the building better
While carrying out the internal work, Marion added a light well to bring natural daylight into the core of the house, reducing reliance on artificial lighting. They also created bespoke timber storage units in the entrance hall and living area. In the summer months they are ready for a heatwave and have a pop-up fabric that they can stretch across the windows. Though simple DIY, this method effectively cuts solar gains, keeping the interior temperatures comfortable.
Passivhaus Open Days allow you to understand more about low energy homes
Ben and Marion are both big fans of the ‘Passivhaus Open Days’ event, which enables you to step inside completed Passivhaus or EnerPHit projects. This is a great way to experience the indoor environment and meet homeowners or professionals who have been involved.
“I’d encourage you to take advantage of these events… you can just make up your own mind.”
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.”
Will South from Etude takes us step-by-step through the retrofit of his Victorian terrace house in Brighton, which has improved the health, comfort and energy efficiency of the property.
Interview with Will South
Will is a mechanical engineer, working at Etude to make buildings better through reducing energy consumption and carbon emissions, an approach he has carried over to all the places he’s lived. With his most recent move he had in mind a retrofit project right from the start. He wanted to be in the middle of Brighton, close to the railway station and the coast, in a modest sized home. So he chose a Victorian terrace house, originally a two-up two-down that he could retrofit to the level that would be right for the building.
Many period properties have common traits
Will’s house is a classic Victorian terrace design, where there is space for you to take off shoes and coats just through the front door, the stairs are in front of you, then there’s the living area on one side and a kitchen all the way through at the back. Upstairs, there are two bedrooms and a bathroom. Will describes it as a simple layout with a nice flow and high ceilings, even upstairs, which is more unusual.
Deciding how deep to go with your retrofit is a personal choice
A big part of Will’s day job is working on Passivhaus projects, but here there was a question around the right level of retrofit for this type of property.
This project has also made him realise that maybe not all homes need to have as deep a retrofit as he used to think, and that heat pumps are more than capable of heating the majority of homes with few changes. However, he knew he wanted to take it further and make the house as comfortable as it could be. With his experience in ventilation systems, glazing specifications, insulation, etc. nothing was off the table.
“We’ve done everything: walls, roofs, floor, new ventilation system, new windows, but a lot of it is as an experiment to show how it could be done.”
Watch an EnerPHit retrofit of this Victorian home in The Hub.
Decarbonise your heating by moving to a heat pump
One key move Will wanted to implement to lower his carbon emissions was to switch from a gas boiler to a heat pump. For most homes, some fabric measures may be needed, often topping up loft insulation, improving windows and upgrading radiators. Anything after that will continue to improve comfort levels, reduce bills and increase resilience for the future.
Create an overall plan for your retrofit
Will started with his dream end point, irrespective of budget, to come up with an overall plan for the retrofit. For example he has not installed solar PV as part of the project but he has made allowances for it as a future addition.
With a solid retrofit plan, he then examined the priorities. There was leak in the roof and a rear extension, where the floor and the roof were in a sorry state. Generally, starting with the roof is a good strategy, which for him meant redoing a parapet then making sure the party firewalls and the chimneys were in a good state before he did anything else, so that the building had a good chance to stay dry.
Having a good ventilation strategy is essential on a retrofit project
Will tells us that although it took a long time he ended up doing everything in almost one go. If you have to do a phased retrofit, you need to make sure that the building is left in a good working state at the end of each stage. The main risk is around ventilation and managing moisture in the building, and you need to be careful you don’t seal up the building or insulate it in one part without dealing with the ventilation. A ventilation strategy is often a priority and if the end goal is a mechanical system, then get that in early, so you’re already managing the moisture levels.
SketchUp was used to create drawings for the project
As a consultant Will knew what was needed but he wasn’t used to doing the drawings himself, so that was a bit of a challenge. He used SketchUp to create a model of the house, including drawings for the existing building and the proposed building. Being in a conservation area, planning permission was needed for replacing windows at the back of the house, and altering the roof-scape slightly to fit the heat pump at the rear, which led to more drawings for the planners. In the end he made a full set of drawings to help him work out what he was going to do and allow him to communicate easily with the contractor.
Will imagines that in the future there will be templates for every building type, that you would alter to reflect your own situation. There’s already a good website called retrofit.support, which has a pattern book of details that give you a place to start.
Create a detailed cost plan
Will paid a cost consultant who specialises in housing to provide him with a full cost plan. This gave him a long list of everything needed, with a starting point for all the prices, giving a good feel for what it would cost if he did everything. This service cost around £400 but was invaluable.
The costs for construction broadly came in as expected. He imagined that there might be a big cost differential between materials but in actual fact he was dealing with such small areas that the cost was all in the labour.
Budgeting for the internal finishes was much more difficult as costs varied dramatically, particularly for bathrooms and kitchens. So unless you have an example of the product that you want to use, it can be hard to budget accurately for this stage.
Different insulation strategies have been used around the house
Will explains that even though it’s a small building a variety of strategies have been needed to insulate the roof, walls and floors. Initially they planned to live in the house while the retrofit took place. The first phase was going to be the rear return, with the kitchen and the bathroom. The kitchen ceiling was falling in and the bathroom ceiling looked worrying, so both needed attention.
The building has all sorts of roof types, which is not ideal. There is a small parapet at the front which has got insulation between the rafters and a lead roof. Then there’s the main loft which has mineral wool insulation, the rear return which has Larsen trusses, filled with insulation in the pitch, and a small flat section of fibreglass roof which has got the heat pump on it.
“Generally, starting with the roof is a good strategy. An old fashioned building has a good hat and good boots.”
Planning restrictions also had an impact on the insulation strategy
Being in a conservation area an internal wall insulation strategy was adopted at the front of the building, using 80mm wood fibre board. There is a bonding coat underneath, made by Baumit, which helped create a flat base for the boards to go on to and added an extra 10-20mm of insulation.
At the rear of the property they were allowed to insulate externally and have used cork, which was chosen because it deals with moisture better than some other products. The external wall insulation is taken below ground to the foundation with EPS.
The floor became a bigger project than originally planned
While Victorian properties are highly desirable today, when they were built they were thrown up quickly and cheaply so there were a few surprises lurking. When they took up the floor, they found slim 3-inch joists sitting on continuous bungaroosh, which was just piled up chalk. As they were able to move out of the house for a few weeks they could tackle the floor in a more significant way.
It involved taking out all the timber and the dwarf walls, scraping back the chalky base to create the space needed for the floor build-up. EPS was directly put onto the sand blinding, then a plastic DPM, and finally a concrete screed with underfloor heating. It’s a simple build-up as there’s no structure sitting on it. Will would have liked to have reused the original floorboards on top of the screed but was advised against it.
The sash windows have been improved
While Will had more freedom at the back of the house, where he installed some triple-glazed units, at the front of the house there were restrictions. He also did not want to get rid of the beautiful period sash windows. So he has replaced the sliding part of the sash and put in vacuum glazing. The boxes are original although he has added a draught strip to improve airtightness. He’s used a foam filled rubber draught strip, a type more often used on a casement window, which is more robust and gives better airtightness, but it makes the windows a bit stiffer. As all the windows are wooden, they will need maintenance and Will suggests to always repaint them just before they look like they need painting!
Having a continuous airtightness layer was easier said than done
The strategy at the beginning was to identify where the airtightness line would be for each part of the construction. Will realised that they would be able to achieve it for some parts, but in other areas they wouldn’t be able to get into places to create an air barrier. He estimates that he got around 75% of the way with airtightness and to get any further would probably have meant gutting the whole house and starting again. The intermediate floor and all the internal walls stayed where they were, but the external wall finishes were removed allowing for bonding of the insulation, creating continuous runs to tie into the next airtightness material and into the windows. The damp proof membrane underneath the screed on the ground floor comes up the inside of the wall and is taped on there. In the loft Will put a membrane over the top of the joists, cut it around all the trusses, taped it and then put the loft insulation on top of it. It was very difficult to tie it into the top of the walls right into the eaves where access was limited. The eaves were extended very slightly at the back of the house to accommodate the external wall insulation. This meant the last two tiles were off for a time and Will used the opportunity to do more taping. His approach was to use membranes only where there was timber, and then everywhere else to use a wet lime product.
Before the sash windows were replaced, Will got an air permeability of 2.1 which is three air changes per hour at 50 Pascals, which he was pleased with and was in fact the number he’d put in his PHPP model. He hopes it will have improved and is going to get another air pressure test and more thermography in the winter.
MVHR can usually be integrated seamlessly into an existing home
Installing mechanical ventilation with heat recovery can really improve the indoor air quality in an existing dwelling. It manages moisture in the home, allowing you to do things like cook or dry washing without consequences. And Will believes that there is always a way of designing a system into an existing home and figuring out the duct runs, etc. It just takes careful planning.
The benefits of MVHR are noticeable even in a leaky home
Even if the airtightness has not been addressed fully Will feels that installing MVHR will improve the ventilation in a house and because it has 90% heat recovery it will give some energy saving no matter how efficient your building is. If you then improve insulation and airtightness you will continue to see benefits.
A heat pump is a good choice for a low energy home
When they bought the house it had a 10-year-old combi boiler, which they could have used for a bit longer, but Will was keen to switch to a heat pump heating system. So when redoing the bathroom he took the opportunity to put a new hot water cylinder where the boiler was, because all the pipe runs link to that point. An extra motivation to get off gas was the massive gas meter that was located in the entrance hallway. He paid extra to get this disconnected and removed.
In Will’s experience it was difficult to buy the heat pump and difficult to get it installed, a view that other people have also expressed. At the time, it was also difficult to find somebody qualified to install heat pumps. In the end, his builder’s plumber carried out the work, and used it to upskill and get his MCS qualification, which was really positive. On the negative side, he hadn’t known they didn’t have MCS and so he was not able to get the boiler upgrade grant.
He had assumed that the heat pump manufacturers would know what was needed for a small home, so hadn’t looked into the detail himself. That was a mistake and the whole process got complicated very quickly. They’d assumed he wanted thermostats in lots of different places, and that the radiators and underfloor heating circuit were on different zones. Will ended up having to change things afterwards, which was frustrating. The other eye opener was just how much equipment you end up with inside the house. He had thought about a cylinder, but with unvented cylinders you also have an expansion vessel for the heating, an expansion vessel for the hot water. Then there are two control boxes, two zone valves and they wanted to put in a buffer vessel as well. It really started adding up, with a lot to fit in a small area. He recommends buying a hot water cylinder with a built-in expansion vessel, which is a bit more expensive but saves on space.
Will’s energy consumption has gone down significantly
The average temperature within the house has increased to a constant 20 degrees, which is where Will wants it to be. Gas consumption has gone from 158 kilowatt hours per metre squared per year, which was quite high, to zero.
This has however been replaced with electric heating, with electricity use now around 40 kilowatt hours per metre squared per year. This went up about 20% after the switch over to electric heating, so a much smaller uplift than the reduction in gas. By Will’s calculations his carbon emissions are down by 85%, which is significant. If solar PV was added at a later date they could be net zero.
Some of this is about how they live, with just two people in the house they don’t have huge consumption and are out a lot of the time. In financial terms their energy costs have more than halved, reducing from £2,500 a year to about £1000 a year.
Sticking to a budget is tricky!
Will sees a budget as a good tool to manage costs although he realises this isn’t necessarily what you’re going to spend. Therefore he put aside a significant contingency. For him this takes away the worry about money, which can so often cause problems.
Buying a smaller house in the first place and securing a bigger mortgage so as to free up the cash was also sensible. They had an inheritance and some savings, which essentially has all been spent on the house. It has come in at around £150,000, but this figure includes a new kitchen, new bathroom, new flooring and a lot of extras.
Replacing the suspended floor was a gamechanger
Amongst the takeaways from the project Will feels the hard work and disruption of putting in a solid floor with insulation underneath has really paid dividends. So if you have enough space to play with then this is something Will would definitely do again as it’s made it draught-free and always comfortable to walk upon.
The installation of the heat pump relied heavily on the plumber
Will was surprised that even though they had a Vaillant heat pump and used their designs, a lot still rested on the plumber. If he was doing it again Will would take more of it under his control.
Airtightness is often about doing the best you can
Will thinks he did a really good job on airtightness, but reflects upon the fact that there is always more that could be done. If he were to do it again he would use more traditional wet materials like lime-based plasters for airtightness when dealing with crumbly brickwork.
The house is now comfortable all year round
When they bought the house the structure was worrying with a bendy beam, ceilings falling down and it was very damp, with slugs behind the kitchen cabinets. During the winter they were just heating whichever room they were in and downstairs they were always aware of draughts from under the floor. There is none of that now, it feels homely and is a constant 20 degrees throughout the house all year round.
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 experience of working on his own retrofit project, and the lessons learnt in the process.
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.
There are always compromises, so how do you best juggle and manage those? Toby bought his house with the idea that it would be relatively straightforward to retrofit. There was nothing (e.g., listed building status) to prevent getting permission to make changes.
The house had deep eaves, making external insulation easier. It was a question of how to execute that while minimising thermal bridging, getting the windows in the insulation zone, and incorporating airtightness.
The original bay was a feature Toby decided to remove
Toby had lots of ideas of what he would like to do. He built an extension and did some minor remodelling first, before starting retrofit work. From a holistic point of view, it would have made sense if the extension had been planned to receive the same external insulation that was added to the house later. The building would have performed better too, as there is a thermal bridge where the extension’s cavity wall meets the new external wall insulation on the main property.
Living in the home first can provide useful information
It takes time living in a home to understand what’s best for how you use it.
There is also thinking about how the home might need to be used in future, and other considerations like whether you’re adding value.
The floor plan of Toby’s house already worked well, so there was little reason to change the layout. He considered taking out a wall with a chimney. That would have been a major structural challenge, so he prioritised keeping things simple and less expensive.
Watch an entire retrofit of this mid terrace home in The Hub.
A truss and fill insulation system can take different forms
There’s a lot of experience with truss and fill systems, especially within the AECB membership. A timber frame is fixed to the outside of the building, with a thin wood fibre board over it to create a cavity. Any blown insulation can then be installed to the cavity; Toby chose cellulose insulation.
There are different ways of doing the frame too. Toby used a Larsen truss with 50 x 50mm battens on the wall, then plywood gussets to support an outboard batten. You could equally do it with an engineered joist, or similar.
There are different ways of finishing the outside. Toby used a 25mm batten cavity with a render-carrying board, and then a render finish. It could also be timber clad. Rendering straight onto the wood fibre is possible but might need more careful thought when it comes to rain robustness.
A second layer of defence between the outside surface and the insulation is comforting, even if you’re not in a very exposed area for rain. If you have render direct on insulation and the render cracks, there’s a higher likelihood of water passing through to the inside.
Plywood window boxes were constructed to take the new windows
The first step was preparing the window openings
To ensure the new windows were in line with the cellulose insulation, Toby built plywood boxes and installed them in the opening so they cantilevered out. They were fixed through the face of the ply into the edge of the cavity wall masonry, and they allowed the new windows to project into the insulation zone.
The bay was removed
New studs for the insulation layer
The cement board going on
A cavity acts as a capillary break in the wall build-up
The cavity in the original wall build-up acted as the capillary break to stop moisture reaching the inside of the building. With the retrofitted wall build-up, Toby made sure the 25mm batten cavity behind the render-carrying board was ventilated and served that function instead.
Below-ground insulation links the floor to the external walls
Toby dug a trench around the building, installed moisture-resistant insulation, then backfilled it and used paving slabs to hold the insulation in place. The 200mm timber frame with the cellulose sits on top of that. The below-ground insulation finishes just above ground level to keep the timber clear of ground level. It required a lot of hand-digging and re-doing drains that are close to the house.
Planning ahead helped with airtightness in the roof
Toby removed the old insulation from the roof. Some leftover mineral wool from the extension went between the ceiling joists, followed by a vapour control membrane over the joists. He installed cellulose insulation over the membrane – it comes in compressed bales which you open and it expands into loose cellulose.
When installing the membrane, Toby left plenty of length on it and folded it back under the rafters. When the scaffolding was up and the end of the rafters exposed, he peeled and then slit the membrane so it could fold down between the rafters. It was then a case of taping that all around, first to the rafters and then down onto the existing render. It felt like it was going to be time consuming, but then proved to be okay once Toby got into it.
To achieve continuity of the roof insulation with the wall insulation, Toby was able to remove just a single row of tiles. He slipped in some insulation that the wall insulation could butt up to. Waterproofing it required some leadwork, which was an unexpected expense, but a nice way to finish it and robust so it won’t let moisture in.
Following established principles can help keep embodied carbon low
Toby hasn’t calculated upfront or embodied carbon. Instead, he has followed general principles that he has learnt from other projects, including using very little concrete or steel.
In terms of concrete, there are small amounts of mortar for repairs and holding the paving slabs in place for the below-ground insulation, plus the 12mm cement board carrying the render. Some stainless steel bars were required as part of a Helifix Helibeam system, which was used to replace a failed concrete lintel.
Retrofit can make it tricky to accommodate MVHR
In an ideal world, Toby would have chosen MVHR, but finding a good position for the unit is always difficult in a retrofit. Put it in the loft, and access for maintenance is harder. Put it in a room and you lose valuable utility space. Installing the ducts is also tricky and can involve a lot of compromise.
Toby chose an Aereco MEV system over MVHR. The system was installed at the time he built his extension, as this was the best time to get ducts running to and from the loft.
MEV is effectively the extract half of a full MVHR system. There’s one fan in the loft, with a duct to each wet room. Each living space has an inlet through the wall, and on each inlet and outlet is a humidity sensitive material that passively operates like a damper. When someone has a shower, the damper opens a bit more to help clear the elevated humidity. The fan in the middle has a pressure sensor which increases the fan speed when the terminal opens more.
Decentralised MVHR, with individual room units, could have been an option but Toby’s experience is that it tends to be relatively expensive to do a whole house that way.
Plan ahead for low carbon heating
Toby’s home still has a gas boiler, a decision he was comfortable with at the time. He will upgrade to an air source heat pump at some stage, and has the infrastructure and electrical supply in place where the hot water cylinder would probably be positioned.
The retrofit industry perhaps overlooks the DIY culture of the UK
Toby has travelled in parts of Europe and recognises the DIY culture we have in this country compared to other places. With motivation, and the right research and guidance (including the House Planning Help podcast!), a proficient DIYer could undertake a retrofit project.
But it’s also important to be aware that it’s a lot of work, with stress and long hours. Toby was working under floodlights until midnight the day before his cellulose insulation was installed, finishing off taping so there was a cavity ready for the insulation.
Standard details are a good starting point for DIY retrofitters
You might have to pay for the information, but it’s less expensive than engaging a consultancy service. If you have building experience and broadly know how a timber frame goes together, or how a masonry wall goes up, then a lot should be fairly obvious. It’s about thinking through the next steps as you work, and coming up with solutions for how X joins to Y.
Toby understands moisture risk and so designed a window detail himself that others thought was overkill. He was happy with it as a belt and braces approach to keep water away from the cellulose insulation. It’s a case of assessing risks and accepting when there is value in paying for advice. Structural calculations are one area that Toby was glad to have professional support.
Every project has unique technical challenges
Toby uses the example of external insulation. There are a host of ways to do it, but you can guarantee that any project will always throw up a detail that no-one has ever come across – or, if they have, that a solution is not well documented. Again, it’s taking time to think about how to do it well using the array of products that are available.
Budget for and prioritise the ‘big ticket’ items
The biggest expense on Toby’s project was new triple glazed windows. The next biggest cost was re-roofing the property, as the bituminous underlay needed replacing. The roofer was able to reuse the original tiles, and the roof work was carried out while scaffolding was up already, which helped. The cellulose insulation was a big undertaking, but not necessarily the biggest cost. Toby did a lot of work himself getting the timber frame ready.
If Toby tried to value the time he has put into the project then it would probably be one of the most expensive retrofits done recently. He has really come to respect the speed at which professional builders work. But he is also interested in the subject, gets enjoyment from doing the work, and can take lessons learnt back to his practice. All of that provides additional motivation.
Junction details and internal finishes take a lot of time
A big lesson for Toby was that lots of time, thought and effort goes into the junctions. The timber frame went up in a few days, which really looked like progress. On the other hand, working at the foundations, taping around the rafters, and sorting out the windows all took a long time.
A lot of functional and structural items don’t have to look nice, they don’t have to be finished. But since Toby has begun to work inside, time has been eaten up finishing window reveals, and sanding and painting so that the finish is to a standard he wants.
It’s hard to capture in words how much more comfortable a retrofit is
In conclusion, Toby observes that he knows the physics of why his home feels more comfortable, but he finds it difficult to express how different it feels when everything’s a more even temperature. You can only truly feel it by experiencing a well-insulated house. It’s a feeling that it just … works.
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.
Florence Collier, founder of Humblebee, takes us step-by-step through the deep retrofit of her 1970s home. She explains the costs relating to each measure and the cumulative total as the project advances.
Interview with Florence Collier
Florence Collier, founder and owner of Humblebee is a chartered mechanical engineer and Passivhaus Designer. A large part of Humblebee’s work is around home retrofits, both modelling and carrying out retrofit assessments. Here Florence takes us through the retrofit of her 1970s house, giving us costs for each part of the project, talking through her decisions and showing us how the different aspects of the work fit together.
Front of house (before the retrofit work)
Reducing carbon emissions was the driver of the project
For Florence reducing carbon emissions was at the forefront of what she wanted to do, but she also had a desire for more community-based living. Cohousing would have been ideal, but she was aware of how much work and time goes into getting a scheme off the ground. When a property came up with a simple footprint, in a good location, that was in need of updating, she thought this was a perfect opportunity to retrofit to the EnerPHit standard, and put into practice everything she had learnt from the Passivhaus training course.
Back of house (before retrofit work)
The simple shape of the building made the retrofit easier
The house was a typical 1970s cavity wall brick construction with a ventilated suspended timber floor. Florence explains that it was basically a box, with a plastic portico stuck on at the front and a single glazed, brick porch at the back. It was clear that to do a proper retrofit both of these structures would need to be removed.
If you have skills and time, you can save money
As a professional in this field Florence was able to model the whole building, make decisions on materials and commission each piece of work as required. She did work with an architect on a few of the details and would advise anyone who doesn’t work in this industry to get a specialist on board to guide you through the process. The material and labour cost of the retrofit to date is just under £106,000, which doesn’t include Florence’s time and other items that could add an extra 10-15% to the total cost.
The plans for the project
All decisions tried to balance efficiency and embodied carbon
The plan was to get as close as possible to the EnerPHit standard, which is the Passivhaus standard for retrofit. This essentially targets a space heating demand of 25 kilowatt hours per square metre per year. Florence knew they couldn’t reduce the embodied carbon already in the building, but they could keep any new features as low carbon as possible and reduce the operational carbon. Moving off fossil fuels was the goal alongside creating a healthy home, free of mould and condensation and with fresh filtered air.
The first step is to get over any mental blocks that you’re having to starting this journey, because it’s so easy to say, well, I can’t do this unless I’ve done this.
Florence took on this step herself
Loft insulation was a small but important first step
When Florence found herself stuck at home during lockdown, it seemed like the ideal opportunity to get started. So they ordered in the materials for the loft insulation and then installed it themselves. Florence explains that although the loft insulation was less than 3% of the total retrofit cost it was an important first step, as between 8 – 10% of their heat loss was through the roof. They chose to use wood fibre and hemp, which aren’t the cheapest insulation products, but sustainable and nice materials to work with. Check out Florence’s blog to go into more detail about this step.
Material costs for loft insulation £2,700 for 56.5m²
The cavity wall was filled with EPS beads
Cavity wall insulation can be added independently of other retrofit works
Whilst waiting for other larger works to begin Florence was able to go ahead with the cavity wall insulation as it didn’t depend on other aspects of the retrofit. She found it was a straightforward step in terms of the material choice, not choosing a natural material in this instance because of the risk of drawing moisture from the ground. In a cavity wall, the cavity is there to conduct any trapped moisture away from the internal walls, so your insulation needs to be moisture repellent. They used EPS polystyrene beads, which have a decent thermal conductivity and contain some adhesive so the beads won’t pour out of your wall if you make a hole in the brickwork. Again, this was a relatively cheap step to take, and the effect was noticeable that winter. Check out Florence’s blog to go into more detail about this step.
Total cost of materials and installation £1,701 (approx. £15.50/ m²)
The new triple-glazed windows go in
Next came the replacement of the old PVC framed windows
The house had double glazed PVC framed windows that were coming to the end of their life. Florence wanted to avoid using plastic in their replacement and opted for timber windows instead, which also looked good aesthetically. They chose triple-glazed windows from 21 Degrees, but from their slightly cheaper range that were not Passivhaus certified. From the PHPP model these windows would still allow them to achieve their energy targets. An external powder coat finish on the windows extended their warranty and protects them from the weather.
Florence explains that a lot of the heat loss through windows is through the frame, so they simplified the number of panes and openable parts. Fixed windows have narrower frames, so it’s about trying to find the right balance between fixed and openable. Having fewer openable windows also helps with airtightness.
The windows sit in a plywood box
Florence was aware that installing their new windows was always going to be a challenge, as they needed to ensure both airtightness and insulation continuity to avoid thermal bridging. Installation ended up taking a day per room, with a plywood box built and fitted to each window before then lifting all of it into place. This was needed to bring the plane of the window in line with the planned external wall insulation. The box allowed the window to be pushed out into that layer, which could then be taped and sealed for airtightness. Check out Florence’s blog to go into more detail about this step.
Total cost of windows and installation £22,033
Florence putting on some external wall insulation
External insulation is like wrapping your house in a blanket
Florence explains that heat gets conducted out of your house wherever there is a junction or fixing structure that is not made of an insulating material. Wrapping the house in external wall insulation (EWI) is therefore an easier approach compared to trying to insulate internally. Florence chose wood fibre as a natural material with low embodied carbon, which would also be good to work with from a general health perspective. The brick construction of the existing house was built to be vapour permeable and wood fibre would maintain this breathability. A lime render was used on most of the house to create a weatherproof layer, with some timber cladding on the ground floor frontage to provide variety and break up the box shape.
Where the walls meet the floor can also create a thermal bridging issue, so Florence wanted to continue the wall insulation down to the sub floor. Wood fibre was used from the damp-proof course upwards and then below that they used a polystyrene plinth, that could be wrapped all the way round. A French drain type system with gravel was used to channel any water away from the building itself. Check out Florence’s blog to go into more detail about this step.
Total cost of materials and installation £26,100
The MVHR unit is in the cold loft
The siting of the MVHR unit was not ideal
Florence explains that she designed everything in parallel, making sure she could see the route for the ductwork before choosing windows. They were clear from the start that the MVHR unit would need to go into the loft. This means the main ducts are running from the unit along the loft floor into a couple of rises, with branches off to minimise the amount of distribution within the house itself. They have a couple of bulkheads dotted around and one or two ducts on display as a bit of a talking point. As they don’t have particularly high ceilings they weren’t going to lower any ceilings to accommodate the ducts and also didn’t want to go through floor joists on the first floor.
In general Florence would not advise putting the MVHR in the loft, particularly if it is a cold loft. It’s fine if you’ve insulated at rafter level, but if you’re trying to hit targets like EnerPHit, all heat loss counts and the amount of ductwork outside the thermal envelope, even insulated does contribute to the overall efficiency of the MVHR. They tried to minimise this by insulating all the ducts with 50mm foil-backed duct wrap. Check out Florence’s blog to go into more detail about this step.
Total cost of MVHR system and installation £ 9,880
Some external shading was built into the design
Shutters could be used more as a simple shading solution
Being half French, half Italian, Florence is used to shutters as a shading device. She sees them as an easy way to slash air conditioning loads in commercial buildings and to make domestic settings more comfortable. Florence was aware that her southwest facing bedrooms were going to overheat in the summer and the initial plan was for simple outward opening shutters, that you pull to and latch. Florence then realised that their windows were quite large with only a third of the window openable, so that type of shutter wouldn’t be possible. They quickly moved to the idea of sliding shutters, but it was still a challenge to find the right dimensions and the right sliders. The next issue was how to fit the solid wood frame to the external wall installation. The answer was to fit lintels above and below the windows made of a composite foam material, that would be less of a thermal bridge than timber.
The wood fibre insulation on the outside of the building has a good thermal mass so this also helps to keep the building cool in the summer. The MVHR can be put in ‘summer bypass’ mode, so when it is cooler inside than out, it can reduce the temperature of the incoming air. But the shutters have increased the comfort level and provide privacy, a sound barrier and security if you want to have the window open. Check out Florence’s blog to go into more detail about this step.
Total cost of shutters and installation £5,547
The suspended timber floor
The finished earth floor
Improving the thermal performance of their floor
Florence tells us that when she first started planning the floor there just seemed to be two options, insulate their suspended timber floor or replace it with concrete. The floor joists could be insulated with an airtight and windtight membrane but would still need to be ventilated which would compromise on the thermal efficiency. She was not keen on concrete because of the embodied carbon and saw it as a cold and hard finish. After hearing Jeffrey Hart talking about earth floors she spoke to another retrofit assessor who’d had an earth floor put in with underfloor heating. They had a deep void, so room to build up the layers needed for an earth finished floor.
They started with an insulating layer of recycled foam glass aggregate which is a lightweight, inert and hydrophobic material. This was put on a geotextile membrane to a depth of 450mm which gives you most of your insulation. Soil goes on top. Florence used MOT type one, which is compacted down to create a solid base. The earth floor finish is a clay, lime and sand mixture with added dust to give the pigment. This layer is only 20mm and is then oiled, waxed and buffed to create a smooth finish.
The original estimate was around £20,000, with most of the material cost going on the glass aggregate. However, they experienced a few delays, initially because the amount of aggregate needed had been miscalculated and so more had to be brought in. The MOT soil was then very wet, so extra time and large fans were needed to dry it. Some miscommunications over how the floor joined the interior wall led to more delays and this work had to be done retrospectively once the earth floor was finished. This all led to extra costs. Check out Florence’s blog to go into more detail about this step.
Total costs of materials and installation £30,318
Want to go in-depth on retrofit? Check out the Kinver story in The Hub.
Finally the electric hot water tank allows them to be independent of gas
Florence explains that they always planned to get off gas and had been keeping their old boiler going until they could get their new cylinder installed.
The hot water cylinder is a Mixergy tank, which Florence describes as an intelligent hot water tank. It has been installed in the loft and will have an insulated cupboard built around it. The tank heats from the top down, separating the cold from the hot, so you’ve always got an amount of hot water ready to use. It uses machine learning to heat water when needed and a PV diverter so any excess solar that is being generated goes to the immersion heater and gets stored as hot water.
Total cost hot water tank and installation £6,859
Extra heating will be from electric panel radiators when required
As the work progressed, radiators were removed from the house and once the water tank was working there was no further need for their combi boiler. The plan is to use electric panel radiators where extra heat is required. More heat may be required at the moment as the floor is still drying out. Florence also realised after a cold winter that the MVHR system hadn’t been on the right setting for full heat recovery.
Replacing plasterboard ceilings is one of the last things to do, but it’s a messy job
One key task remains which is the airtightness of the ceiling on the upper floor. Although they’ve insulated the loft, they’ve still got cracks in the plasterboard ceilings, and a leak in the condensate from the MVHR means they now have a hole in the bathroom ceiling that is just covered up with a piece of ply. They plan to take the plasterboard ceilings down, which is going to be a messy job. That will be the final stage and should reduce their heat losses to EnerPHit levels.
Are heat pumps enough or are fabric improvements needed first?
Florence says there is a lot of talk about heat pumps and whether you should do fabric improvements first or just swap your boiler out for a heat pump and be net zero ready. Florence designs heat pump systems and thinks they’re an incredible technology but feels that reducing it to either this or that option means you’re not taking into account the benefits that come from a well-executed retrofit. You might be able to get the numbers to stack up, but for Florence it is about more than just reducing the carbon emissions. A really important part of her retrofit has been about health, comfort and the general quality of life that has been achieved through the fabric improvements. Even temperatures throughout the house, no draughts, moisture or condensation and constant fresh air all add to the comfort levels. The MVHR helps asthma sufferers and allows you to dry clothes inside as the moisture is constantly being removed. Together these aspects are incredibly desireable.
Howard Johns, founder of One Zero, outlines their home energy transformation service which includes topping up insulation and adding solar PV, battery storage and a heat pump.
Interview with Howard Johns
Howard shares his extensive knowledge of renewable energy, and how he has moved from managing a large portfolio of solar and wind farms to enabling people to generate their own energy. Over the years it has become apparent to Howard that the real sticking point with the large scale projects is the National Grid, which is currently struggling to take new connections. His new company One Zero energy, is focusing on how we retrofit homes in an efficient and light way that can enable them to become part of a new, flexible, distributed grid.
The UK grid is no longer fit for purpose
Electricity companies began as regional entities, with one power station supplying each local area, which worked until there was a problem with that station or too much demand. With no back up in the system, some areas of supply would have to be shut down to enable other areas to keep going. Our current grid system was a national project to join all these power stations together with transmission lines so that it had more inherent flexibility. But essentially, it was built as a one-way system, with power being generated in a small number of locations and then flowing to a large number of supply locations.
With the advent of renewables, particularly wind and solar, distributed generation emerged. So generation is happening in all sorts of locations and Howard explains how we are rewiring the grid backwards to allow this, which has caused a lot of issues. In fact, in many areas it is just not possible to connect any more large-scale renewable generation because the infrastructure cannot take it. If we can’t do that and need energy transformation now to tackle climate change we need to look at other options.
The current energy system is extremely inefficient
Howard explains that for every 100 units of gas used only 40 of those turn into useful units of electricity. During transmission to our homes another 10% is lost and then we might lose more through inefficient appliances. The usable energy you’re getting in your home from that primary energy is around 30%. So if you start to generate energy where you need it you can massively reduce the wastage.
With heat pumps we can be getting 400% more energy out of it than we put in, compared to maybe 90% efficiency with a gas boiler. It is similar with transport, a standard fossil fuel car is about 30% efficient, whereas you can double or treble the efficiency for an electric vehicle. In other words the new electric based technologies that are emerging are the future.
A resilient energy system has more back-up options
Howard outlines how we already have at least five interconnectors into this country to build resilience in the system. A European wide grid allows excess energy, say from Denmark, to be supplied to other European countries.
Behind the scenes at the National Grid (NESO) people are looking at live demand, and turning on things like peaker plants and gas plants, and using batteries in some cases to meet that demand.
A resilient system must get past intermittency
As the amount of renewable generation increases, there’s a new challenge – intermittency. So there will be a correlation between the weather conditions and how much is generated. For example, on a sunny day we might generate 20% of our power from solar, but there will be nothing at night. At times we will get really high levels of grid energy powered by the wind, but on a still day, we may get very little. This is one of the key features of the new system and it is causing problems.
Better storage capacity will help solve the problem
As the cost of energy continually changes if we have the access to battery storage then we can benefit. So it makes sense to use energy at key moments, when it’s abundant and cheap, and avoid using it at six o’clock in the evening when the demand is highest and it’s expensive.
It’s always preferable to use energy as it’s being generated
Howard tells us that it is preferable to use energy as it’s being generated because there will always be losses associated with storage. With batteries you lose around 10% of the energy you put in, just in charging and discharging. However if you haven’t got demand for that power, it’s still a sensible thing to do.
Part of One Zero’s strategy is to install a solar PV array in combination with battery storage. This would mean that for eight months of the year you could be virtually self-sufficient for power. With a decent control system you can effectively run your battery as a micro power station profiting from arbitrage and engaging in the flexibility markets.
Import and export to the grid when it makes financial sense
With many big batteries being built across the UK there may be times when power is cheap or even free! So, in addition to your own generation, in these moments it may make financial sense to import from the grid to charge your home battery or discharge power back to the grid.
Innovative companies, like Octopus and Ovo, already have flexible tariffs that will give you different power prices at different times of day.
Howard thinks the next step would be managed services that take into account wholesale prices, which change every half an hour. Again these have periods where energy is very cheap or even negative, and other times when it’s expensive. With an agile tariff you could see prices change from -2p to 50p a kilowatt hour over a 24-hour period. Software is needed for this management, however, and this is a service that One Zero are working on.
“Generating and storing power at your home is probably the most cost effective way of getting power right now. Not many people realise that.”
Heat pumps will work without a deep retrofit
Using heat pumps is another part of the One Zero energy transformation strategy. People may be under the impression that a deep retrofit – improving the thermal performance, and the health and comfort of the building – is required to make these systems work but that is not the case. So at One Zero they suggest a light touch retrofit, topping up insulation levels and doing the easy wins.
Howard points out that putting a heat pump on a really leaky house is going to be inefficient, but heating it with a gas boiler is also inefficient, so on one level there’s no difference. At the same time, if you use a heat pump, you may also knock four or five tons off your CO2 emissions every year.
However, it still makes sense to do as much as you can to reduce heat loss. When One Zero assess a property, they’ll make recommendations around deeper measures, which could be done first or incrementally over the following years to improve efficiency.
Considering payback has been a big part of this approach
One Zero are looking at measures that could start to pay back in 5 years rather than 40 years or more for deeper retrofits. Howard believes there are millions of homes that need to be improved in the next decade or two, so it’s important that this really appeals to people and offers a clear win. By starting with solar PV and a battery people can achieve a cash flow that they then might reinvest in the next steps. One Zero are also looking at finance packages so you don’t need all the capital upfront to get started on this home energy transformation.
One Zero plan is for people not looking to do a deep retrofit
Howard explains that One Zero is aiming to reach people that will not undertake a deep retrofit of their property, either for financial reasons or lack of engagement with this approach. Their approach is to provide a top up for cavity wall and loft insulation and then solar PV, a battery and a heat pump all for around £21,000. You could have a home energy transformation that would drop your bills by around 50-60% and your carbon emissions by 70%, all delivered within two weeks with minimum disruption. They think this is achievable for homes built after the 1930s, which already have some cavity wall insulation, double glazing and some loft insulation.
What is a deep retrofit? Check out the Kinver story in The Hub.
Every technology has an impact, but this should be weighed up against the alternative
Howard has put a great deal of research into the One Zero home energy transformation package. He believes that if we reached 100% renewable generation our rates of extraction of raw materials from the earth would drop to about a 30th of current levels. The backdrop is that we live in a highly extractive society, where 40% of global shipping relates to fossil fuels.
Everything we do has an impact, so with solar panels, there’s polysilicon production and issues around the supply chain, particularly with indentured slavery in some provinces of China, which can be hard to get around. This is also the case with many other products we are using. A solar panel can liberate the energy that went into making it within nine months of it being in use and the panels then have a 25-year warranty and possibly up to a 40-year lifespan. Most modern solar panels don’t contain any rare earths, they are just copper, an aluminium frame, silver for the conductors and glass. Effectively, polysilicon is a form of refined sand, and so all the parts are recyclable. There isn’t a huge recycling market yet, because most of the panels that have been installed are still in use.
People are also concerned about the huge amount of lithium being produced for home and car batteries. Howard explains that there are two ways in which lithium is produced. One is from mining, generally in Australia, whilst in the rest of the world lithium production is from liquid deposits. These exist as brines under the surface of the earth that can be extracted through a bore hole. The lithium is then taken out and the remaining liquid is pumped back down. So there is an impact but it should be smaller than for mining. Also lithium is a recyclable material, which is abundant in the UK.
Many manufacturers are now redesigning batteries so that they don’t contain cobalt, so as we move forward there should be fewer issues around rare minerals.
Heat pumps are generally made from copper and steel, both of which are abundant in a recycled form across the world. There’s a refrigerant involved, with two possible types, one of which has a lot less global warming potential.
All of these technologies will have a lower impact when compared to the extraction of fossil fuels and the resultant CO2 emissions from using them.
A repowered house – taken off the gas network
Your home can help you reduce your carbon footprint
For a typical dwelling One Zero’s home energy transformation could save between 3-6 tons of carbon over the course of a year, which is equivalent to 20 or 30 flights to Spain. For Howard this is incredibly impactful.
Your electric car battery could support your home energy use
It’s still early days but trials in vehicle to grid energy transfer could support our home energy use. An electric car could have a 30-100 kilowatt battery in it, while your home battery may only be 10 kilowatts, so if you could plug your car into your home and use your car battery to support your home that would be valuable.
It’s also a lot of lithium sitting around that could be used more effectively. Nissan is currently the only car manufacturer that allows this but that could change. The challenge is that most car manufacturers want their batteries used solely for powering their cars and consequently you will invalidate the warranty if it is used for other purposes.
However, these batteries have huge potential to increase storage capacity and help with the intermittency challenge in power generation. And Howard suggests that even accessing 10% of these batteries could have a meaningful impact.
One Zero has a community focus to support people on their renewable energy journey
People can be cautious, so One Zero is working to give people the confidence to act and then supporting them to share their experience with others. Currently they are geographically focused, working in communities where they can sign up more than one house at a time, maybe 5-10 houses across two or three streets. So they are reaching out to people who may not have thought they could be part of this journey into renewables, to help them change the way they think about it.
One Zero is operating in Sussex at the moment and has plans to launch in more areas over the course of the coming year. The next steps could be looking at managing a micro grid behind a distribution transformer that serves a street, so there would be enough generation and storage to provide the community with power for a large part of the year.
“It’s turning it from, that me and my castle thing, into us and our street.”
