Elly Deacon Smith and Kirk Rushby from Arbor Architects explain why they consider the embodied carbon of their projects, how they do it and how it is balanced with competing factors.
Interview with Elly Deacon Smith and Kirk Rushby
Elly Deacon Smith is the Founding Director of Arbor Architects with nearly 25 years of experience. She began her career in Bristol at White Design, working on straw and timber projects that emphasised both embodied and operational carbon long before it became mainstream. After moving back to Herefordshire, she joined Architype, deepening her expertise in Passivhaus construction and building physics, before co‑founding Arbor Architects, which she now leads.
Kirk Rushby is an Associate at Arbor Architects with roughly 20 years in the field, fifteen of which were spent at Architype. His background mirrors Elly’s focus on sustainable, low‑energy design and bio‑based materials. He’s also built his own Passivhaus home.
Focusing only on the energy a building uses in operation misses a large portion of its climate impact. Embodied carbon covers emissions from material extraction, manufacture, transport, construction and demolition. Even a Passivhaus can carry a substantial carbon debt if high‑impact materials are used. Measuring both sides lets designers keep total lifetime emissions within targets like the RIBA 2030 benchmark.
“We’re aware that operational carbon is regulated, but embodied carbon is a bigger blind spot,” Elly notes.
Make sure your design team is carbon literate
Elly taught a master’s course at the Welsh School of Architecture. A key aspect she encouraged was to “think in carbon” for every decision. This habit makes it easier to spot high‑impact choices later, because the designer already asks, “What is the carbon cost of this material?” Kirk agrees that years of practice have turned this questioning into second nature. Early education therefore lays the groundwork for systematic carbon reduction and intuitive decisions.
Use specialist tools to quantify embodied carbon
Arbor initially hired an external consultant to run assessments in PH Ribbon, which can use a certain amount of information from the existing PHPP model. The consultant just required quantities for elements like walls, floors and roofs, enabling calculation of kilograms of CO₂ per square metre. More recently Kirk is using a tool from Studio Bark, whose emerging calculator pulls volumes directly from Revit models, delivering near‑instant feedback.
“We’re really keen to be able to model in‑house, and to be able to make decisions quite quickly,” Elly explains.
Tin Barn
Take advantage of real‑world case studies
Arbor’s retrofitted Tin Barn (certified to the EnerPHit standard) achieved about 347 kg CO₂ / m², comfortably below the RIBA 2030 target. By contrast, a newly built garden‑room annex with timber framing and zinc cladding recorded just over 600 kg CO₂ / m². Interestingly, adding a photovoltaic array increased the figure by roughly 100 kg CO₂ / m². Elly says that this is another good reason to take a fabric first approach so that you keep your energy needs to a minimum.
Cwm Barn
Test assumptions: zinc cladding may not increase carbon dramatically
Swapping timber cladding for zinc on the Cwm Barn garden‑room added only a modest amount of embodied carbon, perhaps contrary to expectations. Elly attributes this to the relatively small surface area and the fact that the underlying timber structure already contributed most of the carbon.
“One of the surprises was that the decision to clad the garden room in zinc didn’t have as big an impact as we thought,” Elly comments.
So the lesson is to evaluate material impact in context rather than doing it intuitively.
Quick checks, early in the design process, make it possible to consider embodied carbon for every project
Waiting weeks for external consultants slows down design decisions and is not practical in the long run. So using a simpler embodied carbon tool like the one from Studio Bark means the whole process becomes easier, different options can be evaluated and it can be integrated into the standard workflow.
Using bio‑based materials normally leads to a good carbon outcome
As a general rule at Arbor they like to build with timber. This stores carbon from the tree’s growth, giving it a lower embodied carbon compared to other methods of construction.
Recognise foundations and substructure as the biggest carbon hotspot
Concrete foundations are inherently carbon‑intensive and often difficult to get away from. However, you can reduce volumes with good design or use low‑carbon mixes. Sometimes screw piles can be a way to avoid concrete altogether but these will normally be made of steel which again has a high embodied carbon. That said, they can be removed and reused at the end of the life cycle.
Balance cost, client expectations and carbon goals early on
High‑performance homes and Passivhaus certification typically increase material quantities and costs a little. Arbor addresses this by being transparent with clients from the outset about trade‑offs between carbon performance and budget. Early budgeting conversations help align expectations, and designers explore “more with less” tactics—optimising layout, trimming unnecessary floor area, and selecting durable, low‑maintenance finishes that lower lifecycle costs. Kirk stresses that “you can get more out of less” with the right mentality.
Good decision-making considers many factors
Embodied carbon is just one aspect of a sustainable home. Arbor evaluates each material for moisture handling, indoor air quality contribution, longevity and visual ageing.
Designer experience and instinct are still important
For design teams that aren’t yet modelling embodied carbon, logical decisions can often lead to low carbon outcomes: prioritise low‑impact substructures, favour timber over masonry where feasible, reuse on‑site materials, and keep use of steel to a minimum.
Anticipate a future where measurement is seamless and data is shared
Whereas many design teams are familiar with operational carbon, the future could see quick‑feedback calculators embedded in standard workflows and a shared database of embodied‑carbon benchmarks for common building typologies. More data will enable designers to make confident, low‑carbon decisions early in the design process, accelerating the transition to a net‑zero built environment.
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.”
Jeffrey Hart from the Earthen Floor Company gives us the lowdown on natural earth floors, explaining what the components are, how they are mixed and the process of installing them. He also outlines the benefits and limitations, and highlights what makes them a great choice for an eco home.
Interview with Jeffrey Hart
Jeffrey Hart got into natural building in Oregon, USA, where he was an apprentice of earthen floors expert Sukita Reay Crimmel. He spent five years constructing various buildings and installing earth floors in each of them, before returning to the UK. Although he started out as a ‘generalist’ natural builder, as demand grew, he found himself focusing exclusively on earth floors through the Earthen Floor Company. He credits Sukita’s influence for both the modern earth floor technique and the resurgence of this historic building method.
Earth floors have been used for centuries
Historically many buildings would have had an earth floor just because it was the ground beneath the building. At some point, people realised that cow dung reacted with the earth to make it a harder, more waterproof surface. It was also noted that when blood fell onto the floor of farm slaughter houses it made the earth rock solid, so over the years different methods have been used to make the floors harder and more durable. Sukita developed a way to use linseed oil to seal and harden the clay, making it a lot more palatable for the modern world and the way we live now.
All earth floors have a binder, an aggregate and fibre
Earth floors – also known as earthen, cob or adobe floors – all have a binder, an aggregate and fibre. In Jeffrey’s case the binder is clay, the aggregate is sand and the fibre is either straw or hemp.
The process of making an earth floor is similar to that of a concrete or limecrete floor, but clay’s ability to hold together is lower than lime or cement, which is why linseed oil is added.
“The linseed oil turns the floor from something that’s relatively soft and quite easily damageable, to this hard-wearing, leathery rock.”
Earth floors can be structural or just the top coat finish
An earth floor can serve as a thin finish over an existing concrete, limecrete or even timber floor, or it can be built up as a full‑thickness slab. The minimal approach involves a 20 mm top coat, while the full build‑up typically incorporates foam‑glass insulation, optional under‑floor heating, a rammed‑earth base and a top coat.
Jeffrey’s preferred full build-up
Moisture must be managed to avoid problems
Moisture is a critical concern for earth floors and Jeffrey will use membranes if damp conditions exist and foam‑glass insulation as it acts as a capillary break, preventing upward moisture migration. It is also important that the floor dries out sufficiently before the linseed oil is applied.
Linseed oil is used to cure and harden the surface
Linseed oil is the “magic” component that transforms a relatively soft, clay‑based mix into a hard‑wearing surface. Once the clay dries, the oil polymerises, linking long‑chain molecules to create a leathery, rock‑like finish. Without sufficient oil penetration, the floor remains soft and prone to damage. The oil also imparts the characteristic warm brown hue that many owners associate with earth floors.
The embodied carbon of earth floors is extremely low
Switching from a conventional concrete floor to an earth floor can reduce embodied carbon by roughly 83 % (according to calculations using the BEAM Estimator). The reduction stems from using locally sourced clay and sand, eliminating the need for high‑energy cement, and the floor’s ability to be broken down and returned to the earth at the end of its life cycle.
“It’s about an 83 % reduction if you were to swap like for like.”
It is possible to use clay from your building plot
If you’re digging the foundations of your house and the spoil is clay, one option is to reuse it in your floor. Jeffrey believes this is the ultimate local and sustainable floor, but there are a few considerations. Most important is to test the soil to confirm that it is clay rather than silt, which lacks the necessary binding properties. Tests assess shrinkage rates, strength, and colour consistency. Variability between layers can lead to inconsistent performance, so careful excavation and sampling are essential. Unfortunately, using local clay isn’t necessarily cheaper because of the required testing and processing, so the Earthen Floor Company offers pre‑bagged mixes with clay sourced from reliable brickworks and pottery suppliers.
A lot of the cost is down to the labour
The current rate for a finished earth floor from the Earthen Floor Company is £160 per square metre for the top‑coat package, which includes materials, labour, buffing, and waxing. Prices vary with location because of the extra travel but if money is tight, self-builders can do the final oiling and waxing themselves.
You can’t walk on the floor for a month while it’s drying
Although an installer might be able to cover 10 m² per day the whole floor must be completed in a single day. This means multiple installers coming to site, the number depending on the size of the floor. After the mix is laid, the floor must dry out before the linseed oil is applied. The drying time depends on ambient temperature, humidity, and air flow; heaters and fans can help accelerate the process, but Jeffrey recommends not trying to speed things up too much as there can be unintended consequences. Once dry, the floor is oiled and then left for a further 14 days to harden.
“From install to you being able to use the floor it’s three weeks if everything goes well.”
Underfloor heating works well with an earth floor
Earth floors store heat efficiently, making them ideal for radiant heating systems. The heating pipes are usually positioned between the insulation layer and the rammed‑earth base, though some projects place them higher in the slab for a faster response in properties that may not be used all the time (such as holiday lets). The thermal mass of the earth floor prolongs warmth after the system cycles off, reducing temperature swings and improving comfort.
Repairing minor damage to the floor is quite easy
A well‑installed earth floor would be expected to last at least 30 years, becoming more beautiful as it ages. If you damage the floor, making a dent or chip, it’s actually not too difficult to remedy. By crushing leftover “biscuits” (small pieces of the original mix), mixing them with oil, you can fill the crack. While repairs are relatively seamless, they may be visible in certain light conditions. To protect your floor it’s a good idea to oil and wax it as part of the maintenance cycle; the oil’s polymerisation continues slowly over the years, gradually increasing hardness.
“We make little biscuits from the mix leftovers; they’re used to patch dents.”
It is possible to choose the colour of the floor
The natural colour of an earth floor ranges from earthy browns to greys, dictated by the clay’s mineral content and the colour of the linseed oil. To achieve brighter hues, pigments can be blended into the mix, producing uniform colour throughout the slab. Alternatively, a coloured hard wax applied as a surface finish can modify the appearance without altering the underlying material, though this layer may need to be reapplied every couple of years. Most self-builders prefer the authentic, warm tones, but designers increasingly want something bespoke.
“People are becoming more aware of this material now; it’s no longer just a niche.”
There are lots of reasons to love earth floors!
As natural materials are much more accepted these days, earth floors are seeing a revival. Jeffrey has also been doing his bit to train the next generation of installers. He is trying to make earth floors as accessible as possible as he really believes in them. They look beautiful, they feel incredible under your feet, they soften sounds and are good in terms of carbon emissions.
“And at the end of the product’s lifespan, you can break it all up, chuck it in your garden, and it will break down to be just earth again.”
Ben Adam-Smith shares a few thoughts about what to look for in a plot of land for a low energy house build.
Start with realistic expectations
Ben starts the podcast by sharing a few pieces of advice about the whole exercise of looking for land:
There is no such thing as a perfect plot. Chasing an ideal site can drag the search out for months or even years, so it’s better to accept that compromises will be necessary.
Remember how the design process works. You don’t start with the house design and then hunt for a site that will accommodate it; instead, identify the constraints and opportunities of each site. Good design embraces these and responds to them.
Recognise that this is a lengthy process. Sometimes it can happen quickly but brace yourself for a journey that could take months or even years.
Be flexible. This allows you to make the most of the opportunities.
The good news is that those who stay determined normally find a building plot in the end.
Define your needs versus nice‑to‑haves
Splitting your requirements into “needs” and “nice‑to‑haves” is a great way to prioritise. Also identify what you can do without. This information can help you calculate what size plot will be enough. For example, a good sized house doesn’t need an acre and could fit on a plot a fraction of the size. However, if you also want to be set back from the road, have a driveway, garage, garden, patio or paddock then this will quickly add to the spacial requirements.
Find a neighbourhood that aligns with your values
Choosing a location is deeply personal, and will be about finding somewhere that resonates with you. Consider proximity to work, schools, medical facilities, shops, and leisure activities, as well as the overall safety and vibe of the neighbourhood. Visiting the area at different times of day reveals hidden issues such as traffic noise, smells, or activities that could lead to unruly behaviour. Community values matter too; a plot embedded in a supportive, like‑minded neighbourhood can enhance long‑term satisfaction. Just be careful not to restrict yourself to a tiny search area, as this can dramatically reduce the pool of viable sites.
Make sure the plot has good solar access
If you want to build to Passivhaus standard then solar access is really important. In the Northern Hemisphere south facing sites are ideal because you can maximise the gains in winter whilst also easily managing shading in the summer. Another advantage is that if you plan to install solar photovoltaic panels then these will also be more productive on a south facing roof.
Be realistic about what you can afford
Sometimes our dreams go beyond our financial means, so it’s really important that we take in the whole picture when considering our building plot. Don’t forget that there is an equation to value land:
Land cost + construction cost + 20‑30 % profit margin ≤ value of the finished home
So land price is only part of the equation; you’ll have to factor in professional fees, material costs, groundworks, utility connections and a contingency budget. Regularly updating a spreadsheet with quotes and estimates keeps the project grounded. Ben also highlights the psychological toll of overruns, noting that stress, health, and relationships can suffer if finances spiral out of control.
Don’t buy a site without planning permission
For first-time self-builders it’s a really good idea to buy sites that already have planning permission for a dwelling in place. Normally the drawings can be swapped out for the design you want so long as you stick within the key parameters.
When you’re looking for sites the main types you will come across are greenfield, brownfield, infill, and replacement dwellings. Greenfield sites rarely get planning permission because they are assigned to agricultural use whereas sites within a settlement boundary stand a much better chance. Checking your local authority’s planning portal for recent applications is a good way to gauge what kinds of projects are getting approved in the area.
Understand the additional costs of sloping sites
Flat land is generally cheaper and simpler to develop, while slopes introduce extra costs for foundations, retaining walls, and drainage. Ben references Mark Brinkley’s “slope law,” which quantifies cost increases for every five degrees of gradient. In other words the steeper the slope the more it’s going to cost.
However, a sloping site can also provide attractive views and unique architectural opportunities, such as an “upside‑down” house. Understanding how you’ll manage excess soil (“spoil”) from excavation is essential; disposal or reuse plans affect both budget and environmental impact.
Find out what’s in the ground and around the site
Soil composition—clay, chalk, sand, gravel, or rock—directly influences foundation design and drainage solutions. Ben shares anecdotes about historic mining areas where unexpected subsidence posed serious risks. One option is to commission a geotechnical survey, which involves boreholes, trial pits, and laboratory testing to produce a detailed ground‑water and soil‑strength report. Local engineers and contractors can interpret these results to optimise foundation type and avoid costly redesigns later. Additionally, finding out what the site was used for before may put your mind at ease when it comes to possible contamination. There could also be ecological considerations, such as protected wildlife (e.g., newts, bats), and trees that you can’t remove (tree preservation orders).
Ensure the site has legal access
Legal access is essential; a “ransom strip” or missing right‑of‑way can render a plot unusable without negotiation. Ben recounts the situation at Denby Dale where the Tunstalls were forced to purchase an additional piece of land to gain access to their proposed site.
Assess whether the site has power, water, sewerage and broadband connections as extending services can be a significant expense. Public footpaths, private roads, and other legal challenges must be identified early, and a solicitor familiar with land should verify that all rights are documented. Ben also notes the importance of planning for delivery vehicles and construction traffic when evaluating access routes.
Add in your gut feeling about this opportunity
If you’ve whittled it down to a favourite plot then are you getting excited? If a plot has lingered on the market for a long time, or if a site feels “off” during a visit, it may signal hidden problems. So walking the site at different times, speaking with neighbours, and observing any recurring nuisances such as noise, anti-social behaviour, or flooding will give you a better grasp of the site’s pros and cons. When doubts arise, it’s sometimes wiser to walk away than to convince yourself it’ll all work out. Ben concludes by encouraging listeners to leverage the free “finding land” course and to join The Hub to get more support.
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.
Ben Adam-Smith explains what value engineering is and how it can help us to do more with less. He reflects on a short video about Slate House in Herefordshire that includes some great examples of value engineering.
This video was produced for Coaction, a fantastic training initiative that aims to upskill the UK construction industry and encourage higher quality, more sustainable building practices.
Coaction has allowed us to share this video with you, which explores some of the decisions made at Slate House and how they contributed to making a low budget but high value home.
Value engineering is about doing more with less
Value is defined as function divided by cost (value = function/cost).
If we increase the function the value goes up.
If we decrease the cost and value goes up.
The only thing we need to bear in mind is that performance must not go down.
Value engineering is about creating better solutions
Value engineering is a creative endeavour and as such there is no limit to what you could do, so long as you’re following the principles mentioned. In fact, just building to Passivhaus standard will entail a fair amount of value engineering (orienting your building well would be one example).
However, when it comes to housing there are some common examples which we are going to cover today.
Photo credit – Graeme Deas and Stuart Hedley
Passivhaus standard is within reach on low budgets (if you make good decisions)
This podcast is looking at some of the value engineering examples at Slate House in Herefordshire.
Graeme Deas and Stuart Hedley are the self-builders behind the project. Builder Mike Whitfield delivered an airtight, insulated shell with the ventilation system integrated, while Graeme and Stuart completed the internal fit-out.
Nick Grant from Elemental Solutions was their Passivhaus Consultant and also a strong advocate of value engineering.
Although building to Passivhaus standard brings multiple benefits related to health, comfort and energy efficiency, is it worth doing if the house doesn’t make a statement?!
Constraints were embraced
The budget was a key constraint on this project but this also informed the choices ahead. If you like this idea of leaning into the constraints then check out our podcast with Andrew Michler on how to create beautiful designs through constraints.
Value engineering happens at the beginning of a project
Value engineering sometimes gets confused with cost-cutting but there’s an important distinction to be made: value engineering can only take place at the beginning of the project whereas cost-cutting is something that takes place at the end. The efficiencies that we’re talking about are decided in the early stages of the design process, not when construction is underway and we’re looking to downgrade materials, etc.
Photo credit – Juraj Mikurcik
Design from the inside out
A driving philosophy on this project was to design from the inside out, with windows being sized sensibly and picking out views.
Therefore nobody would be standing on the outside placing the windows where they would look good. The composition was something that emerged from the design process.
However, Stuart had initially stated that he didn’t want asymmetrical windows.
One of the few specifications I had was that the house should look good from the outside. And part of that for me was that it did not have asymmetrical windows. So I’ve really fallen in love with what I initially thought was rather an unexciting box.
Photo credit – Graeme Deas and Stuart Hedley
Keeping the shape of the house simple will make it cheaper to build
You may hear the term ‘form factor’ mentioned a lot. This refers to how much surface area is losing heat (external surface area) relative to the useful floor area.
So reducing this ratio as much as possible and keeping things simple will make construction easier and it will cost less.
Windows without extra frame and mullions and transoms perform better, and because they are simpler they are also cheaper. Although it’s logical, sometimes it can be a hard sell. For example, Ben and his wife Kay completely disagreed over the windows during their self build. It was only after a lot of discussion that Kay agreed to have windows without mullions and transoms. Even today, while she likes the view from the inside looking out she is still not sure from the outside looking in!
Avoid the wall of glass
Avoiding excessive glazing and sizing windows well will be cheaper and help summer comfort.
There may also be opportunities within a house to have a fixed window. Obviously you want openable windows in every room for ventilation and safety but their may be awkward spots, like above the kitchen sink. A fixed unit here will be another cost saving.
A concrete slab can take on two functions
At Slate House there’s a concrete slab which provides structure but because it’s been polished on top it can serve as the finished floor surface.
Photo credit – Graeme Deas and Stuart Hedley
A tin roof is another cost effective solution
So long as it is detailed simply then a tin roof is durable and low cost.
This, along with many of the other value engineering examples we’ve included, have also been a part of Juraj Mikurcik’s self build project Old Holloway.
Jae Cotterell, founder of Passivhaus Homes, explains the importance of the choices we make when considering our construction approach. It is possible to create a high performance home with low embodied carbon that doesn’t cost the earth if we understand the process, where the impacts lie and where optimisation can make a huge difference.
Interview with Jae Cotterell
Jae Cotterell has extensive experience in designing Passivhaus buildings. She is a co-author of The Passivhaus Handbook and a Director at Passivhaus Homes, where she specialises in the PH15 Construction System. In this chat, Jae explains the importance of considering both embodied carbon and energy efficiency when undertaking new build projects.
What is the PH15 Construction System?
The PH15 Construction System is a complete construction solution that enables you to achieve Passivhaus performance whilst using low embodied carbon materials, with a primary focus on timber. Passivhaus Homes use a range of timber variations in their PH15 Construction System, from Laminated Veneer Lumber (LVL) to Low carbon, wood fibre insulations. This combination and variety of materials allows them to use the timber in the most efficient way and ultimately provide a finished product that is incredibly energy efficient with low embodied carbon.
“I don’t think that you go into these areas to make a lot of money. You do it out of interest because you want your work to be meaningful and to contribute in some way. And, I hope that we will have done that”
Join The Hub to access our in-depth video case study of Buckinghamshire Passivhaus, a PH15 build
The story of the three little pigs says a lot about our current cultural attitudes towards timber construction
The UK has a cultural nervousness towards timber which combined with a long history as a brick block country has created a construction culture that is very different to the timber-centric approaches of Canada, Australia and many European countries. This cultural mindset means that there is a resistance to change in large scale construction, but for Jae, it is important that this preconception is challenged. Smaller projects and self builds allow for the freedom to challenge the status quo, which can hopefully pave the way for a timber industry to be built in the UK.
Timber is a precious resource and it is important how we use it
Due to its low embodied carbon, timber is a much better resource to be using in Passivhaus construction than alternative brick block methods. That being said, timber is a precious resource, and we must be looking at contemporary solutions to ensure that we are intelligently engineering timber in order to use it as effectively as possible.
Laminated Veneer Lumber (LVL) allows you to use timber more efficiently
You can use 35% to 50% more of the log when you create an LVL I-joist than when you construct with solid section timber. Furthermore, Cross Laminated Timber (CLT) uses 44% more timber than an LVL I-joist. At present, there is no production of LVL in the UK but there is hope that this production could be introduced with the regeneration of the timber industry.
For a new and successful timber industry to grow, the biodiversity of forests must be at the centre
Instead of a timber industry that favours monoculture planting with low biodiversity, we need to be looking at a timber industry that is planting with a wider range of trees and considering biodiversity gain. Not only will this improve the ecology of these areas but it will also provide more biodiverse sources of wood.
Timber structures do not increase the risk of fire
Timber in its natural state does burn, however there are many ways to treat the wood to ensure that it is incredibly safe to use in construction. There are a large variety of fire related products on the market, including fire resistant membranes that can be wrapped around the framework and the PH15 Construction System doesn’t include any voids within the internal structure, reducing the ability for fire to spread. In fact, timber construction is significantly safer than many plastic or oil based materials on the market, the likes of which were a primary component in the Grenfell Tower Fire.
Embodied carbon is important to consider, but so is energy efficiency
Embodied carbon looks at the present carbon cost of a new build structure but ensuring that this structure operates efficiently will lower the overall carbon footprint of building throughout its life. Jae highlights that the process of decarbonising energy will limit the national supply and put pressure on the grid. It is therefore crucial that we find ways to use less energy and reduce this demand; the efficiency of Passivhaus structures will play a large role in this.
It’s not about building a large amount of new housing, but instead about ensuring that the ones we do build are of high quality and efficient to operate
Passivhaus Homes’ most recent social housing development, Hook Hollow, has emphasised that the PH15 construction system can deliver both high quality and affordable homes. Jae also highlights that a return to modest terrace housing over large stand-alone homes will be an incredibly efficient approach to building Passivhaus structures because there is a smaller envelope to be considered. Not only does this reduce the number of materials required, but will also reduce the overall costs, allowing for more affordable housing to be built
“I feel the disparity between the have nots and the haves is so big, and having a high-quality home, that also has low bills and energy, and is also very healthy. Those are massive benefits to someone who is under pressure.”
Visualisation of Hook Hollow social housing scheme
An all electric system allows for simplicity
Passivhaus Homes have committed to a no more gas campaign and will not put gas in to any of their new homes. This means all of their new builds will be all electric, allowing for low maintenance and low-cost solutions. The focus is on getting the fabric of the house to a really high quality with very simple hot water and heating solutions. The Hollow Hook social housing development does not include any heat pumps either, reducing the money spent on technology and the cost of future maintenance.
Sacrificing your Passivhaus certification is a slippery slope
By focusing on achieving low embodied carbon but reducing the standard of energy efficiency in your new build, you are likely to lose a lot of the benefits that a Passivhaus house can provide. By opting to forgo triple glazing and MVHR systems, you will end up doubling your energy demand. These systems are also what allows you to maintain a constant temperature, ensure you have high indoor air quality and prevent mould from developing.
“You drop the standard, and you drop a whole load! A little a basket of benefits just get thrown away. I think it will be an absolute error to not actually do it properly.”
It is important to be aware of the greenwashing within the construction industry
There are many products that claim to decarbonise certain areas of construction, but if these products are also limited resources, then they are not sustainable solutions. Instead, they are a solution for a small amount of people and in many cases, the fundamental method may need to be reconsidered. For example, a transition from concrete foundations to above ground timber shell cassettes.
Size and materials matter
A Passivhaus certification does not guarantee that a house is sustainable. As Jae points out, a Passivhaus with a large footprint and built from high embodied carbon materials will use more energy and have a larger carbon footprint than a modest sized standard house. Therefore, it is crucial that scale and materials are considered in combination with energy efficiency.
Overall, we need to shift our cultural mindset to value the quality of our homes
Our current cultural psyche places value on the size and the features of our homes, yet it’s not quite reached the point where we value the benefits of a Passivhaus. It will take time for this to shift, but hopefully, in the not-so-distant future, our cultural mindset will begin to value constant temperatures, high indoor air quality and comfort over the number of bedrooms and wood burning stoves.
“I come across people all the time, they’re not willing to compromise on size, but they’re very willing to compromise on quality. And we need that flipping as a culture… I’d love to see that change, because I think it would make a big, big difference and be much better for us. A much more insightful and wiser way of viewing things.”
