This is the first article of a series where we follow Jeffrey Hart as he designs and builds his tiny house. Here Jeffrey introduces the project, explains his intentions and design process, and describes the exterior wall envelope of the house. Throughout this piece there are references to his Building Sustainability podcast for more background information. These episodes are noted as (BS### Guest Name).
Living tiny is not new to me: while training to be a natural builder I lived in North America for five years and moved to a new location roughly every six months. If it couldn’t fit in the back of my trusty Ford Explorer then I couldn’t own it. Once I returned home to England, I bought and renovated an 11m x 2m (38ft x 6ft) narrowboat which would be my home for the next six years. Acquiring “stuff” simply wasn’t an option. I made an exception for books, as I was eager to create a library of all the excellent building books I’d read but not been able to carry on my travels.
When I decided that I wanted a more land-based existence I initially looked for land to buy. However, I was loath to give up the freedom that boat life encourages. I had grown accustomed to being able to move my home when I desired change or situations changed.
Building a tiny house solved so many of my concerns. Not only did it allow me to retain my freedom of movement, but it also fulfilled my lifelong dream of building my own home, not just homes for other people! Having lived in an uninsulated steel boat partly submerged in cold water, I was also overjoyed at the opportunity to practice my preaching and live in a well detailed home wrapped in lovely natural insulation!
Tiny houses are most popular in the areas which have a tradition of timber frame construction: the U.S., Canada, Australia, and New Zealand. A large proportion of tiny homes are built by people who fall in love with the idea, but don’t necessarily have the skills or experience to use natural building techniques. Because of this, tiny homes tend to be scaled down versions of the conventionally built homes of that area. The go-to insulation is usually foam because of its light weight and high insulation value. The framework tends to be either steel or timber, with a sheet metal roof and they are often wrapped internally with a vapour barrier.
While researching I found designs, forums and YouTube videos of people showing their tiny house details. So many of these featured large amounts of thermal bridging, no vapour control, and other poor detailing, not to mention the reliance on toxic foam insulation. My tiny house project became about showcasing natural materials and good detailing in the tiny house setting. For me, natural building can’t just be about using natural materials. We need to detail our structures using the best building science available and with low-carbon, renewable, natural materials. This gives us high efficiency while the building is in use (small amount of energy needed to heat/cool) with low embodied-carbon materials. The gold star is to sequester more carbon in a building than we used to build it! My design goals are to make buildings that are highly insulated (natural of course), airtight, well ventilated (with heat recovery), with little to no thermal bridging, and vapour open construction (some people might call this breathable, but note we are talking about the movement of moisture in vapour form, not air). The final detail is to make the design rich in biophilic, or nature-like, elements.
To be safely towed on UK roads, the house including trailer should be less than 4m (13.1ft) tall. The trailer I have chosen is 6.6m x 2.4m (21.7ft x 8ft). To be road legal the house must be narrower than 2.55m (8.4ft)and must not exceed a total weight of 3.5t (3.9t imperial). Once the trailer is factored in, there are 2.9t metric (3.2t Imperial) left.
These restrictions are an intriguing set of constraints, which have all challenged me while designing the house. The weight constraint is the trickiest of all! Having every design decision underpinned by the constant nagging question “Yeah great, but what about the weight?” has been the biggest reason for compromises.
I also chose to use the Living Building Challenge (LBC) as a guide for best practices. Particularly in it’s guidance on the distances I could import materials and the toxic materials red list that it provides. It also guided my thoughts on many aspects of the design and construction. You could write a whole article on just the LBC, so I’ll leave it there – but, if you haven’t read the standard, make sure to give it a read (BS045 Martin Brown).
I’m not an architect by any means, but I did my undergraduate degree in design. Designing a tiny house to fit your needs is a very tricky thing. With so few square metres to play with, you need to make them all count. It’s very much like a game of Tetris. I spent a long time observing my patterns of movement while living on the boat, what do I enjoy doing inside my home? Where do I work? Where do I relax? What’s missing? What’s important to the way I live? I also interviewed multiple podcast guests who had built their own tiny houses during this design phase. It was fascinating to see what they deemed most important and how they had achieved it (BS048 Emma Pile), (BS010 Flo Hamer), (BS049 Philippa Battye), (BS059, BS060 Georjie Adams).
My observations were:
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I desired more space in my kitchen.
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I wanted a desk to work at – I spend a lot of time editing and recording podcasts on my laptop.
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I wanted my tiny house to have a comfortable sofa area, something the boat was lacking.
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I’m happy for my bathroom to be small.
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I wanted space for my lovely books!
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Having a space within my home to stretch and exercise was important to me.
With these observations in mind I set to work with some squared paper and some scaled paper cutouts of the approximate size of the things I desired. I drew countless variations, pawing over internet search results, seeking ideas to see if they could be remoulded to fit my requirements. I tried asking myself ‘What If’ (BS011 Rob Hopkins) questions to open up new ideas and draw more variations such as, What if the bed wasn’t fixed? What if the door was on the short side? What if the kitchen and bathroom were not close together? As it happens…they ended up right next to each other. If I ever got too attached to a layout, I worked very hard to make one that was completely different but still fulfilled my needs. I tried desperately not to fall into the trap of loving the first one I drew. And you know what? I did all of that work, compared them all side by side, slowly eliminating ones that weren’t as good…and finally chose something very close to the first one I drew!
I’m afraid even a devout straw bale nerd like myself couldn’t give up the space or weight required for straw. There’s a rather serious weight limit to contend with to keep the house road legal.
The obvious choice in the UK is sheep wool, it’s very lightweight and has good insulative properties, plus it’s an industry that needs support. Due to the rapidly falling fleece prices, many farmers have been reported to bury or burn their sheep fleeces. However, I decided to splurge a little of my precious weight budget on wood fibre insulation, because wood fibre also has thermal mass. My speciality is earthen floors, so I’m a big believer in using thermal mass to regulate temperatures. With no clay plaster or earthen floor going into this building I was keen to find a way to include the mass.
When looking for insulation materials that I could wrap my timber frame with to limit thermal bridging, I was overjoyed to discover facade grade expanded cork. This material can be used externally as a rain screen whilst providing insulation. So not only did I rid myself of thermal bridges, but I also saved precious weight by stacking the functions of the cork! The same cork is being used internally on the ceiling to cover the joists and provide a decorative finish (again stacking functions; no extra cladding and I’m pretty sure I’m getting some biophilia goodness from having the cork on show). A lower grade of cork has been used over the floor joists, with the wooden floorboards on top of the cork. By using the cork in this way I have almost entirely removed thermal bridging in this house.
I have gone out of my way to seek out local materials wherever possible. The OSB sheeting is UK made, and crucially, free from formaldehyde. I’ve used a solid ash timber floor that was grown and milled in my neighbouring county. The small family-run saw mill owner was able to tell me the complete history of the trees’ lives. In the UK the ash tree is being wiped out by the ash dieback fungus ending its reign as one of the most versatile and useful trees on the British Isles. So it felt fitting to use this tree in a place of honour in my house. Externally I’ve used some cedar cladding around the entrance. Even though most of the cedar sold in the UK is imported from Canada, with a bit of searching I was able to find UK grown cedar. Many people turn their noses up at the UK variety due to a higher knot ratio than it’s Canadian counterpart. I simply ordered a little more than needed, then clad the front first using the choice cleaner pieces. I then clad the backside with the more knotty material left over.
Now, I’m aware this might be a contentious issue with the natural building purists – but for this build I am using an Intello airtightness membrane and aiming to make the build as airtight as possible. In all my other construction projects I have used plaster or render as my airtightness. Over the years I have learned and developed some nice details for removing the drafts using just the humble earth plaster. In the tiny house I can neither afford the weight of the plaster, nor would I predict any of that air tightness would remain after the first time the house has been towed. I have heard the moving process described as ‘like an earthquake and a hurricane at the same time.’ So if I want to create a high performance building (approximately 20% of winter heat loss is down to drafts) a membrane is a must. I felt a little uneasy about the use of a plastic membrane so I asked Chris Magwood how he feels about it. His paraphrased reply was that it’s probably less plastic than your plumbing pipes, and it makes your building more efficient (BS044 Chris Magwood).
Now it should be made clear, this membrane isn’t a vapour barrier: it’s not a solid plastic sheet. It has microscopic holes which allow vapour to pass through without letting air pass through, much the same as a clay or lime plaster would. Almost all the materials in this build will allow the vapour created internally (showering, cooking and breathing) to pass through the wall system and out of the building.
I designed my wall, floor and ceiling buildups using a free online tool: https://www.ubakus.com/en/r-value-calculator. This allowed me to see not only the U-value and R-values of different insulation types in the wall, but by setting the expected external and internal temperature ranges and humidity values I was also able to see which wall build ups were in danger of interstitial condensation forming in the wall. Another great feature is that I could see which components would be carbon intensive to manufacture and which components would sequester carbon.
With any airtight building, ventilation is essential to maintain a plentiful supply of fresh air. I opted for a simple decentralized Mechanical Ventilation with Heat Recover (dMVHR) system [Ed: known as a ductless heat recovery ventilator or HRV in the U.S.]. This system looks like a standard bathroom fan but it not only extracts air, it brings it in too. When warm air is pulled from inside the building, the heat is removed. This heat is then used to warm the incoming air. This system should recover 80% of the heat in the air it changes and the energy recovered is greater than the energy needed to run the fans.
Often when people talk of natural homes they talk of a calmness or ‘the feeling’ of a space. One explanation might be that when we surround ourselves with natural forms, patterns, and materials, it relaxes us and helps us to concentrate and recover. A famous study of hospital patients found those with a view of trees recovered faster, with fewer complaints and fewer drugs than those with no view (BS047 Bill Browning & Catie Ryan). The study of design elements that cause biophilia is called Biophilic Design. This can be achieved in many ways, but some simple ways are adding in plants, ensuring you have a view of nature (photographs work, but not as well as the real thing), and using natural finishes.
I have tried to use some roundwood timber in every project I’ve done, even before I learned of biophilic design. There is something incredibly special about seeing an unprocessed tree timber holding up a building, standing up just how it was in nature with the bends, knots and the gentle taper on show. The decision to use a chunk of the weight budget on what is a largely decorative sweet chestnut roundwood cruck frame was an easy one to make. I’ll happily remove my entire kitchen to make the house light enough to travel on the roads if it means I can share my space with those lovely trees. Building roundwood framing is amongst my favourite things to do. Dealing with the complexities of organic lines, joining round to round and using the log scribe: what better way to spend the day (BS009 Adrian Leaman)?
Finishes which allow us to see the natural grain of timber are said to provide a strong biophilic response. Even though we wouldn’t see the grain normally in nature, as it’s the result of cutting, the understanding is that we see the grain, think of trees, and feel nature. The ash floor boards, finished in clear hardwax oil do this brilliantly with their streaks of olive and peach colouration. Waney edge window sills, desk, and worktops made from London plane (Platanus × acerifolia – Lacewood) complete the effect. I think it would be quite possible to go too far and make everything have prominent wood grain. Research seems to show that around 50% of the surfaces being wood grain seems to have the right balance, so the rest of the walls will be neutral, or natural tones. The cork, in many ways, is the star of the show. Where I have used it internally on the ceiling as a way to prevent thermal bridging, I will leave it exposed. I’m hoping the subtle patterning of the tree bark will offer some biophilic response. Externally the cork cladding blends the building into the surrounding woodland.
