Natural building techniques
Eco expert Nigel Griffiths looks at three natural, sustainable construction techniques that will help you minimise your project’s carbon footprint
Eco expert Nigel Griffiths reveals how to construct a lowcarbon bespoke home using age-old, sustainable materials
Historically, all construction was carried out with natural building materials. People simply used what was readily available, typically stone, slate, timber, reeds clay or earth. In one sense, of course, all materials are natural – but it takes a great deal more effort, energy and ingenuity to abstract steel from iron ore than to harvest timber from the forest.
For our purposes, then, the best definition of natural is, perhaps, materials that are minimally processed. This is an important distinction to make. The Mesopotamians used unfired clay bricks in the third millennium BC, for instance, whereas the Romans fired theirs wherever they travelled (with each legion having its own imprint).
In this article, we will concentrate on unfired and largely unprocessed materials – though sometimes these are combined with processed binders, such as lime.
The interest in natural construction techniques arises in part because modern buildings typically contain such a large proportion of highly processed materials, like plastics, that they become somewhat alien to their inhabitants. These products aren’t much fun to look at or touch compared to the beauty and tactile qualities of, say, an elm windowsill set into a waxed earth wall.
In an era where climate change is high on the agenda, the other key attraction of using natural materials is that they require minimal energy to produce and can actually lock up (sequester) CO2 – thus delivering buildings that can be classed as carbon negative.
Straw bale construction
Straw is particularly attractive to eco builders because it’s a waste product. So, something that would otherwise decompose or be burnt can be used to construct warm and durable buildings – a perfect example of a win/win. Don’t confuse it with hay, which is fodder. Straw should have no food in it and thus not encourage rodents. What’s more, straw bales are cheap and plentiful. We actually have enough of the stuff to build all the houses we need in Britain each year. Now there’s a thought!
There are two main approaches towards straw bale construction. You can either use it as an insulating infill for a timber frame (so it’s technically a timber frame building) or use the bales themselves as the structural loadbearing elements that hold up the roof. Straw is typically limerendered inside and out for weatherproofing and to give an appealing finish. Deep (overhanging) eaves provide protection from all but horizontal rain. Whatever approach is taken, the bales are always laid on a masonry plinth raised well above ground level, so that there is no risk of them coming into prolonged contact with moisture.
Small bales are best for load bearing wall construction as they’re easier to handle, effectively working like giant LEGO. Much of the work can be done by DIY enthusiasts, so it holds great appeal for hands-on self builders. And a single component (the bale) replaces the arguably complex build-up of bricks, insulation, cavity wall ties, blocks, cavity closures etc that make up typical UK walls.
People are sometimes wary of straw bale construction because their local building control officer may not be familiar with the technique. There’s no need to worry, though. Straw bale can meet all the UK requirements for insulation, fire protection and structural performance durability. There’s an excellent chapter on this in Building with Straw Bales by Barbara Jones, a founder of the School of Natural Building (schoolofnaturalbuilding.co.uk).
Hemp building
Another genuinely sustainable form of construction, as the main material is a waste product. Hemp shiv is made from the stalks of plants that don’t get used for other purposes (medicine, clothing, paper etc). For building works, it is usually combined with lime to create hempcrete. In France, where hemp construction is more common than in the UK, this is known mostly by its trade name: Isochanvre.
Generally, a structural timber frame is built and has shuttering added before the hempcrete is poured into the core of the walls. Once the mix has cured, the shuttering is then removed and the finished coats of lime render/ plaster can be applied. It’s important to recognise that the hempcrete is forming the walling and providing the insulation, but not the structural strength in its entirety – the underlying timber frame is still required. This could be a
primary frame (such as a partially visible post and beam construction) or a lightweight system that’s covered over.
Hempcrete can also be cast into blocks (or other preformed sections) and laid in a similar way to conventional blockwork, using lime-based mortar to match its natural, breathable qualities. Hemp blocks are a lightweight and highly insulative product, with a much lower environmental impact than cementitious versions. Whether using blocks or cast-in-situ hempcrete, a masonry plinth is first built (as with straw bales) to keep the structure well above ground level and avoid contact with moisture.
Cob walling
Cob is one of the world’s oldest construction materials.
The majority of the UK’S cob buildings are located in the South West – especially Devon, where the soil is wellsuited to making it. Comprised of subsoil, straw and water, cob (sometimes known as ‘mass earth’) is perhaps the cheapest and most sustainable form of walling material, as these substances are all plentiful and easy to acquire. This means the main structure of your home, at least, will have almost zero embodied energy. No binders (such as lime or cement) are required, either.
The ingredients are combined by treading the mix; sometimes with the help of animals, whose dung is said to improve plasticity. The cob needs to be turned and remixed several times before it is ready to be added to the wall, using a traditional three-pronged fork. As the technique is fundamentally low tech, it’s naturally attractive to those who want to get involved with the construction process.
Few new cob buildings are constructed these days, however, in part because of the slow build times. Each layer needs to dry before the next can be added, so depending on the mix you can typically lay up to half a metre in a day. There are also challenges in achieving Building Regulations compliance, especially the Part L energy efficiency requirements. You can overcome this with the addition of further insulation material; but if this is done internally, it makes it impossible to use a natural finish, such as waxed earth. Most cob construction is therefore repair works rather than new buildings, although some intrepid self builders do elect to use it.
Again, a plinth is required at ground level to keep any form of earth building (there are others: adobe, cob block etc) away from its great enemy: moisture. For the same reason, you’ll need a good overhang on the roof. This combination is commonly referred to as being a dry hat and a good set of boots, and it’s essential for almost all naturally constructed buildings.