by Matt Muldoon
Common to all natural building materials is their low embodied energy. Embodied energy refers to the energy consumed over the lifetime of a building material, in its manufacture, processing, transportation, application and demolition. Bricks, for example, consume a huge amount of energy in that the raw materials are mined, processed, fired in a kiln, transported large distances, applied with the help of powered machinery and demolished and disposed of with more powered machinery. A contrast would be straw bales, which require relatively small amounts of energy in cultivation and processing, and the carbon cost of this energy is more than offset by the carbon sequestered by the plant itself. Straw bales also require less energy in their transportation, and at the end of their life they can be easily dismantled and left to rot.
Natural building materials also tend to be breathable - and must be used with breathable renders, plasters and paints. Breathable materials absorb and release water vapour, meaning that they regulate humidity levels well. When the air is humid, they absorb moisture; when the air is dry, they release moisture. This breathability circumvents many of the problems caused by water vapour in modern buildings, such as excessive dampness, condensation and mould growth.
A final point to make (whilst trying not to promote natural buildings as some kind of greenwashed eco-lifestyle-concept) is that many people derive enormous satisfaction from being in a building whose components have an obvious link to the surrounding world. People are pleased to know that the earth walls encircling them were dug from the ground beneath their feet, that the straw keeping them so snug was grown a few fields away, or that the trees holding up the roof inside have descendents growing outside.
So, if you decide you want a building made of natural materials, what are the possible choices? It depends what you want the building to do, and what materials are available near enough to the site for their transport to be sustainable.
In designing a building, several key factors are compressive strength, insulation, thermal mass and time and cost in building. Compressive strength refers to a material’s capacity to bear a load. Earth has a high compressive strength and straw bales have a lower compressive strength. Compressive strength is a limiting factor in the weight of your building, and weight is a limiting factor in the height of your building and what you have in it. So, how high do you want the building to be? Is the roof made of something light (like corrugated metal) or something heavy (like turf)? Will the building contain normal household objects, or heavy factory machinery?
Insulation refers to a material’s capacity to trap heat inside a building. Hempcrete and straw bales are relatively good insulators, earth and limecrete are relatively bad insulators. Is it important that the building is warm? Might the building get too warm? Will the building have a heating system or will it try and do without one?
Thermal mass refers to a material’s ability to store heat and release it. Earth has a large thermal mass, straw has almost none. So, if you have a constant low level of heating then thermal mass is less important. If high level heating is provided for a few hours a day, then thermal mass is needed to stabilise temperatures between heated and unheated hours. Thermal mass is a particularly important element in passive solar design. Passive solar design is the idea of building a structure that derives ambient heating from the sun, so that it needs little or no extra heating. In a passive solar design, you need a large area for solar energy to enter the building and insulation to prevent heat escaping the building, but you also need thermal mass to stabilise temperatures between the times when there is sun and the times when there isn’t.
I don’t want to argue that natural materials are a good thing per se; but I do want to explain their advantages, and also the contexts in which these advantages apply - there’s no point in super-insulating a building if you’re only going to keep tools in it and wood is not sustainable if it’s shipped half way round the world.
Earth as a Natural Building Material
Earth building means different things around the world. Sections of the Great Wall of China are made out of earth rammed into shuttering, prehistoric remains of earth brick cities are found throughout the Middle East, and earth brick remains the predominant building material in South and Central America. Earth can also be pressed onto wooden frameworks, as it has been for thousands of years in Africa and Europe, or just layed down in huge lumps and sculpted into wall shapes.
The key is clay. Clay is the binder that keeps the other components of earth (stone, sand, silt) together in a shapeable mass. Clay is an abundant material, but in parts of the British Isles where it's lacking, lime and chalk have also been used as binding agents.
Earth has developed "sustainable" credentials in the last couple of decades. It's true that the embodied energy of earth as a building material is very low, especially if it is extracted from the building site. It's not a fantastic insulator, however, so compensation must be made with a super-insulated roof, or with extra insulation in the walls. What earth provides in abundance, however, is thermal mass. When earth is used as a walling or flooring material, a massive heat store is created. This makes earth a very useful element in passive solar designs.
Earth has also developed a reputation for "healthiness". It will not give off any harmful materials, and as a breathable material it will regulate moisture levels in a building extremely effectively in combination with its natural plasters and renders.
Earth is durable. The oldest cob structures in the UK are 700 years old, and there is no reason they shouldn't last many centuries more. The critical factor is moisture. Earth buildings need to sit on a plinth to keep them out of direct contact with the ground, and the roof should have a good overhang to protect the walls from long periods of driving rain.
In the UK, the four traditional methods of earth building are cob, clay lump, rammed earth and wattle and daub.
Cob is simply 20-30% clay, 70-80% aggregates (sand and stones) and about half a bale of straw per tonne of mix. The clay binds the aggregates together to create a load-bearing material, while the straw acts as natural rebar, giving tensile and sheer strength, and also aids the drying and insulating capacity of the wall.The materials are mixed by hand, by animal, or with the backhoe of a digger, and the resulting dough-like mass is used to sculpt not only walls, but arches, chimneys and indeed any other feature that is needed. Cob is load bearing, with an average compressive strength of 0.77N/mm2, so wall thicknesses are usually between 450mm and 600mm, depending on the structure. Cob is currently enjoying a massive revival in the UK as a building material, partly because of its sustainable credentials, partly because it is a simple option for the self-builder and partly because it can be formed into shapes which are impossible with other materials.
Rammed earth, clay lump and wattle and daub are all similar in their composition to cob, though there are variations in the amount of clay used and the addition of other natural binders such as lime. Clay lump is simply cob made into bricks. The wet mix is put into molds and allowed to dry, not fired. The resulting blocks can be layed like bricks, with a clay slurry used for mortar. Because the density of the bricks tends to be higher than with cob, load bearing capacity is increased and walls can be thinner, but insulate less well. In rammed earth, the mix is wetted somewhat less than you would wet a cob mix, and then rammed into shuttering by hand or with pneumatic rams. Again, because the density is greater, you have greater load-bearing capacity but less insulation. Daub refers to a cob-like mix which is pressed onto a wooden framework, the wattle. In this type of building, a timber frame provides structural support, and the daub merely acts as an infill. For this reason, it can be very thin and quick to build, but again will not perform well from an insulation point of view.
Earth has been used for construction for at least 10,000 years, and 30% of the world's population live in earth homes. Earth's advantages as a building material are simple: it is everywhere, it is basically free and it is incredibly durable.