On Selecting Natural Building Materials
By Matt Muldoon
By natural building materials, I mean building materials which require no or relatively small amounts of processing, and which will return to the earth without causing undue pollution. Common natural building materials are earth, straw, wood, stone, lime and hemp.
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 breath ability 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 green washed 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.
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By Matt Muldoon of the Natural Building Company
On Earth as a NBM here
On Straw as a NBM here
On Lime as a NBM here
On Hemp as a NBM here
On Turf as a NBM here
To check Matt Muldoon and his company out, click here.
Read our article on him and TNBC here.
