Key Properties of Earth Structures
Chemical-Free - pest and rot resistance can be achieved through appropriate material selection
Fire Resistance - earth naturally resists fire
Strong and Durable - and easily repaired should it be damaged
Vapour-permeable/Breathable - for excellent indoor air quality and longevity of building fabric
Exceptional Thermal Mass - regulates building temperature in summer and winter if used inside thermal layer
Flexibility - suitable for domestic or commercial new build and retrofit projects
Natural Resource - suitable soil is often found on site, making the process extremely low impact
Listed Building Approval - often used in traditional and listed buildings
Materials and Process
Earth building comes in many forms, from surface and decorative finishes to insulating infill and full structural systems. But the core of earth building uses clay as a binder for inorganic materials like silt, sand and gravels or organic like straw, hemp and a growing range of bio fibres and fillers.
Clay is a micro particle with a flat plate geometry which absorbs and expands many times with water then wraps around the most complex geometries either as a liquid or through compaction then dries and hardens. The unique platelet geometry and electrical bonds that form in the wetting and drying process give clay the ability to move without cracking. These bonds, once made, can be broken and re-made over and over again. This ‘cradle to cradle’ lifecycle is one of clay’s unique selling points, allowing the same material to be made up as a surface finish, then into a structural material, and then back to a surface finish over tens, hundreds or thousands of years!
The UK has a long history of earth building, and 8,000 houses of just a single earth building type - cob - were bought and sold in the South West of the UK between 2005 and 2016.
Different Uses and Contexts
While clay is endlessly malleable it has other properties which make it particularly interesting as a component in an indoor climate. More airtight buildings meeting increasingly stringent regulations mean a decline in indoor air quality. Both humidity and volatile organic compounds, (VOCs) are now big problems. Clay both regulates both liquid and gaseous humidity while also sequestering VOCs. This reduces the need for complex mechanical extraction, instead requiring just the presence of exposed, beautiful clay surfaces.
Clay is in use in new build domestic, public and commercial buildings. It’s use in historic buildings include cob and rammed earth structures, stone-and- clay-mortar buildings, clay finishes and timber-framed buildings with clay infill.
Clay mixes are all safe to use and require little in the way of personal protection for workers. They work well in modern building site conditions either as products delivered to site or material dug and prepared at site. This second category can give some of the lowest embodied carbon materials available while still achieving remarkable properties from thermal mass, load bearing structures to surface finishes, and insulation when combined in light earth mixes.
Specifiers need to understand the enormous range of clay bound materials and be clear about what they can achieve in a broad range of situations. Costing clay bound products is relatively straight forward but using materials on site in-situ requires some experience. Costs involved are quite often offset against savings from simpler built elements and the on-going running costs of buildings. Plasters can be used in a wide range of situations as they ‘stick to anything’, are available in a wide range of colours and are easily maintained. Structural elements in block, cob, rammed earth or clay mortared elements like brick or stone can be costed most easily with the help of experienced contractors.