Key Properties of Earth Plasters
Chemical-Free - non-toxic and healthy
Vapour-permeable/Breathable - for excellent indoor air quality and longevity of building fabric
Strong and Durable - and easily repaired should it be damaged
Fire Resistance - earth naturally resists fire
Natural Resource - locally available in abundance, or as off-the- shelf products
Flexibility - suitable for domestic or commercial new build and retrofit projects
Coloured Plaster - available in a range of colours, with no painting required
English Heritage Approved - often used in traditional and listed buildings
Main Benefits and Uses
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 plasters regulate 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 plasters are in use in domestic, public and commercial buildings. Clay plasters can be used on cob, stone, brick, concrete, timber framed, straw bale, rammed earth or any combination of these, including gypsum and other kind of substrate board, lathe, reed or cane. The thickness of clay plasters to maximise their effectiveness in buffering humidity and temperature can be calculated to achieve the most effective indoor climate.
Materials and Process
Clay plasters are the fastest growing area in the earth building world. They stick to pretty much anything, so whatever structural method you use, it can benefit from a coat of clay internally to finish the job. Clay plasters can be made from the wide variety of clays that exist, from the highly expansive to the relatively inert. And because of this, clay plasters come in a wide range of colours, from black to white and pretty much everything in between. Clay plasters can also be tinted with oxides to achieve a high level of finish and colour which can be matt, smooth or highly polished. Finished coats start around a millimetre thick while bulking coats can be much thicker.
Like all earth building processes plasters can sometimes be sourced on site, and bagged products are also widely available. Clay plastering is similar to lime or gypsum plastering, levelling and finish all have their own processes but as with the difference between gypsum and lime plasters clay has its own timing and touch; a little training goes a long way.
Clay plasters 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 while still achieving a natural healthy and beautiful surface finish.
Specifiers need to understand the enormous range of clay plasters and colours and be clear about what they can achieve in a broad range of situations. Costing clay plasters and products is relatively straight forward, but using materials on site in-situ requires some experience. 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. Clay plasters like all earth building materials now have a full suite of European and UK training standards ensuring a high level of predictability and finish.