Building with earth—isn't that old fashioned?

Building with earth could be described as old fashioned as it is our most ancient form of building and started with mud huts.  Earth was also used in the construction of some of the worlds greatest architecture, and today it is still used to build beautiful and individually sculpted homes.

One  method of earth building is known as cob.  Traditionally the mixing was done in a pit with animals to trample and mix the earth and straw.  This inevitably meant the addition of waste matter in the mix, and lead to a common belief that cob would only work if a small percentage of dung was used.  This isn’t true at all and no dung is required!  These days cob can be mixed by rotavator, or if on a large scale, with a JCB.  Cob is actually a mix of subsoil, either clay or chalk mixed with straw, and is relatively wet to build with.  Walls are formed by either pressing and sculpting the cob, or shuttering can be used.   Curved walls are easy to build as well as  features and  designs can be carved into the walls.

Another popular method of building is rammed earth, the original  French term being Pisé.  The technique came from the Romans and was taken up in the eighteenth century in England. Rammed earth differs from cob in that you just use damp subsoil, again either clay or chalk.  The earth is literally rammed with shuttering under a lot of pressure to form walls.  The advantage of this method is that that the process is quicker and the building won’t shrink so much when drying.

Historically, earth building has been very popular in the South West, from the chalk hills of Dorset through to the clay sub-soils of Devon, and there is plenty of evidence of this around us in the pretty cob cottages in most villages throughout the Blackmore Vale.  The disadvantage of cob building is that it is relatively slow and labour intensive, and so it was superseded by bricks and blocks. But, in light of the need to reduce the CO2 released during the construction of buildings, known as the ‘carbon footprint’,  earth building has become very viable.

Chalk sub-soil and straw are readily available in this area, often on or near the site almost eliminating the need for transport making this a highly sustainable building method with very little embodied energy.  Earth is an abundant,  recyclable, non-toxic material, and benefits from having a high thermal mass, or in other words, the building stores heat.  Most people are familiar with the concept of many older houses that are cool in the summer and warm in the winter, and these will most likely be cob or mass stone.  This thermal mass can be used to greater effect with the  benefits of solar gain in the design and orientation of the building.  By making the most of winter sun, the need for heating can be reduced.  Unfortunately, it is difficult to make an earth building meet current building regulations on insulation, and they usually require extra external insulation to meet requirements.  But the complex effects of thermal mass are being researched and are showing benefits greater than insulation on its own.

Earth buildings can last hundreds of years if properly maintained. The walls are load bearing and structurally sound.  This has been assumed for years, but now with modern technology structural engineers have proved that this is true.  Sound proofing is better than an average brick build, and lime rendered earth buildings, unlike modern brick buildings, are breathable.

The environmental benefits of earth building are so great that it has to be considered as an alternative building method to cut down current carbon emissions and energy use in the construction industry.  Earth buildings have low carbon footprints and the materials are locally sourced.  Now we have the technology to take advantage of earth building and can use modern manufacturing methods, such as the ability to make cob blocks.  This leads to a natural transfer of skills where most of the workforce in the building and construction industry are already trained to build with blocks.  The answer to creating a low carbon producing construction industry isn’t easy, but with the help of modern technology  applied to  cob and rammed earth building techniques, we are equipped to make a significant environmental change and at the same time, build beautiful, individual houses.

By Rob Buckley
Dorset Centre for Rural Skills

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