Building with earth isn’t just for hot countries; in fact in Ireland building with cob used to be commonplace and in eco-circles, it’s certainly made a comeback. But there are other earth building alternatives, including rammed earth…
Earth when used as a building material is fireproof, contributes to the regulation of the indoor environment (temperature and humidity levels), and when correctly specified and built, is sufficiently strong for buildings up to ten stories high.
Earth building methods are arguably the oldest known to man; the technique varies from region to region as each has been tailored to the type of soil available locally. Mixes were either applied in situ or dried in the shape of bricks and blocks, from the common clay brick to CEBs (compressed earth blocks, essentially rammed earth bricks).
In all cases, the earth based building components are made of ‘subsoil’, which is the inert (free of organic matter) layer beneath the topsoil. It consists of a mixture of gravel, sand, silt and clay. If topsoil is used instead, the integrity of the finished product will be compromised.
Cob vs. Rammed Earth
A lot of the subsoil found in Ireland has a high clay content, which is what is required for cob construction (15% to 40% of the mix). Cob used to be a very common building method in our part of the world with examples in Ireland, the UK and Brittany (France) that are still standing. Subsoil and straw, which acts as a reinforcing mesh, are mixed together with water to get an elastic consistency – a tractor is often used to make large batches. For smaller quantities it can be mixed on a tarp, (with foot/stomping power!), however this method makes it more difficult to keep the water content suitably low.
The rammed earth technique, on the other hand, is better suited to subsoil that has a lower proportion of clay (generally speaking less than 15%) and higher amounts of sand and gravel. Its roots can be traced back to China (think Great Wall!), the Middle East and North Africa, especially in regions where the clay content was low and timber scarce. Rammed earth is stronger than cob and less prone to shrinkage as the mix is compacted and requires less water, shrinkage and subsequent cracks being caused by water evaporation.
Cob and rammed earth can be used for floor and wall construction. In both cases the proportions need to be correct and when done on a DIY basis the only way to determine the right mix is by making lots of samples and test walls, and preferably consult with an experienced cob or rammed earth builder.
Waste arising from earth based construction is minimal and when it does occur it’s easily dealt with (unless it’s been stabilised with cement). When excavating for foundations subsoil often has to be carted off site; in this instance it would be reused in the walls, further minimising waste.