Structural Earth

Earth is an inexpensive building material and learning to build with it is easy.  It uses an abundant resource that does not cause deforestation or pollution, and doesn’t require power tools or industrial manufacturing.  It is non-toxic and 100% recyclable; it can, quite literally, be recycled right back into the garden as the saying goes. 

There exists a much smaller body of research on the structural properties of alternative building materials than their conventional counterparts.  Generally, laboratory testing is expensive and therefore usually performed by individuals or industries that stand to benefit financially or commercially from the data, which is not the case with natural building materials.  Perhaps the very nature of natural building materials resists commercialization.  In lieu of an extensive body of scientific literature, we have vast examples of structural earth in practice, as it was historically, and remains today, one of the most widely used building materials in the world.

Image courtesy of John Nelson

Cob: Cob is one of our favorite, and easiest, building materials to work with.  Cob is a mixture of earthy clay, sand, straw, and water, which can be mixed together and easily molded into monolithic structures with no seams or cracks.  Long straw fibers give cob its tensile strength. Sand gives cob compressive strength and stabilizes the clay because it does not expand or contract when wet.  Clay bonds electrochemically to itself, water and other minerals, thus functioning as the “cement” that holds everything together.  Because clay expands when wet and contracts when dry, too much clay will cause cracking; it must be moist enough to stick together, but dry enough to hold it’s form without a mold.  It is most important to have the right proportion of sand to clay as these two elements have the largest impact on the strength properties.  Mixtures with more sand have a higher thermal mass, and mixtures with more straw have higher insulation value.[2]  Because cob doesn’t require processing or machinery to build, it has lower embodied energy than most other building materials. [3]

Cob is resistant to seismic activity and is fireproof.  It is a common misconception that cob structures are only suitable for dry climates, but cob actually originated in England and cob structures there have lasted for hundreds of years.  In Southern England tens of thousands of structures are 500-700 years old.[4]  Cob can absorb large amounts of water without damage if it is allowed to dry out.  In general, cob can endure most climates, even very wet climates, if protected properly with a good roof, large overhangs, and a tall waterproof foundation.  Rubble trench foundations are particularly well-suited for very rainy climates and provide extra protection against flooding.

Adobe: Adobe is made from the same mixture as cob, but instead of being applied wet to build structures, adobe is formed into bricks, allowed to dry, and bricks are used to construct the walls. 

Rammed Earth: Rammed earth uses a mixture similar to cob, but unlike cob, it requires temporary forms.  Forms allowing for a wall thickness of 4 to 10 inches are used temporarily, while the earth is poured in and mechanically compacted to roughly 50% of the original height.  As with other earthen mixtures, the clay content absorbs moisture, typically holding the humidity in a room to 40-60%.[5]  Rammed earth has a high thermal mass, with 12-14” thick walls, but is not the greatest insulator.