Cob construction is a type of earthen construction that uses a clay-like material. The material consists of 15-25% clay and 75-85% sand/aggregate as well as straw and water. The material is typically mixed by hand, then it is applied by chunks, and the walls are built up like any other masonry structure.
Because cob construction is 100% from the earth, it is completely safe for humans, animals, and the environment. It is non-toxic, and less likely to cause health problems; the natural forms also promote psychological well-being. The dirt can often be taken from right on site, so the energy of transporting the material is low; only a small tractor or manpower would be needed. The tools required to produce a decent building are simple tools most people already have. Spades, pitchforks, shovels, are the more common tools, but one of the most important is the club. The club can easily be made right on site out of scrap wood. By building with cob, you reduce pollution, deforestation, and energy use, and it eliminates the need for the mining of materials for gypsum, and iron.
One of the biggest benefits is the cost; one can build a house for under $5,000, and a modest cottage for under $500.
There are potential risks to taking materials from on site, however. Precautions must be taken to ensure that the land has not been misused by former owners. If the ground is contaminated, those contaminants could end up in the building itself, and come in contact with the inhabitants.
A downfall to using cob construction is its insulating qualities. Cob walls have a high U-valule. The higher the U-value, the less efficient the material is for insulation. The cob itself typically has the highest value that is allowed (0.7W/m2k), and these buildings typically compensate for the high U-values by super-insulating other elements of the building (i.e. roofs, doors, windows). The material does not serve as a good insulator because it is heavy and dense. Because it is clay, the mixture contains no air pockets that serve as an insulating barrier.
Cob buildings, however, have excellent capacitive insulation properties. The walls are typically thick, and are able to store heat, and when the temperature drops, release it over a period of time. This aspect usually is effective enough to make up for the material's poor insulating qualities.
The materials used for cob building are completely sustainable. Cob buildings have been around since the beginning of time; ones dating back to the 1830's have been found in California and Pennsylvania, and in the UK, there are several from the 1500's.
Cob can be recycled indefinitely, and is not "down-cycled". Cob itself is not always the only material used in cob construction, however, sometimes a plaster-like material must be applied to the walls to make them waterproof, and more suited for everyday life. This material might not be 100% recyclable, so when the building has served its purpose and must be torn down, it is important to separate these materials.
Here are some useful links to organizations, builders, and general information:
Building With Cob (book)
Dirt Cheap Builder (books)
Cob balls waiting to be applied
Roof framing for a small cob studio
Using the club to pack walls
A house found in Devon, England that dates back to 1539.
< Fig. 5
A farmhouse in Richmond, North Otago, NZ from the 1860's.
The Hand Sculpted House
By Ianto Evans, Michael G. Smith, Linda Smiley, and Deanne Bednar
Building with Cob: A Step by Step Guide
By Adam Weismann & Katy Bryce