The reasons to build with cob are infinite and vary with each builder. For us, we build with cob because it is low impact, sustainable, and reusable (yes, cob walls can be knocked down, remixed with water, and reused). It is non-toxic, all natural, locally soureceable (for many), and affordable. There are few places on Earth where clay sub soil can not be found making it among the lowest impact building materials, from a transport perspective, available. It also consumes scant amounts of water (compared to concrete and other engineered materials) and returns what it consumes purified and resuable via ground water and evaporation. If you plan your cob timing right, with moderate rain fall, you can make cob without adding any water at all beyond what comes with the rains.
You can learn how to cob the first time you try it. If you do it wrong, or do something you don’t like, it is easy to repurpose what you have used and start over. It allows for infinite amounts of creativity and craft and encourages an intimate relationship between the builder and the structure being built. We have yet to run short on ideas when it comes to earthen building detail.
Clay is naturally deodorizing and air purifying creating fresh homes. It is also a phenomenal sound buffer creating an interior stillness like we have never experienced in industrially constructed buildings. Cob is fireproof and rot proof. Insects do not chew it to the ground and rodents don’t nest in it. Since we live in Vancouver BC, earthquakes are a part of life here. Cob performs exceedingly well when challenged on shaker tables, such as at the UBC Engineering Earthquake Laboratory.
It passively adsorbs, stores, and radiates back heat which can be capitalized on for heating efficiency by making cob-rich south facing walls and incorporating wood burning rocket mass heaters in to the structure. It likewise buffers high heat temperatures much like concrete parkades and sub ground housing.
Finally, if a day ever comes that we no longer want our cob structure, we can smash it down back in to the very earth from which it came, leaving no trace of its existence.