Building with cob; a home made out of mud.

Cob is a mixture of clay, sand, straw and water. Cob building is an old technique. Next to Adobe brick construction it is the most common earthen building technique. Cob construction involves layering a mix of clay, sand and straw directly onto the wall without the use of mortar or a framework. The rough surface is later trimmed and usually rendered, which leads to a soft, natural and warm look to the walls.

Cob provides extra thick and sculptured walls but they may be prone to shrinkage while drying, calling for careful selection of materials and construction detailing. Cob walls are usually load-bearing. A cob home will have a very distinct and natural earthy quality to it that you will not find in a conventional home.

Energy Efficient

Cob buildings are massive monolith structures suited to moderate and warm climates. Their mass provides excellent thermal storage. The cob itself is a thermal mass that absorbs sunlight and warms the building over the course of the day. This is called passive solar heating, and it will keep the inside of the building warm in the winter and cool in the summer. For colder climates with less solar exposure cob may not be an appropriate building method because of its lack of insulation.

Healthy Living

Unlike conventional homes which are largely built with synthetic, industrial materials, cob homes are built almost entirely out of natural and clean materials. Industrial homes are often filled with indoor air pollutants and off-gassing of chemicals contained in the materials. Cob homes do not have this problem. In fact, cob actually “breathes” through tiny pores and keeps air fresh and healthy. If you suffer from indoor allergies then living in a cob house could significantly improve your quality of life by eliminating the toxins that may cause them.

Will my home wash away in the rain?

One of the most commonly asked questions about cob is “What happens when it rains?” Cob building originated in England and Wales, which are not dry places in any way. There are areas with whole villages constructed with cob, many of them several centuries old.

The solution to this is the same as that of any building style: A good roof and a good foundation is the key to surviving a wet climate. A stem wall with sufficient height to protect the cob walls from splashing water when it rains and large roof overhangs are time proven methods to ensure the survival of the building. Added protection in the form of a lime plaster is often found on cob buildings. Do make sure that any render you use to protect your walls does not limit the permeability or breath-ability of your walls [ discussion] as this may lead to moisture build up and lead to catastrophic failure of the walls.