Cob 101 / Is cob a good insulator?

No! However, cob homes are still very thermally efficient. This is because cob, like rock, is capable of absorbing and storing immense amounts of heat that it can then radiate back. Think of the days when stones were heated in a fire and then transferred in to a bed to keep people toasty warm overnight. Cob does the same thing. Cob, like rock, takes a long time to heat up and a long time to cool down. If built strategically, cob can passively absorb all sorts of heat from solar energy to the heat of wood fires and rocket stoves and then slowly radiate it to back to you, such as overnight when solar energy disappears and the fire has gone out.