If you live in a temperate climate where the sun shines during the cold months, use the heat of the sun to heat your home. Using passive solar design means you'll use less energy, money and fuel to stay warm. Let the sun shine in! This is an important part of designing your home. (Read the sections on passive solar design on pages 58 and 114-117 very carefully.)
This means it is dense stuff that holds the heat from the sun or the fire for a long time, radiating it back out into a room slowly. It also takes a long time to heat up. In most climates where humans live, this helps create a pleasant indoor temperature. Cob is naturally cool in hot summers, and can absorb solar warmth in the winter. It tempers the climate throughout the year and does the same on a smaller scale throughout the day/night cycle. Cob is ideal in desert areas where it holds the night's cool throughout the day, slowly heating up to release the day's warmth at night. Pay attention to the weather in your area through the seasons. The more sun you get during the colder months, the more practical a solar designed home will be.
If you live where it's really hot or cold around the clock, you may want to insulate or 'outsulate' your cob walls with something that is full of air pockets, and synthetically regulate the temperature by heating or cooling the interior. Adding more straw, pumice, vermiculite, or even styrofoam packaging chips to the outer or inner layer of cob may increase the insulation value.