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Thermal mass on the interior of the building is able to store heat or cold, and then slowly release it back out to the surrounding air. Heat from a sunny winter day as well as the coolness of a summer night can be retained inside the insulated shell of the exterior walls if the building incorporates adequate thermal mass. Thermal mass is needed to prevent daytime overheating of the structure and to stabilize its ambient temperatures through nights and periods of cloudy weather. The more mass is available, the more stable the interior temperature. Also, the more directly the winter sun hits the mass, the higher the solar heat gain.
Floors can provide a good source of mass. Concrete, brick, flagstone, or other masonry materials work especially well. Any of these can be layered over with cork — an organic material that insulates well, and has the advantage of creating a softer surface underfoot. Earth floors can be used, but do not perform as well. Insulating beneath the floor helps to return the heat gains to the interior of the building more quickly. Interior walls and houses are often framed with wood, but if built out of masonry materials such as rock or adobe, these can provide excellent mass, especially those interior walls hit directly by the sun. Concrete poured between studs is a quick way to add mass.
Water is one of the best thermal mass materials. Water has a very high heat capacity (specific heat), which means it can hold a lot of heat or cold before releasing it to the surrounding air. It was common in early solar buildings to see 55 gallon drums filled with water and painted black. Stored solar gain water walls or seats can be fabricated out of metal and incorporated into the south wall of the building, or placed near a wood burning stove. Water storage has the disadvantage of possible leaks.
Although its heat capacity is less than that of water, stone can also be used for thermal mass. An easy way to create an instant rock wall is to use gabions or heavy gauge wire baskets filled with rock. They make nice partitions, and can also be used for cooling in the summer if water is set up to trip down the rocks.
There is one other thing I need to mention here: thermal mass is useless if not surrounded by properly insulated exterior shell. It would reduce the rate of temperature drop inside the house when you turn the heaters off, but would not reduce the total heat loss!