In terms of building and construction, a thermal mass is essentially a solid that absorbs heat from the sun during the day and slowly radiates the heat at night. It may take the form of a thick wall or floor slab, made of either stone, concrete, clay, adobe, brick, rammed earth, or even a volume of water. A thermal mass offers a much more energy efficient alternative to using a standard, forced-air heating system.
Heat is transferred from a thermal mass by radiation, convection and conduction. In the winter, the heat moves from the wall to the interior space, and in the summer, it works in the opposite direction and is expelled outdoors. Therefore, a consistent and comfortable temperature may be maintained throughout the day. The right orientation for a thermal mass depends on the climate in which a structure is built. In a cold climate, it should face the winter sun, whereas in a warm climate, it should not be exposed to direct sunlight. In general, the larger the mass, the more effectively it performs.
Thermal masses are a common strategy used in passive design.
A trombe wall is a type of thermal mass.