what is thermal mass

Thermal mass is a property of a material that enables it to absorb, store, and release heat. It is the ability of a material to absorb and store heat energy, and it is typically referred to by the symbol Cth, with its SI unit being J/°C or J/K. Materials such as concrete, bricks, and tiles have high thermal mass because they absorb and store heat, while lightweight materials such as timber have low thermal mass because they do not absorb and store heat. Thermal mass is used in building design to provide inertia against temperature fluctuations and to store heat. It is ideally placed within a building where it is shielded from direct solar gain but exposed to the building occupants. Thermal mass can be used in floors, walls, or ceilings, and it needs to be isolated from the influence of external air temperatures. The efficacy of thermal mass depends on the placement of the material with respect to direct irradiation of the sun. Appropriate use of thermal mass can make a big difference to comfort and heating and cooling bills, but it should not be used as a substitute for insulation.