Thermal Mass Systems
Thermal mass systems are components or materials that allow a structure to absorb heat under certain conditions and times, e.g., from sunshine during the day, and then slowly release it over time when it is wanted, e.g. after the sun goes down and there is no direct solar gain to warm the indoors.
Thermal mass is often used "defensively" to absorb unwanted heat which would otherwise make the indoors uncomfortably warm. For example, thermal mass added to a roof will allow the roof to absorb the heat of the summer sun without getting as hot and radiating so much heat down into the living space. Then, at night, the thermal mass in the roof will radiate the heat up and away from the living space--out to the night sky which can absorb unlimited amounts of radiation.
Another application is adding thermal mass on the inside of the home. This mass will absorb heat coming from the winter sun during the daytime so the air in the home doesn’t overheat. After the sun goes down, the thermal mass radiates the heat it absorbed back out to other surfaces of the room, the furniture, and even the occupants, thereby keeping them and their home warm and comfortable overnight when there is no direct solar gain and the outdoor air is colder so that the house loses heat faster. The classic example is a concrete slab floor or an interior wall made of dense material or a special material designed to absorb heat, especially the heat of direct solar gain. Thermal mass can hold heat without getting too hot and then release the heat slowly. Thermal mass can create a "heat battery," storing energy from electricity available at low cost at particular times when heat is not needed to keep the home comfortable. The thermal mass then slowly releases its heat to the indoors later when electricity is more expensive.