Thermal mass refers to the mass of a structure that can act to retain and release heat. This is an important component of passive solar designs.
Dense materials such as concrete, rammed earth, and bricks all have good thermal mass properties.
Thermal mass works to regulate and dampen down temperature fluctuations in buildings.
In summer, the thermal mass of a building can “soak up” excess heat from within the building. This works if the thermal mass surface is shielded from direct sunlight by blinds, sails, a pergola or other means.
In winter, the thermal mass of a building can store heat during a sunny day if direct sunlight is allowed on it. During the cooler evenings, this “heat store” then releases its stored heat back into the building.
A building with little thermal mass (or incorrectly installed thermal mass) can experience large internal temperature fluctuations over a 24 hour period – heating up during the day and cooling down overnight.
A building with well designed thermal mass would experience smaller temperature fluctuations over the same period in the same location. It would stay cooler during the day and warmer overnight.