Thermal Mass Design in Buildings - Heavy or Light?

Thermal mass, including the building envelope, the interior partition, the furnishing, or even the air inside building, is defined as the mass that can store thermal energy (heat or cooling energy). For storing heat in buildings, there are two important thermal properties of the materials that need to be considered, i.e. the heat capacity by volume and the heat-absorption rate. The first property determines the ability of the element to store thermal energy, and the second property determines the ability of the element to conduct the thermal energy. The combined convective and radiative heat transfer coefficient and the surface area of the thermal mass determine the rate of heat transfer between the thermal mass element and the air. One of the good examples in using thermal mass is night cooling, which can avoid or minimize the need for mechanical cooling in buildings. We present a simple design formula for use by architects and engineers which involves only three design related parameters, i.e. the time constant of the system, the dimensionless convective heat transfer number and the Fourier time constant. The present method allows the fast determination of the amount of thermal mass as well as key design parameters when the phase shift of indoor air temperature and the attenuation of the indoor air temperature fluctuation are specified.