Designing Thermal Mass in Naturally Ventilated Buildings

According to its location, thermal mass can be approximately classified into two types: external and internal. Internal thermal mass, such as furniture and purpose-built internal concrete partitions, does not expose to ambient temperature directly, while external thermal mass, such as walls and roofs, expose directly to ambient temperature variation. A virtual sphere method for modelling thermal mass is introduced and its application in building thermal mass design is discussed. When the ventilation rate is constant, three key design related parameters are identified, and both the phase shift and fluctuation of the indoor temperature are determined by the time constant of the system, the dimensionless convective heat transfer number and the Fourier time constant. A simple design method for architects and engineers is described. The present method allows the rapid determination of the amount of thermal mass, as well key design parameters when the phase shift of indoor air temperature and the attenuation of the indoor air temperature fluctuation are specified. This paper also presents an explicit formula for predicting phase shift when the air flow is driven by buoyancy force alone.