This study examines the development of rules of thumb based on relationship between thermal capacity and spatial volume; it explores the following research questions:(1.) Can you have too much mass in a building?(2.) Is there a point at which adding additional mass to a space will not reduce the internal diurnal temperature range during summer beyond its current range? The review of existing rules of thumb (Slee and Hyde, 2011) concluded that an effective measure for the quantity of mass in a space is represented as a relationship between the amount of thermal capacity in a space and the volume of the space. It is known that adding thermal capacity (thermal mass) to a space reduces the diurnal variation of the ambient air temperature in that space. Is there a point when additional mass will not reduce the diurnal variation further? This paper reports work using Thermal Simulation Modeling where the quantity of thermal capacity in a space is changed incrementally and the ambient air temperature is observed. The experiments demonstrate that there is an exponential relationship between the quantity of mass (thermal capacity) in the space and the diurnal variation of the ambient air temperature in that space.