The apparent thermal capacitance of a building is obtained by adding the distributed specific heats of all building elements. It differs considerably from the effective thermal capacitance, which is calculated in the present study by forcing the solution of a lumped-system differential equation to follow the experimentally validated, finite-difference solution of a rigorous set of coupled differential equations describing the heat transport and energy balance in buildings. The effective thermal capacitance is calculated for various characteristic cases of Greek buildings and provides a simple procedure for approximating the transient thermal behaviour of buildings.