Given the importance of buildings on the energy balance in Greece, an attempt has been made to study their energy behaviour and thermal comfort. Our primary purpose is to provide an estimation of the building's energy consumption and examine how this affects the comfort conditions. This includes the definition of thermal conditions acceptable for various activities at different times of day during each month of the year. We cannot underestimate the value of real measurements and observations of the building's energy systems, but such data are not always available. The best opportunities for improving energy performance occur early in the design process. Our simulation results can give an indication on which end uses are the most energy consuming, the "weaknesses" of a building and thus urge the owner or engineer to take effective conservation energy measures. (C) 2007 Elsevier Ltd. All rights reserved.