Points out that the contribution of thermal mass to the control of the indoor environment in terms of keeping the inside air temperature fluctuations within the comfort range can be improved if mass location inside the building is designed to receive the appropriate quantity of solar heat. To this end, the first step is to identify the area and geometry of sunlit internal surfaces. Presents a theoretical procedure to achieve this based on vectorial analysis to locate the direction and position of solar rays in space and to determine the angular boundaries of the sun's rays for each configuration. Introduces the concepts of window ratio and corner ratio to distinguish between the various sets of configurations falling inside the space. Classifies all configurations in terms of profile angles and gives formulas for calculating the sunlit areas.

KEYWORDS: Passive, insolation, windows, thermal comfort, calculating, buildings, air temperature, solar heat gain.