Fuzzy reasoning is used for visual comfort control in buildings. Natural ventilation is used to save energy and contribute to the achievement of thermal comfort. The present paper investigates the impact of natural ventilation on the thermal-comfort index, assuming the implementation of fuzzy reasoning for visual comfort control. Mathematical models are used to calculate the outdoor climate, the indoor climate and the thermal-comfort index. Finally, a fuzzy-reasoning expert-system, using a linguistic type algorithm, is presented.