This paper presents the design of a fuzzy reasoning expert system for the achievement of thermal and visual comfort in buildings. This system does not demand the precise mathematical model of the building to achieve the control law but uses high-level control Variables such as thermal and visual comfort. The powerful interactions of the passive components and of the comfort subjectivity match with the application of the fuzzy control theory entirely. Mathematical models are presented, where the actions of the actuators are applied. The design of the rule base is described and, finally, the system is evaluated by using extensive, worst-case, simulation results.