Abstract: A system for regulating the ventilation of a building works automatically to reduce heating and air conditioning in unoccupied rooms by making the output of air removed from each room or service area vary as a function of the amount of humidity in the flow of air leaving such service area, the overall delivery of air introduced to replace the air removed being the greatest in the service area the loosing air from which exhibits the greatest humidity. To control the flow of air to various service areas, suitable intake and exhaust openings, having valves therein, such as flap or butterfly valves are provided.