The invention is related to a method and an apparatus for controlling a ventilating system or an air conditioning system of buildings. This controlling is used predominantly in industrial real estates and serves for the establishment and maintenance of the desired room conditions. The method for controlling a ventilating system or an air conditioning system according to the invention uses the air density as the essential parameter for the controlling. The air density depends on the temperature, the air humidity and the air pressure. The air density difference between the introduced air and the room air is kept small.