The Gira hygrostat detects a room's humidity and temperature using internal sensors. If the humidity exceeds the setpoint on the rotary knob, the Gira hygrostat switches on an externally connected fan to dehumidify the room. In the process, the device treats the setpoint dynamically, i.e. it takes account, within certain limits, of the fact that warmer air can absorb more moisture than colder air. For example, if the Gira hygrostat measures a higher room temperature than the reference temperature of 21°C, it permits a correspondingly higher degree of humidity. This avoids unnecessary ventilation and saves energy.