A ventilation controller including at least one humidity sensor for controlling the humidity of a room. The ventilation controller incorporates a housing sized and shaped to replace, or be placed in, a standard electrical junction box. Circuitry in the controller receives data from the sensor(s). The controller automatically switches on power to an exhaust fan when either the humidity exceeds a manually set humidity level and/or a rapid increase in humidity is observed. When a plurality of sensors is employed, the humidity level from a first sensor is compared by logic circuitry to the humidity levels detected at a reference sensor(s). When the humidity at the first sensor exceeds the humidity at the reference sensor(s), the ventilation controller switches on power to the exhaust fan.