THERMOBOX isolates the exhaust system of the extractor hood. The heat retention system consists of three consecutively arranged pairs of flaps in a defined spaced. In the closed system, two insulating air chambers form. A reflective layer on the first pair of inwardly aligned flaps decreases the transmission factor at the entrance to the system. With a U-value of only 2.2 W/(m²K), the THERMOBOX is significantly better than comparable systems. The system opens and closes by means of magnetic force. The advantage is a graduated force curve which is ideal for use in backflow flaps. THERMOBOX is Blower Door certified.