These all aluminum, self-acting back draft dampers open only when fans are in operation, and automatically close when fans are turned off. Aluminum, multi-leaf, interconnected blades with stainless steel sleeve bearings are set into a square frame. Designed to mount inside of curb or framed in opening between the exhaust fan and the ductwork. IMPORTANT Dampers are not designed for use with fans that exhaust grease-laden vapors