Premium Back Draft Damper is installed into ductwork to prevent unwanted outdoor drafts from entering into your bathroom fan, kitchen fan, clothes dryer, etc. Ultimate one way air flow valve allows air to flow out but not back in. It does not rust or require any maintenance. Non-mechanical in-line one way air flow valve that requires minimal pressure to operate, has no moving parts. Sized slightly less than the internal diameter of the duct to slip easily into duct. Rubber gasket provide a friction fit and air-tight seal. Eliminates cold air back drafts in the winter saving heating loss. Eliminates hot air back drafts in the summer saving air conditioning loss. The damper consists of a specially formulated fabric sleeve attached to an outer metal sleeve. The fabric sleeve will allow air to pass through unrestricted in the desired direction of flow without reducing system efficiency. It is silent, produces no metal clanging sounds, tested to UL723 standards and can be installed in solid or flexible ductwork in any orientation. Installs easily - simply slide into duct. Specifically designed to stop back drafts from your clothes dryer, bathroom exhaust fan, kitchen fan, pellet stove, direct vent water heater and direct vent furnace. Available in 4 in., 5 in., 6 in., 7 in. and 8 in. sizes.