Abstract: A back draft damper for exhaust fans and hoods such as those used in kitchens or bathrooms; the damper comprising a stationary valve plate having a pair of openings and an elongate slot between said openings with a pair of flapper valve plates having curved edge portions extending through said slot and forming pivotal mounting for said flapper valve plates on said stationary valve plate; said flapper valve plates having extremities overlapping said openings in said stationary valve plate; the flapper valve plates being openable by air under pressure and closeable by gravity, and being on an upper surface of said stationary valve plate.