Continental Fan Manufacturing BDD100 Spring Loaded Backdraft Damper for 4" Ducti, N/A Neoprene damper seal minimizes leakage 20 gauge steel housing, aluminum spring loaded butterfly damper blades Simple installation in any position - entire assembly inserts into duct Exterior foam gasket assures a tight seal against duct 4", 5", 6" & 8" models include stamped metal sleeve with center pin design