57 1/4 In.H-W Aluminum Wall Backdraft Dampr, Frame Material Galvanized Steel, Number of Panels 2, Maximum Air Velocity 2500 fpm, Overall Height 57 1/4 in, Rough Opening Shape Rectangular, Fits Fan Diameter 48 in, Free Area 16.7 sq ft, Overall Width 57 1/4 in, Frame Material Thickness 0 ga, Frame Depth 2 1/2 in, Rough Opening Length 54 in, Maximum Pressure 1 in wc, Rough Opening Width 54.5 in, Blade Material Aluminum, Closure Type Gravity, Flange Width 1 1/2 in