41 In.H-W 4.5 In.L Aluminum Backdraft Damper, Damper Application Backdraft, For Nominal Duct Diameter Not Applicable, Frame Material Galvanized Steel, Number of Panels 1, Maximum Air Velocity 2250 fpm, Overall Height 41 in, Rough Opening Diameter 39 1/2 in, Rough Opening Shape Square, Fits Fan Diameter 36 in, For Duct Shape Square, Free Area 8.9 sq ft, Overall Width 41 in, Frame Material Thickness 20 ga, Overall Length 4 1/2 in, Frame Depth 2 in, Rough Opening Length 38 1/2 in