29 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 29 in, Rough Opening Diameter 27 1/2 in, Rough Opening Shape Square, Fits Fan Diameter 24 in, For Duct Shape Square, Free Area 4.1 sq ft, Overall Width 29 in, Frame Material Thickness 20 ga, Overall Length 4 1/2 in, Frame Depth 2 in, Rough Opening Length 26 1/2 in