37 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 37 in Rough Opening Diameter 35 1/2 in Rough Opening Shape Square Fits Fan Diameter 36 in For Duct Shape Square Free Area 7.1 sq ft Overall Width 37 in Frame Material Thickness 20 ga Overall Length 4 1/2 in Frame Depth 2 in Rough Opening Length 34 1/2 in