21 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 2000 fpm Overall Height 21 in Rough Opening Diameter 19 1/2 in Rough Opening Shape Square Fits Fan Diameter 18 in For Duct Shape Square Free Area 1.9 sq ft Overall Width 21 in Frame Material Thickness 20 ga Overall Length 4 1/2 in Frame Depth 2 in Rough Opening Length 18 1/2 in