21 In.H-W 5.75 In.L Alu Wall Backdraft Dampr Damper Application Backdraft For Nominal Duct Diameter Not Applicable Frame Material Galvanized Steel Number of Panels 1 Maximum Air Velocity 2500 fpm Overall Height 21 in Rough Opening Diameter 18 1/2 in Rough Opening Shape Square Fits Fan Diameter 18 in For Duct Shape Not Applicable Free Area 2 sq ft Overall Width 21 in Frame Material Thickness 20 ga Overall Length 5 3/4 in Connection Type Not Applicable Frame Depth 2 1/2 in