21 In.H-W 6.5 In.L Fiberglass Backdraft Damper Damper Application Backdraft For Nominal Duct Diameter Not Applicable Frame Material Fiberglass 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 1.8 sq ft Overall Width 21 in Frame Material Thickness 0.1 in Overall Length 6 1/2 in Connection Type Not Applicable Frame Depth 3 in