27.8 In.H-W 4.5 In.L Alu Spring Bkdrft Dampr Damper Application Backdraft For Nominal Duct Diameter 28 in Frame Material Galvanized Steel Number of Panels 1 Maximum Air Velocity 2000 fpm Overall Height 27 3/4 in Rough Opening Diameter 28 in Rough Opening Shape Square Fits Fan Diameter 28 in For Duct Shape Square Free Area 4.9 sq ft Overall Width 27 3/4 in Frame Material Thickness 20 ga Overall Length 4 1/2 in Frame Depth 2 in Rough Opening Length 28 in Maximum Pressure 1 in wc