Backdraft Damper / Wall Shutter Fits Fan Dia 18 in Shutter Overall Size 21 in x 21 in Shutter Construction Material G-90 Galvanized Steel Frame and Tie Rods Shutter Opening Required 18-1/2 in x 18-1/2 in Shutter Free Area 2.051 sq ft Vertical Shutter Mounting Position Shutter Mounting Location Wall Frame Depth 2 1/4 in Open Depth 6 1/4 in Flange Width 1 1/2 in Mounting Hole Size Prepunched Max Velocity 2000 FPM Blade Material Mill Finish Aluminum