Backdraft Damper / Wall Shutter, Fits Fan Dia. 18 in, Shutter Overall Size 21 in x 21 in, Shutter Construction Material Extruded Aluminum Frame and G-90 Galvanized Tie Rods, Shutter Opening Required 18-1/2 in x 18-1/2 in, Shutter Free Area 2.043 sq ft,…