Manual Damper, Damper Application Backdraft, For Nominal Duct Diameter Not Applicable, Frame Material Fiberglass, Maximum Air Velocity Not Rated, Overall Height 21 in, Rough Opening Diameter Not Applicable, Rough Opening Shape Square, Fits Fan Diameter 18 in, For Duct Shape Not Applicable, Free Area Not Applicable, Overall Width 21 in, Frame Material Thickness 2.6 mm, Overall Length Not Applicable, Connection Type Not Applicable, Frame Depth 3 3/8 in, Rough Opening Length Not Applicable