21 In.H 21 In.W Drnbl Extrd Alu Combo Dmpr Louver Type Combination Frame Type Flanged Outside Height 17 1/2 in Outside Width 17 1/2 in Rough Opening Height 18 in Rough Opening Width 18 in Louver Maximum Intake 779 cfm Maximum Exhaust 1 114 cfm Flange Width 1 1/2 in Free Area 0.7 sq ft Beginning Point of Water Penetration 1 118 fpm Maximum Air Velocity 3 000 fpm Blade Angle 45 Degrees Material Aluminum Inside Height 16 1/2 in Inside Width 15 3/4 in Number of Blades 3