MaxxAir IF24UPS Heavy Duty 20-Gauge Galvanized Steel Exhaust Fan, 24-Inch Promotes excellent air circulation and quickly removes unwanted odors and humidity Great for barns, greenhouses and other heavy duty or commercial applications PSC energy efficient thermally protected motor, Rough Opening: 28.25 x 28.25 Balanced rust-free 4-blade fan assembly totally enclosed for protection and OSHA compliant Integrated, precision-formed exterior shutters are draft-free and open and close automatically