Features: * Injection molded frame with smooth aluminum louvers that automatically open when fan turns on, and close when fan turns off to keep out air, insects, and dust * Balanced shutters for complete automatic service-free operation * Installs centered on joist with 18 inch or 24 inch centers with 2 cut openings, 1 on each side of center joist per shutter model * 35-3/4 inches L x 30-1/4 inches W overall, 33-3/4 inches L x 28-1/4 inches W opening * White enamel finish What"s In The Box: * Ll Building Products S30 30-Inch Automatic Fan Shutter