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" or 24" centers with 2 cut openings, 1 on each side of center joist per shutter model. 35-3/4"L x 30 1/8"W OD, 33-3/4"L x 28-1/4" W opening.
