Whole House Fan Shutter - 24" - Interior Use 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. Fits Fan # WHFS24M - 24" Opening Size Required: 27.25" x 28.25" White Enamel Finish - Interior use.