The U.S. Department of Energy recommends covering older models of whole house fans when they are not in use. A typical older whole house fan has a 30-inch-diameter blade with a sheet metal cowling of 31 inches to allow for blade clearance.
An attic-side box cover may be constructed from a 4-by-4-foot piece of 1-inch rigid fiberglass duct board. The box will be 33 inches square with 1-inch-thick walls (inside dimension of 31 by 31 inches). It will be 6 1/2 inches deep. Adjust the dimensions if you are purchasing a newer whole house fan.
Fan box cover materials list:
• 48-by-48-inch piece of 1-inch fiberglass duct board
• Silver duct tape or house wrap tape
• Measuring tape, straight edge, utility knife
• Permanent marker to label box
• Gloves and eye protection
• H brackets
When installing a whole house fan, be sure to provide proper support and seal the unit into the rough opening in the ceiling. Never cut a truss chord; wooden H brackets installed between the trusses create a framed box to raise the fan above the truss system. The louvers must be able to operate freely, and care must be taken to prevent binding or misalignments.