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Whole House Fan Installation Tips

Whole House Fan Installation Tips

Most of the newer whole house fans are designed to be installed with minimal alterations to attic framing. Special models are made that can actually fit over and around attic trusses that have been placed at 24 inch centers. Don't make the mistake of buying a fan that fits your framing but NOT your house. An undersized fan simply might not move enough air.

Exhaust Air Ports

Pay attention to the instructions that come with the fan. They will undoubtedly mention attic exhaust area and will represent this number in square feet. Do whatever is necessary to achieve or exceed this number. There is a very good chance that you will need to install extra roof vents to meet this important requirement. If you don't know how to install a roof vent without causing a leak, you will need a roofer.

The size of the hole cut in the roof is not the true ventilation area. Often the instructions refer to net free area. This is usually 50 percent of the actual hole that is cut. Why only 50 percent? The screening, louvers, rain covers, etc. that are a part of roof and gable end vents create resistance to air flow.

Dealing With Insulation

Many attics have blown-in insulation. Powerful attic fans can create a virtual snow storm in attic spaces, if you don't plan ahead. Often, you simply need to construct a simple plywood box or sides around the attic fan. This will deflect air up towards the roof and stop insulation from being sucked into the high speed air flow immediately adjacent to the fan.

This box can also serve another valuable purpose. If you take your time and cut all of the pieces so they are flush and level once installed, it makes a superb platform to support an insulated cover for when the fan is not used. Many of the ceiling louver kits allow vast amounts of cold air to drop down into the house during winter months. You can cover your attic fan from the top with strips of plywood that are then covered with batt fiberglass insulation. If you are really handy you will glue pieces of foil faced foam to the sides of your attic fan deflector box to stop heat loss through the sides of the structure.

Electric Needs

If you purchase a low powered fan, you might not need a separate circuit for the fan. In any case, you need to follow the National Electric Code with respect to fans like this. It may say or your local inspector demand that a separate circuit be installed. If your builder or electrician was thinking ahead, then it will be easy. I always installed several blank or empty conduits from the electric panel to the attic area to accommodate future wiring needs. If you don't have one, try to find where the large plumbing stack travels up from the basement area to the attic. Often the plumbers leave enough space between the pipe and the framing to allow you to drop a string and pull a wire.


If you are getting ready to cut a truss to make your fan fit, STOP! You can't cut a prefabricated truss without seriously compromising your roof structure. Go to plan B, or call in an experienced carpenter who can tell you your options.

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