How to install an attic fan | Pro Construction Guide

How to install an attic fan

To properly ventilate an attic,  you’ll probably need to install an attic fan. As heat builds up in the attic, the attic can reach temperatures as high as 150 degrees Fahrenheit. Even if the attic is insulated, the hot attic air will gradually heat the home and increase the amount of air conditioning and energy needed to keep it cool.

Drill a pilot hole through the roof from inside the attic

Drill a pilot hole through the roof from inside the attic in the approximate location you plan to mount the fan. Then, cut a round hole that matches the diameter of the attic fan.

In dry climates, a hot attic can dry out the roof framing and shorten the life of the roof shingles. In colder climates, poor attic ventilation can lead to a moisture build-up that creates a place for mold to grow and increases the possibility of wood rot. In extreme cases, ice can form in the attic.

An attic fan works by drawing in cooler outside air through vents in the roof or the gable end, and blowing out the hotter air within the attic.  If you install an attic fan properly, it will be quiet, use little electricity and can save as much as 30 percent on cooling costs. The added benefits of increasing the life of the roof and prohibiting mold growth make the attic fan the important addition to the home.

Solar-powered attic fans are gaining popularity, although engineers and manufacturers recommend they be used in smaller attic spaces because of the limited volume of air they can move. Installation is similar to a standard attic fan with the exception of the wiring.

Step 1: Select the correct fan size

To install an attic fan, first measure the attic and then use the chart below to select the proper size fan. Note: this chart is based on a roof pitch of 4/12. If the roof is framed more steeply, you have more cubic feet of air to move, and you should consider increasing the attic fan size to the next larger fan in the chart. Be sure to check the fan sizing guide provided by the manufacturer of the attic fan you plan to use.

Attic fan sizing chart

Fan diameter CFM  Attic size
inches cubic feet per minute  square feet
8 550  up to 500
10 625  up to 800
12 1200  up to 1300
14 1800  up to 2500
16 3000  up to 3500
The fan slips under the shingles around the top half of the hole

The fan slips under the shingles around the top half of the hole, and rests on top of the bottom-half shingles.

Step 2: Position the fan

The two most common locations to install an attic fan are on the roof or behind a gable end vent. Since the reason you install an attic fan is to draw as much air as possible through the attic, position the fan as far away as possible from a roof or gable end vent.

If the new fan is installed near a vent (incoming air source), the fan will draw in the air from that source and immediately exhaust it. This is sometimes called “short-cycling” or “short-circuiting.” It defeats the purpose of the fan and won’t make much of a difference in the attic air temperature. By keeping the new fan and vent far away from each other, cooler air will pass through the entire attic space, as hotter air is exhausted.

At a louvered opening in the attic, secure the fan

At a louvered opening in the attic, secure the fan by driving screws through its mounting brackets and into studs.

Step 3: Frame the opening

Typically when you install an attic fan you fit it between and attach it to rafters for a roof-mounted application or between wall studs for a gable-end application. Either way, most fans will mount to the studs or rafters, which are 16 inches on-center (o.c.). Additional framing should only be required if the rafters or studs are not spaced 16 inches o.c. or if the fan is too large to fit between the framing members. In this case, it may be advisable to install two smaller fans rather than alter a load-bearing member to make a larger fan fit.

Attach the fan's thermostat to a nearby joist

Attach the fan’s thermostat to a nearby joist in the direct path of the air flow.

Step 4: Install an attic fan

Prior to starting any work, turn off the electrical circuit you will be working on.

  • To install an attic fan that is roof-mounted, drill a pilot hole through the roof from inside the attic in the approximate location you plan to mount the fan. Then from on top of the roof, cut a round hole that matches the diameter of the attic fan. Spread a generous amount of tar or roof mastic around the opening, slide the flange of the fan housing underneath the upper row of shingles, and press it firmly into the mastic. If necessary, replace shingles around the flange. Note: if the roof is clay tile, metal or flat, consult a professional roofer; these types of roofs require more complex techniques to make an installation that won’t leak When the fan housing is in position, nail down the four corners of the flange, and dab tar or mastic over the nails to make them watertight.
  • To install an attic fan at the gable end, mount it between studs behind the gable-end vent. Install short wood studs (2 x 4 or 2 x 6) above and below the fan between the studs on which the fan is mounted to block any areas around the fan housing that air can pass through. If the gable-end vent is square or rectangular, use rigid insulation to cover the gaps around the fan housing.
Wire the grounds, the whites, and the black wires

Wire the grounds, the whites, and the black wires, fold the wires into the thermostat box, and replace the cover.

Step 5: Install the thermostatic fan regulator

Mount the thermostatic fan regulator to the stud or rafter nearest the fan and remove the cover. Then remove the knockout hole for the incoming power supply. Consult the manufacturer’s instructions before proceeding. Typically, you’ll connect the three white wires together. Make all the connections with wire nuts – not electrical tape. Then connect the black power supply wire to the black wire from the regulator, connect the black wire from the fan to the red wire from the regulator and connect the copper or green ground wires to each other or to a ground screw.

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Step 6: Test the fan

Install the cover on the thermostat and set it to 95 degrees Fahrenheit. Turn the circuit breaker on. If the fan doesn’t start, lower the thermostat setting until it does. If the fan still won’t start, turn off the power and recheck the wiring connections.

Warning: Before you install an attic fan, determine whether the home has gas appliances, such as a water heater or furnace, installed in the attic. If it does, the attic fan can cause a negative air situation that can draw exhaust fumes back into the attic. This can result in a fire or fill the attic with carbon dioxide. You may need to make additional intake openings elsewhere in the roof or at a gable end that doesn’t have a vent already.


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