How to Install an Attic Fan

Attic fans do the same thing that properly ventilated attics do on their own; fans just add power to the natural system of ventilation to increase the rate of air exchange. The primary benefit of a fan is that is can help lower the temperature in the attic during hot weather. Secondarily, attic fans also can reduce moisture in an attic and can lower temperatures in the winter to help prevent ice dams.

It's fair to say that manufacturers of attic fans often exaggerate their benefits, so you should take this into consideration. It's also important to note that an attic fan does its job effectively only when the ceiling below the attic is well sealed. Air leaks in the ceiling between the living space and the attic allow conditioned (heated or cooled) indoor air to escape in the attic when the fan is running, resulting in higher heating or cooling bills.

Just as important is the need for adequate intake ventilation in the attic itself. Attic fans exhaust hot air near the peak of the roof, while they pull in outdoor air through vents in the eaves of the roof. The number of vents—or more specifically, their total "free ventilation area"—must meet the minimum requirement of the fan for maximum efficiency.

Standard (non-solar) attic fans typically connect to a 120-volt household circuit, which can be rated for either 15 or 20 amps. The fan can tap into an existing circuit in the attic as long as there is enough capacity on the circuit so it won't be overloaded by adding the fan. If there is no suitable existing circuit for the fan, you must have a new circuit installed from the home's service panel (breaker box). A new circuit can also be used to power new lights or other electrical devices in the attic.

Most fans have an internal thermostat that turns the fan on and off automatically based on the attic temperature. Because of this, you can connect the fan directly to an electrical circuit without a switch, although it is better to install a standard single-pole (on/off) switch so you can turn the fan off manually, if desired. Fan circuits can also include an optional humidistat (which works like a thermostat for humidity levels instead of temperature); in this case, the circuit includes a separate humidistat unit and an additional switch that can override the humidistat.

A roof-mounted fan is installed over a hole in the roof decking and has a large, flat base of flashing that slips under the roof shingles. The shingles and flashing are then sealed for a weather-tight installation:

1. Start in the attic, and select a fan location that is close to the roof peak and the middle of the attic space. If the fan is on the rear side of the roof, it won't be visible from the street.
2. Make a mark on the underside of the roof sheathing, centered between two roof rafters (or trusses), to mark the center of the fan.
3. Drill a hole at the mark, up through the sheathing and all the way through the roof shingles.
4. Move to the top of the roof and draw a cutting line for the hole, using the template provided with the fan, centering the template on the drilled hole.
5. Cut away the shingles along the outline with a utility knife.
6. Mark an inner hole on the roof decking, using the template, then cut out the decking with a jigsaw.
7. Remove all nails from the shingles in the square area around the hole where the fan's flashing will slide in.
8. Slide the fan flashing under the shingles on the upper portion of the hole, and secure it with roofing nails. The lower half (or so) of the flashing rests on top of the lower shingles. Seal the shingles and flashing with roofing sealant.
9. Install an electrical box in a convenient location in the attic or in the living space below. Run circuit cable to the box and install an approved switch for manual fan control.
10. Run more circuit cable to the fan and make the final wiring connections, following the manufacturer's diagram.
11. Set the temperature setting on the fan's thermostat, and test the fan and switch for proper operation.

Solar attic fans are installed just like standard roof-mounted attic fans (see above) but without all of the wiring steps. Also, when determining a location for a solar fan, choose a spot that receives full sun during the afternoon in the hottest time of the year (for example, at 4 p.m. in July). This is when the fan will be operating most frequently. Be sure to cover the solar panel on the fan with cardboard during the installation to prevent the fan from running before it is completely installed.

Gable fans are designed to mount directly behind louvered attic vents in gable walls. If you don't have an existing gable vent that is suitable, you can install a new gable vent, but it might be better to install a roof-mounted fan instead. Roof fans are more effective than gable fans.

Gable fans work best when mounted to a piece of plywood with a hole that matches the fan housing; the remainder of the plywood covers any exposed portions of the gable vent for maximum efficiency (some fans have an integrated plate or panel that does the same thing as plywood):

1. Cut and install a 2 x 4 cross brace at the top and bottom of the vent opening to create a square or rectangular frame around the opening. Most vents are installed between two vertical wall studs, and the cross braces can simply be screwed into the studs.
2. Cut a piece of plywood to match the outer dimensions of the frame around the opening. Place the fan housing facedown over the center of the plywood and trace around the housing. Cut out along the outline.
3. Mount the fan to the plywood, using the provided mounting brackets and screws.
4. Mount the plywood (with fan attached) to the frame around the gable vent, using wood screws. The fan should face out toward the vent.
5. Install an electrical box in a convenient location in the attic or in the living space below. Run circuit cable to the box and install an approved switch for manual fan control.
6. Run more circuit cable to the fan and make the final wiring connections, following the manufacturer's diagram.
7. Set the temperature setting on the fan's thermostat, and test the fan and switch for proper operation.