How to Install an Attic Exhaust Fan

Multiply the attic floor's width and length to get the attic floor's area. Multiply the attic floor's area by .7 to figure how many cubic feet per minute, or cfm, of air the fan must push. Buy a fan that exceeds the total by 15 to 20 percent.

Multiply the fan's cfm rating by 144 and then divide that number by 300 to find the attic's intake-air louver size. Most homes have soffit and ridge vents that exceed this amount. If not, then installing an intake-air vent in the opposite gable, which is the exterior wall portion found in the attic just below the roof, will increase the fan's efficiency.

Measure the inside of the fan's mounting flange with a tape measure. The mounting flange will encircle the fan's sleeve, the metal housing that the fan resides in, and will connect to the gable's framing. Some fans do not use a sleeve; instead, they use cross-member bars that screw to the gable's framing. The air-flow direction arrow, stamped on the fan motor or sleeve, will point towards the mounting flange.

Set a ladder against the house in order to reach the gable.

Draw a layout of the fan's opening, using the fan's sleeve or cross-member measurements, on the exterior of the gable with a tape measure and pencil. Installers often use the cardboard box the fan comes in as a template. Transfer the fan's opening measurements to the box and cut the box with scissors. Place the cardboard cutout against the gable and trace the cutout with a pencil.

Cut the fan opening's layout lines on the gable with a reciprocating saw. Remove the cutout section.

Measure the distance between the gable's wall studs, found inside the attic, with a tape measure. Cut a 2-inch by 4-inch wood board these same measurements with the reciprocating saw. Toe nail, which means to nail at an angle, the cut wood boards to the gable's wall studs. When complete, the opening will have framing above and below the opening.

Measure the distance between the top and bottom framing at the sides of the fan's opening. Cut a 2-inch by 4inch board to these lengths. Toe nail the boards to the top and bottom framing. When complete, the entire fan opening will have a wood board frame.

Nail the gable to the wood frame from the outside with 1-inch nails. Drive at least three nails on each side.

Mount the fan using the manufacturer's instructions. Most models screw or nail through pre-drilled installation holes into the inside of the framing from inside of the attic. Some models slide into the frame from the outside and attach to the frame from outside the house through holes inside of the fan's sleeve. Use a drill and 1-inch screws or 1-inch nails to secure the fan.

Screw the fan's louver to the gable from outside of the house. The louver will have pre-drilled installation holes. Use the supplied screws or the same 1-inch nails or screws used to hold the fan in place.

Mount the fan's controller, basically a thermostat, to a gable stud. Follow the manufacturer's instructions. Usually the controller will connect to a wall stud within 1 foot of the bottom of the fan with screws through pre-drilled holes, located on each side of the controller.

Open the controller's access cover with a flat-head screwdriver or nut driver, depending on what the manufacturer uses.

Run a length of 14-2 wire from the fan's control box to an attic light. A 14-2 wire has two strands of insulated 14-gauge wire and a bare copper ground wire wrapped in a plastic coating. Use an attic light with a pull-chain switch. A light with a pull-chain switch will have constant electricity supplied to it.

Remove 6 inches from the plastic coating of each end of the 14-2 wire with a razor knife.

Strip half an inch from each insulated strand on both ends of the 14-2 wire.

Push the 14-2 wire into the fan's control box through the wire-access hole, usually located on the side of the control box.

Connect the 14-2 wires to the control box's wires with wire nuts. Connect the black wires together, the white wires together and the green wire to the copper wire. Close the control box.

Turn the pull-chain light on.

Turn the electricity to the pull-chain light off at the circuit breaker box. When the correct circuit breaker has turned off, the pull-chain light will no longer glow.

Open the pull chain-light's cover. Usually two flat-head screws hold it in place.

Feed the 14-2 wire into the pull-chain light's electrical box. Use the same hole that the wire that feeds the pull-chain light uses.

Connect the 14-2 wire to the pull-chain light's wires with wire nuts. Remove the existing wire nuts, one at a time. Connect the black wires together, the white wires together and the bare copper wires together.

Replace the pull-chain light's cover. Turn the circuit breaker on. The light should glow and the fan will turn on if the attic has a high enough temperature.