Attic Exhaust Fan — Thermostat, Sizing & Replacement

An attic exhaust fan is a thermostat-controlled or humidistat-controlled powered fan — typically 12–20 inches in diameter — mounted in the gable end or roof deck of a home to expel heat and moisture from the attic space.

What It Is

An attic exhaust fan is the specific residential unit most homeowners install when they add powered attic ventilation. While "attic blower" is the broad category term for any power attic ventilator, an attic exhaust fan refers to the self-contained unit with a built-in thermostat (and often a humidistat), a direct-drive or belt-drive motor, and a fan blade diameter in the 12–20 inch range — sized to move 800–1,600 CFM of air through a single-family attic.

The built-in thermostat is the key feature that distinguishes an attic exhaust fan from a simple vent fan. The thermostat monitors attic temperature and switches the motor on when conditions reach the set point — typically 90–100°F — and off when the temperature drops below it. Many models include a second control: a humidistat that activates the fan in winter when attic humidity exceeds 60–70%, preventing moisture buildup, condensation on roof sheathing, and mold growth.

Makeup air enters through soffit vents at the eaves. The fan draws that cooler outdoor air upward through the attic cavity and exhausts it through the fan housing. This convective replacement cycle is far more effective than passive ventilation alone during peak summer heat.

Types

Gable-mounted attic exhaust fans are mounted in the triangular gable vent opening at either end of the attic. The fan housing replaces or mounts inside an existing gable vent. These are the easiest to install and service because no roof work is required — all access is from inside the attic or through the gable from outside. They are the most common type for DIY installation.

Roof-mounted attic exhaust fans sit on the roof deck and exhaust through a louvered cap. They require cutting the roof deck and installing flashing — more involved than gable mounting, but more effective for hip roofs or homes without accessible gable vents.

Solar-powered attic exhaust fans integrate a photovoltaic panel directly on the unit, eliminating the need for electrical wiring. They operate only during daylight hours, which coincides with peak attic heat, but they cannot run at night or on heavily overcast days.

Where It Is Used

Attic exhaust fans are most effective in single-family homes in hot climates where attic temperatures regularly exceed 120°F in summer. They are also used in cold climates year-round when a humidistat is included, to exhaust winter moisture that would otherwise condense on cold roof sheathing.

How to Identify One

A gable-mounted attic exhaust fan is visible as a fan housing set inside the gable louver, with a motor and blade visible from inside the attic. A roof-mounted unit appears as a low-profile domed or louvered cap on the roof surface. Both will have a thermostat box — a small rectangular control unit with a dial — mounted nearby on a rafter or the unit housing itself.

Replacement

Attic exhaust fans typically last 10–15 years. The thermostat and capacitor often fail before the motor. Before replacing the whole unit, test whether the motor runs when the thermostat is bypassed — if it does, replace only the thermostat ($15–$30). Full unit replacement costs $80–$200 for the unit plus labor. Hardwired units require an electrician; solar units are typically DIY-installable.

Attic Exhaust Fan — FAQ

What is the difference between an attic exhaust fan and an attic blower?

Attic blower is the general category term for any powered attic ventilation fan. Attic exhaust fan refers to the specific self-contained residential unit — typically 12–20 inches in diameter with a built-in thermostat — installed in the gable end or roof. In practice, most homeowners use both terms interchangeably, but technically an attic exhaust fan is the specific product while an attic blower describes the broader function.

What temperature should I set my attic exhaust fan thermostat to?

Most manufacturers recommend 90–110°F. A setting of 100°F is a practical starting point for most climates. Setting it too low causes the fan to run during mild weather when the attic isn't significantly hotter than outdoors, wasting electricity. Setting it above 110°F allows excessive heat buildup before the fan activates. Adjust down by 5–10° if your attic consistently runs very hot.

Does an attic exhaust fan need a humidistat?

A humidistat is valuable in any climate with cold winters. Without it, the fan only runs in summer. In winter, moisture from daily household activities migrates into the attic and can condense on cold roof sheathing, promoting mold and rot. A humidistat activates the fan when relative humidity exceeds the set point (typically 60–70%), exhausting that moisture before it accumulates.

My attic exhaust fan stopped running — is it the thermostat or the motor?

Test the thermostat first: locate the thermostat control box and temporarily bypass it by disconnecting the thermostat wires and connecting them directly together. If the fan runs, the thermostat is faulty and can be replaced for $15–$30. If the fan still won't run with the thermostat bypassed, the motor or capacitor has failed and the unit needs replacement.

How big does my attic exhaust fan need to be?

A common sizing rule is 0.7 CFM per square foot of attic floor area. A 1,500 sq ft attic requires approximately 1,050 CFM. Most 14–16" residential exhaust fans move 1,000–1,500 CFM, making them adequate for typical single-family attics. In very hot climates or dark-roofed homes, size up. Ensure adequate soffit intake area: roughly 1 sq ft of free vent area per 150 CFM of fan capacity.

Do I need a permit to install an attic exhaust fan?

Electrical permits are typically required for hardwired attic exhaust fans because you are adding a new circuit or extending an existing one. The permit requirement varies by jurisdiction. Solar-powered units that require no wiring often fall below the permit threshold but check with your local building department. Roof-mounted units may also require a roofing inspection.