Bathroom Exhaust Fan: Selection, Installation, and Duct Routing

Sizing the Fan

Fan capacity is measured in CFM (cubic feet per minute). The minimum rule: 1 CFM per square foot of bathroom floor area. A 50 square foot bathroom needs at least a 50 CFM fan.

For bathrooms over 100 square feet, add 50 CFM for each toilet, shower, and bathtub. A large master bath with a separate shower and tub might need 150+ CFM.

Longer duct runs reduce effective airflow. A fan rated at 80 CFM at 0 inches of static pressure delivers significantly less through a 15-foot duct with two elbows. Size up if the duct run is long or has multiple bends.

Noise Level

Fan noise is measured in sones. Lower is quieter. For reference: 1 sone is the sound of a quiet refrigerator. 4 sones is a normal conversation level.

Cheap fans run 3-4 sones and sound like a jet engine in a small bathroom. Spend slightly more for a 1.0-1.5 sone fan — the difference in daily comfort is significant, especially for fans that run on a timer or humidity sensor.

The fan is not the only noise source. A poorly routed duct (too small, too many elbows, flexible instead of rigid) creates air turbulence that adds noise. A quiet fan on a bad duct is still loud.

Duct Routing

The duct must terminate at the exterior of the house — through the roof or a side wall. Never into the attic, soffit, or crawl space. Moisture exhausted into enclosed spaces causes structural damage.

Use rigid metal duct (4-inch round is standard for most residential fans). Rigid duct flows better and collects less lint and dust than flexible duct. Flexible duct is acceptable for short runs (under 6 feet) where rigid duct cannot fit.

Keep the duct run as short and straight as possible. Every 90-degree elbow adds the equivalent of 5-8 feet of straight duct in airflow resistance. Two elbows on a 10-foot run perform like a 20+ foot straight run.

Insulate the duct in unheated spaces (attics, crawl spaces). Warm moist air in an uninsulated duct condenses in cold weather, and the water drips back into the fan or collects in the duct. Duct insulation wrap or pre-insulated duct prevents this.

The exterior termination needs a dampered vent cap — one with a flap that opens when the fan runs and closes when it stops. This prevents cold air, insects, and animals from entering through the duct when the fan is off.

Installation: Replacing an Existing Fan

Turn off the circuit breaker. Remove the existing fan cover and disconnect the wiring. Most fans mount to a bracket attached to a ceiling joist. Unscrew the bracket and lower the fan housing.

If the new fan is the same size, mount it in the same bracket position. If bigger, you may need to enlarge the ceiling hole. Use a drywall saw to cut carefully — check for wires and pipes first.

Connect the duct to the new fan's outlet port with foil duct tape (not cloth duct tape, which degrades over time). Connect the wiring: black to black, white to white, green to ground. If the new fan has a separate exhaust-only and light circuit, you may need to run additional wiring.

Mount the housing, attach the cover, and test. The fan should draw a tissue against the grille when running. If it does not, the duct has a blockage or disconnection.

Installation: Adding a Fan Where None Exists

This is a bigger project. You need to: cut a ceiling hole, mount the fan housing, route a new duct to an exterior wall or roof, install an exterior vent cap, and run electrical wiring from a switch to the fan.

Locate the fan between two ceiling joists. Cut the hole and install the mounting bracket spanning between the joists. Position the fan near an exterior wall to minimize duct length.

Route the duct through the attic or ceiling cavity to the nearest exterior wall. Cut a hole through the wall or roof, install the vent cap with appropriate flashing, and connect the duct.

Run 14/2 NM-B wire from the fan to a new wall switch. If adding a fan/light combo, you may want a double switch (one for the fan, one for the light) which requires 14/3 wire. This electrical work requires a permit in most jurisdictions.

Controls and Timers

A basic on/off switch works but relies on the user to run the fan long enough. Most people turn it off too soon.

A timer switch (Lutron, Leviton) lets you set 10, 20, or 30 minute run times. Press the button and the fan shuts off automatically. This runs the fan long enough to clear moisture after a shower without wasting energy.

A humidity-sensing fan or switch turns on automatically when humidity rises and off when it drops back to normal. Best option for households that forget to run the fan.

Some modern fans include motion sensors, Bluetooth speakers, and night lights. These features add cost but the humidity sensor is the one genuinely useful upgrade over a basic switch.

Tools for Exhaust Fan Installation

Drywall saw for the ceiling hole. Drill/driver for mounting screws. Wire strippers and wire nuts for electrical connections. Foil duct tape and 4-inch rigid duct. Duct insulation wrap for attic runs. Exterior vent cap with damper.

For new installations: fish tape or cable for routing wire to the switch. Circuit tester to verify the breaker is off. Reciprocating saw or hole saw if cutting through an exterior wall for the vent cap.

Frequently Asked Questions