Installing a Bathroom Exhaust Fan Step by Step
Before You Start
Turn off the power to the bathroom circuit at the breaker, not just the wall switch, and confirm it's dead with a non-contact voltage tester. If you're replacing existing wiring, adding a new switch, or running a new circuit, hire a licensed electrician — bathroom circuits have specific code requirements (GFCI protection, damp-rated fixtures) that are worth getting right the first time.
Step 1: Size the Fan
Measure the bathroom's square footage and match it to CFM using a sizing table (roughly 1 CFM per square foot up to 100 sq ft, or add fixture CFM for larger rooms). Buying a properly sized fan is the single biggest factor in whether the installation actually solves the room's humidity problems.
Step 2: Plan the Duct Route
Decide how exhaust air will exit the building — through the roof or an exterior wall — before cutting anything. The duct run should be as short and straight as possible and must terminate outside, never into the attic.
Step 3: Cut the Ceiling Opening
With power off, mark and cut an opening sized to the fan housing, working between ceiling joists. Check for wiring, plumbing, or framing in the way before cutting.
Step 4: Mount the Housing
Secure the fan housing to the joists per the manufacturer's instructions, keeping it level so any condensation drains toward the duct outlet rather than back into the motor.
Step 5: Run and Connect the Duct
Connect insulated flexible or rigid duct from the housing outlet to the roof or wall termination point, sealing joints with foil tape or mastic and sloping the run slightly toward the exterior.
Step 6: Wire the Fan
Connect the fan according to its wiring diagram and local code, using wire nuts rated for the connection and a properly grounded circuit. This is the step where hiring a licensed electrician pays off if you have any doubt about the existing wiring, switch leg, or grounding.
Step 7: Install the Grille and Test
Attach the grille cover, restore power at the breaker, and test the fan. Confirm it runs smoothly, check that the exterior damper opens while it's running, and feel for airflow at the roof or wall cap to confirm the duct connection is solid.
Skipping the electrical safety steps or terminating the duct in the attic are the two mistakes that turn a straightforward fan installation into a much bigger repair down the road.