How to Install a Bathroom Exhaust Fan

Bathroom humidity is the enemy of walls, paint, and grout. Left unchecked, it settles into drywall, feeds mold, and rots wood framing from the inside. An exhaust fan pulls that moisture straight outside before it becomes a problem—but only if it's installed correctly. The difference between a fan that works and one that doesn't hinges on three things: a clear path to exterior air, proper ductwork (rigid, not kinked plastic), and a switch wired to run during and after showers. This is a project that sits at the intersection of rough carpentry, electrical work, and HVAC thinking. You'll be comfortable doing it if you've run wiring before or patched drywall. The payoff is immediate and measurable: windows stop fogging, mold doesn't take hold, and your bathroom stays dry.

1. Find Your Vent Route First. Stand in the attic or crawlspace above the bathroom and select a spot on the ceiling directly above the toilet or sink. Avoid running duct directly over insulation—air needs to move around it. Drill a pilot hole up through the ceiling to mark your cutout location on the bathroom side. From the attic, check for joists, pipes, or wiring in your planned ductwork path to the exterior wall or roof. If obstacles exist, shift your location or plan to route around them.
2. Secure the Fan Housing. Use a drywall saw or reciprocating saw to cut a hole matching your fan housing diameter (typically 6 or 8 inches). Remove drywall, insulation, and any blocking. Position the fan housing in the opening from below, center it, and secure it to the ceiling joists with the mounting brackets that came with the fan. Ensure the flange sits flat against the drywall on all sides—gaps will leak conditioned air. The housing should not sag or shift when you release it.
3. Run Rigid Duct Straight Out. Measure the run from your fan housing to your chosen exterior exit point (roof vent, gable vent, or wall cap). Use rigid ductwork—smooth aluminum or galvanized steel—not flexible plastic accordion duct, which catches lint and collapses. Secure ducts with duct tape at each joint and use metal clamps every 3 feet. The duct should slope slightly downward toward the exterior exit to prevent condensation backup. If the run is longer than 20 feet, size up to an 8-inch duct to reduce resistance.
4. Seal the Outside Exit. Cut an opening through your exterior wall or roof at the end of the ductwork run. Install a roof vent cap (with flashing and weatherproofing) or a wall-cap vent, depending on your location. Secure the cap with the provided fasteners and seal all gaps around it with roofing cement (for roof) or caulk (for walls). The cap should have a backdraft damper that closes when the fan shuts off, preventing cold air or pests from entering. Test the damper by hand to ensure it moves freely.
5. Wire to a Timer Switch. Turn off power to the bathroom circuit at the breaker. Run a new 14-gauge electrical wire from the breaker (if you have capacity) or from an existing bathroom outlet to your fan housing, using a junction box or integrated wiring terminal. Use a timer switch rated for your fan's amperage (usually 0.5–1.5 amps). Wire the hot to the timer, the neutral to neutral, and ground to ground. Wrap all connections with electrical tape. Set the timer to run 15–30 minutes after a shower to fully expel humidity. Do not daisy-chain the fan off another circuit without a dedicated breaker.
6. Caulk All Leaks. From the attic, pack fiberglass insulation around the fan housing but not directly on top of it; leave a 1-inch air gap for heat dissipation. Seal the gaps between the housing flange and the ceiling drywall with paintable silicone caulk to prevent air leakage. Caulk any gaps where ductwork passes through attic framing or insulation. This prevents warm, moist air from leaking into the attic instead of out the vent.
7. Patch and Power Up. Apply joint compound around the drywall opening, sand smooth, prime, and paint to match. Cut a soffit or return-air grille to fit the opening if your fan housing doesn't have an integrated trim ring. Flip the breaker back on and test the fan at full speed. You should feel strong suction at the bathroom grille and hear steady air movement in the ductwork. Check the exterior damper opens fully and closes when the fan stops.
8. Test Moisture Control. Once patching compound is cured, take a hot shower with the bathroom door closed, turn the fan on, and set the timer. Feel for air movement at the grille and listen for steady operation. After the shower, check that walls and mirror are clear of fog within a few minutes. The goal is a dry bathroom within 15–20 minutes of stopping the shower. If you see condensation forming on walls or windows over the next few days, the fan may be undersized or the ductwork blocked.