A bathroom without a working exhaust fan accumulates moisture with every shower and bath. That moisture leads to peeling paint, mold growth, and rotting drywall — damage that is expensive to fix and hazardous to your health. Installing or replacing a bathroom exhaust fan is a moderate DIY project that protects your home from moisture damage.

How to Install a Bathroom Exhaust Fan

Choosing the Right Fan

Exhaust fans are rated in CFM (cubic feet per minute). The formula is simple: multiply the bathroom’s square footage by 1.1. A standard 8-by-10-foot bathroom needs an 88 CFM fan, so choose a 90 or 100 CFM model.

For bathrooms over 100 square feet or with a separate toilet enclosure, you may need a larger fan or multiple fans.

Also consider:

Sone rating: Measures noise. Lower is quieter. Fans rated 1.0 sone or less are barely audible. Budget fans at 3.0 to 4.0 sones are noticeably loud.
Features: Humidity sensors (auto-on when moisture is detected), motion sensors, integrated LED lights, and timers are available. A humidity sensor is the single most useful feature because it ensures the fan runs long enough to clear moisture even if you forget to leave it on.
Duct size: Match the fan’s duct outlet to your existing ductwork — typically 3 or 4 inches.

Replacement vs New Installation

Replacing an existing fan in the same location is straightforward — the hole, ductwork, and wiring are already in place. Installing a fan where none existed before is more complex because you must cut a ceiling hole, run new ductwork to the exterior, and run a new electrical circuit.

This guide covers both, but if you are running new electrical wiring, hire a licensed electrician unless you have experience and your local code allows homeowner electrical work.

Replacing an Existing Fan

Step 1: Turn off power at the breaker panel. Verify with a multimeter or voltage tester that the circuit is dead.

Step 2: Remove the old fan. Pull off the grille (it usually unclips). Disconnect the electrical wire connections inside the housing. Remove the screws or brackets holding the fan housing to the ceiling joist. Lower the housing.

Step 3: Inspect the ductwork. Look up into the opening. The flexible duct should run to an exterior wall or roof vent. If it terminates in the attic without venting outside, you must extend it — exhausting moist air into the attic causes mold and rot.

Step 4: Install the new housing. Most fans mount to a ceiling joist with screws or to an expanding bracket that spans between joists. Slide the housing into the ceiling opening and secure it. The fan’s intake opening should sit flush with the finished ceiling surface.

Step 5: Connect the duct. Attach the existing flex duct to the new fan’s outlet port with a metal clamp or foil duct tape (not standard cloth duct tape, which degrades in humid conditions).

Step 6: Wire the fan. Connect black to black, white to white, green or bare to green or bare. If the fan has a separate light circuit, it will have an additional wire — typically blue for the light. Wire it to the switch leg for the light switch.

Step 7: Install the grille, restore power, and test.

New Installation

A new installation requires cutting a ceiling hole, mounting the fan housing, running new ductwork to the exterior, and running electrical wire from a switch to the fan.

Locate and Cut the Ceiling Hole

Choose a spot between two ceiling joists, centered over the shower or tub if possible. Use a stud finder to locate the joists. Trace the fan housing template on the ceiling and cut with a jigsaw or drywall saw.

Run Ductwork

Use rigid or semi-rigid metal duct for as much of the run as possible — smooth walls move air more efficiently than corrugated flex duct. Route the duct to the nearest exterior wall or up through the roof. Install a wall cap or roof vent with a backdraft damper to keep outside air, rain, and pests out.

Keep duct runs short and straight. Every bend and every extra foot of length reduces airflow. Insulate the duct in unconditioned spaces (attic, crawlspace) to prevent condensation on the outside of the duct.

Run Electrical

Run 14/2 NM-B (Romex) cable from the switch location to the fan, and from the switch to the existing bathroom circuit. Install a single-pole switch or a combination switch (one toggle for the fan, one for the fan’s light) in a standard electrical box.

Important: If you are not experienced with electrical work, hire a licensed electrician for this step. Bathroom circuits have specific code requirements including GFCI protection, and incorrect wiring creates fire and shock hazards. For any work involving your home’s electrical panel, always hire a licensed professional.

Finish

Mount the fan housing, connect the duct and wiring, install the grille, and test. Verify airflow by holding a tissue near the grille with the fan running — the tissue should pull firmly against the grille.

Venting Best Practices

Always vent to the exterior. Never into the attic, soffit, or crawlspace.
Use a backdraft damper at the termination point.
Slope ductwork slightly downward toward the exterior vent so any condensation drains out rather than back into the fan.
Seal all duct joints with foil tape or mastic to prevent air leaks.

Maintenance

Clean the fan grille every few months — dust buildup reduces airflow. Vacuum the fan blades and motor annually. Test the fan’s airflow with the tissue test — if suction has weakened, the duct may be blocked or the motor may be failing.

A functioning exhaust fan is one of the most important systems in your bathroom. It is not glamorous, but it prevents thousands of dollars in moisture damage.