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How to Install a Bathroom Exhaust Fan

Home/Resources/Bathroom/Installing a Bathroom Exhaust Fan
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Written & researched by Priya Anand DIY & Tools Editor
Updated August 2026
Golden-hour view of a person's hands fitting a new shower head in a tiled bathroom

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Installing a bathroom exhaust fan is an intermediate DIY project: a straight swap into an existing opening and duct takes 1 to 2 hours, while a brand-new fan with fresh ducting and a roof or wall cap is a full-day job. Budget $40 to $150 for the fan and another $30 to $60 for duct and a vent cap. The tricky parts are venting it to the outdoors correctly and the electrical connection. Reusing an existing switched circuit is DIY-friendly; running new wiring or a new circuit is where local code, a permit, and often a licensed electrician come in.

How to Install a Bathroom Fan — Ask This Old House

Step-by-step: installing an exhaust fan

This sequence assumes attic access above the bathroom. Cut power at the breaker before touching any wiring.

  1. Size the fan and plan the vent path. Use 1 CFM per square foot, 50 CFM minimum. Decide whether you are going out through the roof or a sidewall, and pick the shortest, straightest route. Never plan to end the duct in the attic.
  2. Kill the power and remove the old fan or mark the opening. Switch off the breaker and confirm with a non-contact tester. For a replacement, disconnect the wiring and duct and drop the old housing. For a new install, mark the housing outline on the ceiling between joists and cut the drywall with a drywall saw.
  3. Mount the housing to a joist. Position the housing so its edge is flush with the finished ceiling and screw its bracket to the adjacent joist. It must sit tight to the drywall so the grille seals with no gap.
  4. Run the duct to the exterior. Connect smooth-wall rigid or insulated semi-rigid duct at the size the fan specifies (usually 4 inches) from the housing collar to a dedicated roof or wall cap fitted with a backdraft damper. Keep it short, minimize elbows, and slope horizontal runs slightly downhill to the outside.
  5. Seal and insulate the duct. Band and foil-tape every joint, and wrap the duct with insulation where it passes through unconditioned attic space to stop condensation from forming inside it and dripping back.
  6. Make the electrical connections. Match the fan leads to the supply cable, white to white, black to black (or to the switched leg), ground to ground, secured with wire nuts, and clamp the cable at the housing. If you are adding a new circuit or new cable, this is the point where code and permitting apply.
  7. Restore power and test. Turn the breaker back on and run the fan. Check that air moves out of the exterior cap: hold a tissue to the outside vent and confirm the flapper opens.
  8. Fit the grille. Squeeze the mounting springs into the housing slots and push the grille up flush to the ceiling.
Wide environmental view of a person's hands fitting a new shower head in a tiled bathroom, in soft overcast light

Tools and materials

You need a drywall saw, a drill/driver, a non-contact voltage tester, wire nuts, foil duct tape and metal banding, rigid or insulated duct at the specified diameter, an exterior vent cap with a damper, and duct insulation for attic runs. A fan with a humidity sensor or a companion timer switch pays for itself in mold prevention. If poor ventilation has already left you fighting moisture and hard-water film in the shower, our exhaust fan roundup ranks the quietest high-CFM units owners keep, and a shower filter helps with the scale that humidity bakes onto glass and tile.

Close-up detail of the material being worked in a bathroom project, in warm evening lamplight

Venting, electrical and code

Two rules protect your house. First, the duct must terminate outdoors, at a roof or wall cap, never in the attic or a soffit; venting into the attic drives mold and rot and fails inspection. Second, treat the wiring seriously: cut power at the breaker, verify with a tester, and stop if you would be running a new circuit rather than reusing an existing switched one. Electrical near a wet room and new ductwork through the building envelope are governed by local mechanical and electrical codes, which vary by jurisdiction and frequently require a permit. When new wiring or a new penetration is involved, bring in a licensed electrician or contractor.

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More from this hub: Bathroom · Resealing a shower · Regrouting tile · Installing a vanity light · Best exhaust fans · How we rank

Straight Answers

Frequently Asked

What size exhaust fan do I need for my bathroom?

The rule of thumb is 1 CFM per square foot of floor for bathrooms up to 100 square feet, with a 50 CFM minimum. So a 5-by-8 bathroom (40 square feet) wants at least a 50 CFM fan, and a 9-by-9 (81 square feet) wants roughly 80 CFM. For rooms over 100 square feet, the Home Ventilating Institute suggests adding 50 CFM per toilet, tub or shower and 100 CFM for a jetted tub. A separate enclosed toilet or a steam shower should get its own fan. Undersizing is the most common reason a fan fails to clear fog and control mold.

Can you vent a bathroom fan into the attic?

No. A bathroom fan must exhaust to the outdoors through the roof or a sidewall, never into the attic, soffit cavity or crawlspace. Dumping warm, moist air into an attic is a leading cause of mold, rotted sheathing and ruined insulation, and it violates building code in most jurisdictions. Run insulated rigid or semi-rigid duct to a dedicated roof or wall cap with a backdraft damper, keep the run as short and straight as possible, and slope any horizontal section slightly toward the exterior so condensation drains out rather than back into the housing.

Do you need a permit to install a bathroom exhaust fan?

For a like-for-like replacement using the existing duct and wiring, usually not. Adding a brand-new fan that requires new electrical circuits or cutting a new duct and roof or wall penetration often does require an electrical or mechanical permit, and the work must meet local code. Because rules vary by city and state, check with your building department before you start. The wiring side in particular is where a permit and, in many places, a licensed electrician are required if you are running new cable rather than reusing an existing switched circuit.

Why is my new bathroom fan still not clearing steam?

Usually the ductwork, not the fan. A correctly sized fan will still underperform if the duct is too long, kinked, crushed, or made of ribbed flexible tubing that adds friction, or if the exterior damper is stuck shut with lint. Measure the run and count the elbows: each 90-degree bend costs the equivalent of several feet of straight duct. Use smooth-wall rigid duct at the size the fan calls for (typically 4 inches, larger fans 6), keep it short, and confirm the outside cap flapper opens freely when the fan runs.

How long should you run a bathroom fan after a shower?

Leave it running about 20 minutes after you finish. The fan needs time to pull the lingering humidity out of the room and the towels, not just the visible fog off the mirror. A simple way to guarantee it is to wire the fan to a countdown timer switch or a humidity-sensing switch that shuts off on its own, so it is not left running all day or, worse, switched off the moment the shower stops while the room is still saturated. Twenty minutes of runtime is the single cheapest defense against bathroom mold.