How to Install a Bathroom Exhaust Fan

Measure the length and width of the bathroom and multiply to get square footage. ASHRAE 62.2 sets a flat baseline of 50 CFM of intermittent mechanical ventilation for any bathroom with a tub or shower, regardless of floor area. The Home Ventilating Institute (HVI) recommends sizing fans at about 1 CFM per square foot of floor area (with a 50 CFM minimum) as an industry rule of thumb, which generally lands you on a standard 50, 80, or 110 CFM fan — a 7×10 bathroom (70 sq ft) typically ends up in the 80 CFM tier. For bathrooms larger than about 100 sq ft, or if you have a jetted or whirlpool tub, follow the tub manufacturer’s published CFM recommendation and check local mechanical code, which can require 80–110 CFM or higher. While you're shopping, look for a sone rating of 1.0 or lower for bedrooms and master baths — anything louder is miserable to listen to twice a day.

Flip the breaker that feeds the bathroom fan switch at your main electrical panel and turn the fan and lights on in the bathroom to confirm they're dead. Climb the ladder with a non-contact voltage tester, pop the grille off the old fan, and hold the tester against every wire inside the housing — it should stay silent. Only then do you touch anything.

Unplug the motor from the integrated outlet in the housing (most modern fans plug right in) or unscrew the wire-nut connections if it's direct-wired, noting which color went where. Slide out the motor assembly, then disconnect the flexible duct from the exhaust port and pull the Romex out of the internal clamp. Finally, unscrew the housing from the ceiling joist it's fastened to and lower the whole box out of the hole.

Test-fit the new housing in the existing hole. If it drops in with a little slack, you're golden. If the hole is too small, trace the new housing footprint on the ceiling and enlarge the opening with a drywall saw, cutting just shy of the line and filing to fit. If the hole is too big, screw a scrap of drywall to the joist above to shrink the opening — you can always mud and paint later.

From above (in the attic if you have access) or by working through the ceiling hole, slip the flexible or rigid duct over the new fan's exhaust collar. Secure it with a hose clamp, then wrap the joint tightly with UL 181 foil HVAC tape — not cloth duct tape, which fails within a year. A zip tie alone is not a code-compliant seal; you can use a zip tie temporarily to hold the duct in place while you tape, but the finished joint must be foil-taped. If you're running new duct, use rigid 4-inch metal pipe and keep the run as short and straight as possible; every 90° elbow cuts airflow by 30%.

Feed the existing electrical cable through the internal strain-relief clamp on the new housing and tighten the clamp screw. Position the housing in the ceiling hole with the flange tight against the joist and drive 1-1/4 inch drywall screws through the mounting flange into the wood. The housing should feel rock-solid — any wobble will transmit vibration and make the fan sound loud regardless of the sone rating.

Inside the housing's wiring compartment, strip 3/4 inch of insulation off the house wires if needed. Connect black-to-black (hot), white-to-white (neutral), and ground-to-ground (green or bare copper) using yellow wire nuts, twisting clockwise until tight, then give each wire a firm tug to confirm it won't pull loose. Tuck the connections neatly into the compartment and replace the cover plate.

Plug the motor assembly into the integrated outlet inside the housing and screw it into place with the provided mounting screws. Squeeze the grille's spring-steel mounting wires together and push them into the matching slots in the housing until the grille seats flush against the ceiling. Run a thin bead of silicone caulk around the outside edge of the grille only if required by the manufacturer — most modern grilles seat cleanly without it.