How to Install a Bathroom Exhaust Fan

Why Bathroom Ventilation Matters

Without adequate ventilation, moisture from showers and baths accumulates on surfaces, promoting mold growth, peeling paint, warped wood trim, and deteriorating drywall. Building codes require all bathrooms to have either an operable window or a mechanical exhaust fan. Even bathrooms with windows benefit from an exhaust fan, as windows are rarely opened during cold weather when ventilation is most needed. A properly sized and installed exhaust fan removes moisture before it can cause damage.

Sizing the Fan

Exhaust fans are rated in CFM (cubic feet per minute). The minimum recommended CFM equals the bathroom square footage — a 50-square-foot bathroom needs at least a 50 CFM fan. For bathrooms with large tubs, steam showers, or above 100 square feet, increase the CFM rating accordingly. Also consider the sone rating (noise level) — 1.0 sones is quiet conversation level, while 3.0 or higher is noticeably loud. Look for ENERGY STAR-rated fans that are both efficient and quiet, typically producing 0.3 to 1.0 sones.

Installation Steps

For a new installation, choose a location between the shower and toilet (the primary moisture sources), ideally centered in the ceiling. Cut an opening in the ceiling drywall to match the fan housing. Position the housing between ceiling joists and secure it with the provided brackets. Connect the wiring (typically tied into the existing light switch circuit or wired to a separate switch). Run 4-inch rigid or semi-rigid duct from the fan housing to an exterior wall or roof vent — never exhaust into the attic, as this deposits moisture into the attic space.

Ductwork and Venting

The duct run should be as short and straight as possible — every elbow and extra foot of duct reduces the fan's effective CFM. Use rigid metal duct when possible; if flexible duct is used, pull it taut to minimize ridges that restrict airflow. The duct must terminate at a proper exterior vent cap with a damper that closes when the fan is off to prevent backdrafts, pests, and cold air from entering. Insulate the duct in unheated spaces (attics) to prevent condensation from forming inside the duct and dripping back through the fan. Seal all duct joints with foil tape (not regular duct tape, which deteriorates over time).