How to size and vent a bathroom exhaust fan

Sizing the fan

For a normal bathroom under about 100 square feet, use one CFM for every square foot of floor, with 50 CFM as the minimum. A 60-square-foot bathroom gets a 60 to 80 CFM fan. Going a little bigger costs almost nothing and gives you room to spare once the duct takes its share of the airflow.

For a bathroom over 100 square feet, or one with several wet fixtures, size by fixture instead. Add a general allowance for the room plus an allowance each for the shower, the tub and the toilet. A large ensuite often works better with two fans, or one fan with two grilles, than with one big fan at one end.

The duct matters as much as the fan

Fans are rated on a test bench. In your ceiling, every foot of duct, every elbow and every ridge in flexible pipe cuts the airflow. A big fan on a bad duct run can move less air than a small fan on a good one.

• Vent it outside. Through a roof cap or a wall cap, never into the attic, a soffit or a joist space. Venting into the attic is the most common serious mistake, and it puts warm, wet air straight into cold insulation.
• Insulate the duct where it runs through unheated space, or moisture condenses inside it and drips back into the fan.
• Keep it short and straight. Every elbow costs as much airflow as several feet of straight pipe.
• Use smooth-walled duct where you can. If you use flexible duct, pull it tight so it doesn't sag.
• Slope it slightly toward the outside so any condensation drains away from the fan.
• Check the damper at the outside cap opens and closes. A stuck damper shuts the whole system down without anyone noticing.

Letting air in

A fan can only pull out as much air as the room can replace. In a well-sealed bathroom with a tight door, a strong fan works against a partial vacuum and moves far less air than its rating. That's why a new fan sometimes disappoints in a newly renovated bathroom.

• Leave a gap under the door of about three quarters of an inch above the finished floor. A thick new tile floor can eat up that gap.
• Try the tissue test. Hold a tissue up to the running fan. If it doesn't stick, the room isn't getting enough air in.
• Be extra careful in tight new houses. The better sealed the house, the more deliberately you have to plan for incoming air.
• Add a transfer grille if a door gap isn't enough or you don't want one.

Older houses are where venting gets hard. A top-floor bathroom has a short run to the roof. A bathroom on the middle floor of a two-storey house, or in a condo, may have no easy path at all. Then the options are a wall cap through the nearest outside wall, an inline fan mounted away from the room in the joist space, or, in a condo, a shared system you're not allowed to change.

Electrical rules in a wet room

Decide on the controls before the drywall goes up. A timer switch that keeps the fan running for twenty minutes after the light goes off fixes the most common problem, which is people shutting the fan off with the light. A humidity-sensing switch does the same thing automatically. Both need the wiring planned at rough-in.

Questions homeowners ask