Renovation guide · Bathrooms
Bathroom exhaust fans: size, ducting and controls
A fan’s rating is measured in a lab. What it moves in your bathroom depends on the duct behind it, and that’s the part nobody sees.
6 min readUpdated By SS Carpentry and Renovations
Key takeaways
- Ontario’s Building Code asks for a bathroom fan rated at least 25 L/s (about 53 cfm), on a switch in the room.
- Size it by floor area: about 1 cfm per square foot, never under 50, with more for a large bathroom.
- The duct decides what the fan really moves: at the pressure fans are rated at, a 4″ duct is allowed only 2 m, a 6″ duct 16 m.
- It must end outside, never in the attic, and run about 20 minutes after each shower.
Why does a bathroom need an exhaust fan?
A shower fills a small room with water vapour, and moisture is what mould needs. Health Canada’s advice is to keep indoor humidity between 30% and 50%. In the room that makes the most moisture, the exhaust fan is what keeps it there.
It protects the waterproofing too. Whatever is behind the tile, the membrane maker in our guide to what goes behind shower tile still calls for ventilation to manage water vapour and condensation.
What does Ontario’s Building Code ask for?
| Article | What it requires |
|---|---|
| 9.32.3.7 | An exhaust fan rated at least 25 L/s in every bathroom (unless the house’s main ventilation system draws air from it), on a manual switch in the room. A humidity control can be added, but the switch must still turn the fan on. |
| 9.32.3.10 | Fan capacity is rated at 25 Pa of pressure. A fan within 1 m of duct of the room is limited to 3.5 sones on HVI’s scale (2.5 on CSA’s). |
| 9.32.3.11 | The duct never discharges into the attic or any enclosed space, is insulated to RSI 0.5 through unheated space, has sealed joints and doesn’t sag or get crushed. |
| 9.32.3.13 | The outlet has a backdraft damper and sits at least 1.8 m from air intakes and vented soffits, and at least 100 mm above the ground. |
- A: Fan in the ceiling, over the shower or tub, rated at least 25 L/s (about 53 cfm)
- B: Duct insulated to at least RSI 0.5 through the cold attic, joints sealed
- C: Out through the roof or a wall, with a backdraft damper: never into the attic
- D: Outlet at least 1.8 m from vented soffits and air intakes
- E: A gap under the door (about ¾″) lets replacement air in
- F: A manual switch in the room, which turns the fan on whatever a timer or humidity control is set to
How big a fan does a bathroom need?
The Code’s floor is 25 L/s, about 53 cfm. The fan makers’ trade body, the Home Ventilating Institute (HVI), sizes by floor area: 1 cfm per square foot, and never under 50 cfm, for a bathroom up to 100 sq ft. A larger room adds 50 cfm for each toilet, shower and tub, and 100 for a jetted tub.
- A 5′ × 8′ bathroom is 40 sq ft: HVI’s 50 cfm minimum, and the Code’s 25 L/s (53 cfm) as the floor.
- An 8′ × 10′ ensuite is 80 sq ft: about 80 cfm.
- A 12′ × 10′ ensuite with a toilet, a shower and a tub is over 100 sq ft: 150 cfm.
Put the fan over the shower or tub where you can, using a fan approved for that wet location, and give an enclosed toilet room its own. For noise, check the sone rating on the HVI label: one sone is about a quiet fridge in a quiet kitchen, and lower is quieter. The Code caps a bathroom fan at 3.5 sones on HVI’s scale.
Why does the duct matter as much as the fan?
A fan’s airflow rating is measured against a set resistance: 25 pascals, or 0.1″ of water, for a bathroom fan. Every metre of duct adds to it. Ask more of the fan than it was rated against and it moves less air than the number on the box.
The Code’s duct table shows how quickly this bites. For a fan moving 25 L/s at that rated pressure, the longest duct it allows:
So a 4″ duct run 5 m across an attic to a gable is too long for that fan. Either the duct gets bigger or shorter, or the fan is chosen for a higher pressure. Through the cold attic the duct is insulated, so the moist air doesn’t condense inside it, and it ends outside, through the roof or a wall with a damper. Never into the attic.
How long should a bathroom fan run?
HVI recommends at least 20 minutes after each use, with a timer or another control that keeps it running after you leave. A humidity sensor that starts the fan automatically is a good addition; the Code just asks that the manual switch can still turn it on whatever the sensor is set to.
The fan also needs air to replace what it pulls out. HVI asks for at least ¾″ under the bathroom door. A thicker new floor can quietly close that gap, so it’s worth checking when the flooring changes.
When is the best time to fix a bathroom fan?
When the ceiling is open. In a bathroom renovation the old fan comes out with everything else, and a larger duct, a better route to the outside or a new outlet is far easier than through a finished ceiling. Moving a fan or adding a circuit is electrical work: almost all of it needs a notification to the Electrical Safety Authority, which our guide to permits for renovation work in Ottawa explains.
On the bathrooms we renovate, electrical work goes to licensed electricians, and a bathroom typically takes 3 to 4 weeks on site. If the bathroom is one part of the home renovations we carry out in Ottawa, the fan and its duct belong on the list at the estimate.
Guidance only, not a design or code ruling. The Ontario Building Code and each fan’s installation instructions set what a bathroom fan must meet.