Bathroom Ventilation in Ottawa: Fan Sizing, Duct Runs and Stopping Mould
The fan is only half the job. The duct behind it decides whether mould ever starts.
The catch is that a fan rated on the box loses 20 to 40 percent of its airflow to a long or bendy duct, so we size up when the run is long. Ventilation is built into a bathroom renovation rather than bought on its own, and our complete bathroom renovation scope starts from $20,000 CAD, with a free in-home assessment for your room.
Ottawa bathrooms fight moisture harder than most rooms in the house, and the exhaust fan is the one defence between a hot shower and mould behind the tile. Most fan problems are not the fan, they are the sizing and the duct. We handle ventilation as part of bathroom renovations across Ottawa, Kanata, Nepean, Barrhaven, and Orleans. For the full pricing picture, our bathroom remodeling cost guide sets out every project size.
Why Bathroom Ventilation Fails
A fan can be permitted, installed, and still do almost nothing. The reason an Ottawa bathroom stays damp is usually one of three things: it is undersized for the room, the duct is too long or too twisty, or it dumps into the attic instead of outside. Any of those leaves warm damp air sitting in the room and the wall cavities, and that is exactly where mould starts.
The Ontario Building Code sets a floor of roughly 50 CFM for a switch-operated bathroom fan, or about 20 CFM where ventilation runs continuously. That is a minimum for code, not a target for a room that actually stays dry, which is why proper sizing matters more than just passing inspection.
How to Size the Bathroom Fan
1. Use 1 CFM per square foot
For a bathroom up to 100 square feet, multiply the floor area by one. A 6 by 8 foot bathroom is 48 square feet, so it needs about 50 CFM. An 8 by 10 foot primary bathroom is 80 square feet and needs 80 CFM. The 50 CFM minimum applies even to a tiny powder room.
2. For a large bathroom, size by fixture
Over 100 square feet, add it up by fixture instead: about 50 CFM each for the toilet, the shower, and the tub, or 100 CFM for a jetted tub. A large master bath with all three can need 150 to 200 CFM, often split between two exhaust points.
3. Add for a high ceiling
The sizing assumes an 8 foot ceiling. For every foot above that, add roughly 10 percent to the CFM, since there is more air volume to clear.
4. Size up for the duct run
A fan hits its rated CFM only in a lab. In a real duct with length and elbows it delivers 20 to 40 percent less, so when the run is longer than about 15 feet we choose a fan rated 25 to 40 percent above the minimum so the air actually leaving the room meets the target.
5. Choose a quiet fan
Fan noise is measured in sones. A loud fan gets switched off, which defeats the point, so we fit quiet units at 1.0 to 2.0 sones for anything next to a bedroom. A humidity sensor or timer that runs the fan for 20 minutes after a shower removes the moisture people leave behind.
Duct Runs Done Right
The duct matters as much as the fan. Use 4 inch smooth rigid metal, not the ribbed flexible kind, because flex duct creates two to three times the resistance and chokes the airflow. Keep the total run short and as straight as possible, since every 90 degree elbow adds the resistance of another 5 to 10 feet of pipe.
A useful rule is to keep the total equivalent length under about 25 feet. A 15 foot run with two elbows already behaves like 65 feet, which will strangle most fans. The duct terminates at an exterior wall or roof cap with a backdraft damper, and it never ends in the attic, a soffit, or a wall cavity.
Stopping Mould for Good
Ventilation is the first defence, but mould control is really moisture control across the whole room. The fan clears the humid air, a humidity-sensing switch makes sure it runs long enough, and proper waterproofing under the tile keeps water out of the surfaces where mould feeds. Miss any one and the other two work harder.
That is why we treat the fan, the duct, and the shower waterproofing as one system on every renovation. A quick field test tells you a fan is pulling: hold a single sheet of tissue to the running grille and it should hold there by suction. If it falls, the fan or the duct is not moving real air.
What Ventilation Costs in an Ottawa Renovation
A fan and its duct are a small part of a renovation, and because the wall and ceiling are already open, that is the time to do it right rather than fight it later. We quote ventilation as part of the whole bathroom, not a line item on its own.
| Scope | 2026 price (CAD) |
|---|---|
| Complete Bathroom Renovation, ventilation included | From $20,000 CAD |
| Tub-to-Shower Conversion, with the wet area re-sealed | $13,000 to $15,000 CAD |
| Shower Installation, waterproofed and ventilated | From $13,000 CAD |
| Bathtub Remodel, updating the wet area | $13,000 to $15,000 CAD |
| City of Ottawa permit, if wiring or fixtures move | A few hundred CAD, by fixture count |
Upgrading a fan and its duct is folded into any of these while the room is open. Our full cost guide sets out the range for every size, and a free assessment gives the exact number for your bathroom.
Why Ottawa Families Choose Bytown Bath
We serve Ottawa homeowners and we renovate bathrooms only. On ventilation that focus matters, because a fan is only as good as the duct behind it and the waterproofing beside it, and those are the parts a rushed job hides.
Every quote is written and upfront, our installs are handled by professionally trained installers, and we back the product and labour with a written warranty. We size the fan to the room, run the duct short and straight to the outside, and pair it with proper waterproofing so the room stays dry. Payment is 50 percent on booking and 50 percent on completion.
Frequently Asked Questions
What size exhaust fan do I need?
For a bathroom up to 100 square feet, use 1 CFM per square foot with a 50 CFM minimum, so a 48 square foot bathroom needs about 50 CFM and an 80 square foot one needs 80 CFM. For larger bathrooms, add about 50 CFM per fixture. Size up when the duct run is long.
Can a bathroom fan vent into the attic?
No. It must vent outside through a wall or roof cap. Venting into an attic dumps warm moist air onto cold sheathing, which condenses, rots the roof, and grows mould. It is also a code issue in most places.
Why does my bathroom still get damp with a fan running?
Usually the fan is undersized, the duct is too long or bendy, or it is flexible duct choking the airflow. A fan rated on the box can lose 20 to 40 percent of its airflow to a poor duct, so the air actually leaving the room falls short.
How long should the fan run after a shower?
About 20 minutes, to clear the moisture left behind. A humidity-sensing switch or a timer does this automatically so it is not left to memory, which is the most reliable way to keep humidity down.
What does bathroom ventilation cost in Ottawa?
It is built into a renovation rather than priced on its own. A complete bathroom renovation starts from $20,000 CAD, and upgrading the fan and duct is folded in while the room is open. A free assessment gives the exact number for your bathroom.
Worried About Moisture or Mould? Book a Free In-Home Assessment
We check your fan, your duct run, and your waterproofing, tell you plainly what is undersized, and leave you with a written, upfront quote.
Bytown Better Bathtubs and Showers | 3894 Russell Road, Ottawa, ON K1G 3N2
(613) 746-8055 | info@bytownbath.ca | Monday to Friday, 8am to 5pm
Related Guides from Bytown Bath
| Guide | What you will learn |
|---|---|
| Complete Bathroom Renovation | Scope and inclusions when a bathroom is built out fully |
| Shower Installation Ottawa | Waterproofing and ventilating a new shower |
| Tub-to-Shower Conversion Ottawa | Re-sealing the wet area during a conversion |
| Bathtub Remodel | What a tub-focused remodel covers |
| Bathroom Remodeling Cost Ottawa | The full pricing range for every project size in the city |
Helpful Outside Resources
| Resource | Why it helps |
|---|---|
| City of Ottawa: Do I Need a Building Permit | Which ventilation and wiring work needs a permit |
| CMHC | Guidance on moisture, mould, and home ventilation |
| Canadian Home Builders Association | Standards and advice on hiring renovation professionals |
| Parachute Canada | Home safety guidance including damp and slip risks |
| Electrical Safety Authority | Why fan wiring needs a licensed contractor |
