For Ontario bathrooms covered by the prescriptive Part 9 mechanical ventilation rules, a supplemental exhaust fan rated at least 25 L/s, approximately 53 CFM, is required where the principal ventilation system has no exhaust intake in that bathroom or water-closet room. A window is not the exception named in that rule.
The important first question is which ventilation system serves the room. A separate bathroom fan and an exhaust intake connected to the home's principal system are different arrangements. Confirm the design before replacing equipment or closing a ceiling.
What does the current Ontario code say?
The references below are from the provincial 2024 Building Code Compendium, Division B, Part 9, checked October 18, 2026. Existing-building renovations can involve different compliance paths; confirm applicability with the building department.
| Check | Reference | Requirement in brief |
|---|---|---|
| Bathroom without a principal-system exhaust intake | 9.32.3.7.(4) | Supplemental fan rated at least 25 L/s |
| Supplemental-fan control | 9.32.3.7.(6), (8) | In-room manual control; automatic control must not prevent manual activation |
| Exhaust destination | 9.32.3.11.(2) | No discharge into an enclosed heated or unheated space |
| Duct in an unheated space | 9.32.3.11.(3) | At least RSI 0.5 insulation |
| Duct condition | 9.32.3.11.(7), (8) | Prevent crushing and sagging; seal joints with specified materials |
Is a 50 CFM bathroom fan enough?
Do not round the 25 L/s requirement down to a product labelled 50 CFM. Twenty-five litres per second is about 53 cubic feet per minute. Compare the actual equipment rating and the designed installation, not a rounded shopping description.
A fan's advertised capacity also does not answer every ducting question. Ask the installer to identify its model, proposed duct size, route and exterior termination. A short, direct route and a long route with several changes in direction are different installations.
For a combined tub and shower room, discuss how the room is used and what the design calls for. Buying a larger fan without checking the rest of the system is not a complete ventilation plan.
Does a bathroom with a window still need mechanical exhaust?
Under the rule above, the relevant exception is a principal-system exhaust intake in the room. An operable window is not a substitute named in that provision. Ask the designer to identify the bathroom intake and show which system it connects to.
That matters when an HRV or another central arrangement is already present. Do not assume that a grille is a working exhaust intake because it looks like one. Trace the connection and check the ventilation design before deciding that a separate fan is unnecessary.
Plan the duct before the ceiling is closed
A bathroom quote should identify where the moisture-laden air leaves the building. A grille in the ceiling tells you where air enters the duct, not where the duct ends. Request a route that can be inspected before the assembly is covered.
Record the exterior termination location on the drawing or scope. Ask how the route passes through the attic or other cold spaces, how it will be supported, and how the outlet relates to nearby openings and air intakes. Those details belong in the plan rather than being left for the last day of tile work.
If access reveals moisture-damaged insulation or staining, have the cause investigated before replacing the finish. A new grille does not establish that the old moisture problem is resolved. Coordinate that inspection with the bathroom waterproofing work, so both sides of the ceiling and wall assembly are considered.
What to put in a bathroom renovation quote
| Scope item | Ask the contractor to specify |
|---|---|
| Existing system | Separate fan or connection to principal ventilation |
| Equipment | Model, rating, controls and included electrical work |
| Route | Duct path and exterior outlet location |
| Access | Ceiling openings and how finishes will be restored |
| Concealed work | Who checks the installation before closing it |
| Handover | Product instructions, maintenance access and inspection records |
For a bathroom renovation in Cambridge, settle the route while the layout is being priced. If you're planning a bathroom remodel in Kitchener, include access and finish repairs in the same comparison. These are parts of a complete renovation scope, not just the price of a fan.
Frequently asked questions
Can a fan discharge into the attic?
No. An attic is an enclosed space. The exhaust needs a properly designed exterior termination, rather than ending above the insulation.
Does a humidity switch remove the need for manual control?
No. Where the cited supplemental-fan provisions apply, the automatic control must allow manual activation. Ask the installer to demonstrate the controls during handover.
Does replacing a bathroom fan need a permit?
Describe the actual work to the municipal building department and the electrical contractor. A replacement using the existing route differs from adding wiring, changing the ventilation arrangement or relocating equipment. Confirm the required approvals before starting.
Can the HRV serve the bathroom?
The code distinguishes a principal-system intake from a separate supplemental fan. Have the mechanical designer confirm that the proposed arrangement satisfies the applicable design, rather than treating any HRV connection as an automatic exemption.
What should I check after installation?
Ask for a demonstration of operation and controls, the equipment instructions and any required inspection records. Keep maintenance access clear. If steam or moisture problems continue, request a diagnosis of the complete installation rather than assuming a higher fan rating will solve it.
Our Ontario bathroom renovation guide covers the wider planning sequence. To include ventilation in a renovation scope, tell us about the bathroom before selecting the fan or finalizing the ceiling layout.