How Much Ventilation a Remodeled Bathroom and Kitchen Need
A remodeled bathroom without an openable window needs an exhaust fan that moves at least 50 cubic feet per minute (cfm) when it runs on demand, or 20 cfm if it runs continuously, and the air has to go outside, not into the attic. In the US that comes from the International Residential Code (IRC); in Canada, Part 9 of the building code sets similar minimums in litres per second. Kitchens follow their own numbers, and large range hoods can trigger a makeup air requirement that catches a lot of homeowners off guard.
Below is what the rules say, where they differ between the US and Canada, and what an inspector usually looks at when a kitchen or bathroom remodel is signed off.
Why ventilation gets checked during a remodel
Once you pull a permit, the remodeled room generally has to meet the code in force today, not the code from the year the house was built. The old fan that hums but barely moves air, or the one that was never ducted anywhere, often gets replaced as part of the job.
Timing matters too. Late fall and winter are when ventilation problems show up: hot showers meet cold exterior walls and a cold attic, and moisture condenses on window frames, ceilings and inside ductwork. Peeling paint above the shower, a musty vanity cabinet or a brown ring around the fan grille are all signs the existing setup is not doing its job.
US rules for bathrooms (IRC)
The IRC gives bathrooms two options. The first is natural ventilation: a window with at least 3 square feet of glazing, half of which opens. The second, which almost every remodel uses, is mechanical exhaust under the mechanical chapter of the code:
- 50 cfm for a fan that runs intermittently (switched on when the room is used)
- 20 cfm for a fan that runs continuously
Many local amendments require a fan in any bathroom with a shower or tub, window or not.
The bigger rule is where the air goes. Exhaust has to discharge to the outdoors. It cannot dump into the attic, a crawl space, a soffit cavity that is not vented, or the space between floors. Terminations also need to sit a set distance away from operable windows, doors and property lines, and they need a backdraft damper so cold air does not pour in when the fan is off.
The 50 cfm figure is a floor, not a target. A small powder room is fine at that level. A large primary bathroom with a separate shower and soaking tub often gets a larger fan or two fans, especially when the shower is enclosed in glass up to the ceiling.
US rules for kitchens
Kitchens are handled differently. The IRC table for local exhaust lists 100 cfm intermittent or 25 cfm continuous for a kitchen, and many jurisdictions and energy programs expect kitchen exhaust to be vented outside. A recirculating hood (one that filters and blows air back into the room) removes some grease and odour but does nothing for moisture, so check with your building department before counting on one.
The rule people run into on high-end remodels is makeup air. When a kitchen exhaust system can move more than 400 cfm, the IRC requires makeup air, meaning a way for replacement air to enter the house at roughly the rate being exhausted above that 400 cfm. Without it, a powerful hood in a tight house can pull combustion gases back down a furnace or water heater flue. Makeup air is usually a motorized damper on an outside duct, interlocked with the hood, sometimes heated. It adds real cost and needs planning before cabinets go in.
The duct itself has rules as well. Range hood ducts are generally required to be smooth-walled metal (galvanized steel, stainless steel or copper), airtight, and fitted with a backdraft damper. Flexible foil duct, which is common on bathroom fans, is not accepted for a range hood.
Canadian rules: Part 9 of the building code
In Canada, houses fall under Part 9 of the National Building Code, adopted with changes by each province (the Ontario Building Code, the Quebec Construction Code, the BC Building Code and so on). Instead of cfm, Part 9 works in litres per second (L/s) and treats kitchen and bathroom fans as supplemental exhaust on top of the house's principal ventilation system.
The values commonly used are:
- Bathroom: about 25 L/s (roughly 53 cfm)
- Kitchen: about 50 L/s (roughly 106 cfm)
As in the US, the air must be exhausted to the outdoors, ducts running through unheated attics need to be insulated to limit condensation, and exterior hoods need dampers. Many provinces also have rules about depressurization when large exhaust fans share a house with fuel-burning appliances, which is the Canadian equivalent of the US makeup air question. Because each province edits the national model, confirm the exact figures with the municipality issuing your permit.
In condos, the building often sets stricter limits than the code. Some towers run bathroom fans into a shared vertical exhaust shaft with a fixed airflow, and the board may not allow a new exterior wall penetration for a range hood. Read the renovation rules before you choose a hood.
What inspectors tend to check
When a bathroom or kitchen remodel gets its final inspection, ventilation is usually part of the walk-through. Common items:
- The fan's rated airflow on the label or spec sheet meets the minimum for the room.
- The duct ends outside at a proper roof or wall cap, not in the attic.
- A damper is installed at the fan or the termination.
- Duct size matches the fan outlet. Reducing a 4 inch outlet to 3 inch duct strangles the airflow.
- Fans over a tub or shower are listed for that location and wired as the manufacturer requires.
- Makeup air is present for a kitchen hood above 400 cfm (US) or where the province requires it.
Some jurisdictions also want the fan on a timer or humidity control. Even where it is optional, a humidity sensing switch is one of the cheapest upgrades in a bathroom remodel.
Choosing a fan that actually works
Rated airflow is measured under test conditions, and real duct runs with elbows and long horizontal stretches cut it down. A few practical points:
- Keep duct runs short and straight, and use rigid or semi-rigid duct where you can.
- Size up when the duct run is long or has several bends.
- Look at the sone rating. A fan at 1.0 sones or less is quiet enough that people actually leave it on.
Our bathroom remodeling page covers what else gets checked when the walls are open, and the kitchen remodeling page explains how hood placement fits into layout and cabinet planning.
Frequently asked questions
Can a bathroom fan vent into the attic?
No. Both the IRC and Canadian building codes require bathroom exhaust to discharge outdoors. Venting into the attic puts warm, wet air against cold roof sheathing, which leads to frost, dripping and mould on the underside of the roof. If an existing fan vents into the attic, a remodel is the time to extend the duct to a roof or wall cap.
When does a range hood need makeup air?
In the US, the IRC requires makeup air when a kitchen exhaust system can move more than 400 cfm. Many large professional style hoods exceed that, so the requirement often comes up during high-end kitchen remodels. In Canada, the trigger depends on the province and on whether the house has fuel-burning appliances that could backdraft, so ask the permit office.
Is a recirculating range hood allowed?
It depends on the jurisdiction. A recirculating hood filters grease and some odour but returns the moist air to the room, so some building departments do not accept it as kitchen exhaust. In condos that forbid new exterior openings it may be the only option, so confirm with both the building and the municipality.
How big should a bathroom exhaust fan be?
The code minimum is 50 cfm intermittent in the US or about 25 L/s in Canada. A common rule of thumb is about 1 cfm per square foot of floor area for bathrooms up to roughly 100 square feet, with more for larger rooms, enclosed showers and long duct runs. Fixtures like a jetted tub or a steam shower usually call for extra capacity.
Planning a remodel?
If your kitchen or bathroom project includes a new fan, a bigger range hood or a duct that finally needs to go outside, put it in the scope from the start. Send us the details, and the written quote will spell out the ventilation work along with the rest of the job.