What size bathroom fan do you need?

The short answer

For a bathroom up to 100 square feet, the Home Ventilating Institute recommends 1 CFM of airflow for every square foot of floor. For larger bathrooms, it recommends adding up the fixtures instead, at 50 CFM each for a toilet, a shower and a tub, and 100 CFM for a jetted tub.

Building codes in Canada and the United States both set a floor of 50 CFM for a bathroom fan that runs only when someone switches it on. So even a tiny powder room should have at least 50 CFM. In practice, most full bathrooms are better off with 80 to 110 CFM, because real ducts never deliver the airflow printed on the box.

Fan sizes for common bathrooms

These sizes follow the Home Ventilating Institute's method with a little extra to make up for duct losses. A ceiling higher than 8 feet, a long duct run or a long, hot daily shower is a reason to go up a size. Steam showers need their own ventilation plan.

What the building codes say

In Canada, house ventilation is covered in Section 9.32 of the National Building Code and the provincial codes based on it. Municipal ventilation forms based on Alberta's edition, for example, list minimum supplemental exhaust of 50 CFM for a bathroom and 100 CFM for a kitchen. In the United States, the International Residential Code asks for 50 CFM for a bathroom fan that runs on demand, or 20 CFM if it runs continuously.

Many Canadian homes built in the last few decades also have a principal ventilation fan, which runs to bring fresh air into the house year round. Often it is one of the bathroom fans, with a labelled switch, and it is interlocked with the furnace fan so the fresh air is spread through the house. If you replace that fan, the new one has to meet the principal fan rules, which the code sets by the number of bedrooms.

These figures are from the Alberta Edition of the National Building Code, and other provinces may differ. If your house has an HRV, it is usually doing the principal ventilation job instead, and our guide on whether your home needs an HRV explains how they work. When in doubt, find out which fan is the principal one before you change anything.

Why the box rating is not what you get

Fans are tested at set levels of resistance, called static pressure. Every metre of duct, every elbow, a crushed section of flexible duct or a cheap wall cap with a stiff flap adds resistance, and the fan moves less air. A fan sold as 80 CFM can easily deliver 40 or 50 in a real house with a long, bendy duct.

Look for the airflow the fan delivers at 0.25 inches of static pressure rather than the headline number. It is the more honest figure, and good manufacturers publish it.

• Keep the duct as short and straight as possible, with as few elbows as you can.
• Use the duct size the fan was designed for, and never neck a 6 inch fan down to 4 inch duct.
• Rigid or semi-rigid duct moves more air than floppy foil duct.
• Seal every joint with mastic or metal foil tape, which Canadian codes require for exhaust ducts. Cloth duct tape dries out and lets go.

Venting a fan through a cold attic

This is where most of the trouble starts, and it gets worse the colder the winter. A bathroom fan must vent outside, never into the attic and never just pointed at a soffit vent. Warm, wet bathroom air that reaches a cold attic condenses and freezes on the underside of the roof. On the first warm day, the frost melts all at once and shows up as stains on the ceiling or water dripping out of the fan grille. Here in Calgary, where chinooks bring several thaws every winter, we see it often.

Even a fan that is ducted outside can drip if the duct is not insulated. The moist air cools inside the duct and condenses, and the water runs back down into the fan. Through any unheated attic, the duct should be insulated, as short as possible, and sloped so water drains toward the outside rather than back into the bathroom.

• Use an insulated duct through any unheated attic.
• End it at a proper roof or wall cap with a damper, not at a soffit.
• Fit a backdraft damper so cold air does not pour back in when the fan is off.
• In very cold weather, a flap can freeze shut with frost. A better quality cap with a lighter damper helps.
• If you see frost on attic nails, stained ceiling patches around the fan or ice at the vent, have the duct checked.

Air leaks around the fan housing also let warm air into the attic, which feeds ice dams. Our guide to designing a home for cold winters covers air sealing and attics.

How quiet should a bathroom fan be?

Fan noise is measured in sones. Under 1.0 sone is very quiet, about the hum of a modern fridge. Around 3 sones is noticeable, and older builder-grade fans were often louder than that.

Noise matters more than people think, because a loud fan is a fan nobody turns on. Canada's building code even sets sound limits for bathroom exhaust fans for that reason. For a bathroom used every day, especially an ensuite next to a bedroom, aim for a fan rated at 1.0 sone or less.

Switches, timers and humidity sensors

• Run the fan for about 20 minutes after a shower, longer after a long hot shower or a bath. Most mirrors clear well before the moisture in the room has gone.
• A timer switch or a humidity sensor takes the guesswork out. A humidity-sensing fan turns itself on when moisture rises and off when the room is dry.
• Set a humidity sensor for the season. Indoor air in a cold-climate winter is very dry, so a setting that works in summer can leave the fan running too little in January or too much in August.
• Do not wire the fan to the light alone. People switch the light off as they leave, which is exactly when the fan needs to keep running.
• A fan inside the shower enclosure or over a tub has to be rated for wet locations and protected by a ground-fault circuit.

Signs your fan is too small or not working

• The mirror is still fogged ten minutes after a shower.
• Paint is peeling or bubbling on the ceiling, or there are black spots in the corners.
• Grout and caulking go mouldy quickly.
• Doors and trim swell or stick in the bathroom.
• A square of toilet paper held to the grille does not get pulled up against it.

A bathroom without a window relies entirely on its fan, so these signs show up there first.

Replacing a fan during a renovation

Changing a fan for the same size in the same spot is usually simple. Moving it, upsizing the duct, adding a new circuit or adding a fan where there was none involves electrical work, which normally needs a permit, and in a bathroom renovation it is part of that job's permits anyway.

If you are renovating, put the fan where it does the most good, which is usually over or right beside the shower, and plan the duct route at the same time as the plumbing. In a basement bathroom, the duct often has to travel through a joist space to the rim joist, so plan it before the ceiling is closed.

Questions people ask

• Can a bathroom fan be too big? It can, if it pulls so much air that it depressurizes the house. That is rare with a normal bathroom fan, but it is worth checking if you also have a big range hood. Our renovation dictionary explains make-up air.
• Should the fan be over the shower or the toilet? Over or near the shower, where the moisture comes from.
• Is it OK to run the fan all night? Yes, a quiet fan rated for continuous use can run as long as needed, and the principal fan in many homes runs for hours every day.
• Why does my fan drip water in winter? Usually because the duct in the attic is not insulated, is disconnected, or slopes back toward the fan.

Planning a new bathroom

Nobody shows off their bathroom fan. But a fan that is sized for the room, quiet enough to get used and vented through a short insulated duct does more for how long the tile, paint and ceiling last than almost any finish you pick. When we design a bathroom, the fan and its duct route are planned along with the plumbing, not left for the last day. If you are in the Calgary area, see our bathroom renovations page for the rooms we build.