Bathroom exhaust fan: how big it should be and where the duct has to go

A bathroom makes more moisture than any other room in the house, and in a Toronto winter that moisture goes looking for the coldest surface it can find: the mirror first, then the window, then the ceiling paint, and, if the fan sends it there, the underside of the roof. The exhaust fan is the one part of a bathroom that decides where that water ends up. It is also the part most often picked by price, mounted wherever the joists allow and ducted by whoever is closest to the attic hatch.

This guide covers the two things worth getting right: how much air the fan should move for your room, and where its duct must end. Then it shows how to test the fan you already have.

How many CFM does a bathroom fan need?

For a bathroom up to 100 square feet, the Home Ventilating Institute recommends 1 CFM (cubic foot per minute) of exhaust for every square foot of floor, which works out to about eight air changes an hour. Above 100 square feet, size by fixture instead: 50 CFM each for a toilet, a shower and a tub, and 100 CFM for a jetted tub.

The numbers come from HVI's page on bathroom ventilation. Two rooms of the size we usually see in GTA houses show how the rule works in practice.

Ceiling height changes the picture too. The per-square-foot rule assumes an ordinary 8 foot ceiling, so a bathroom with a 10 foot or vaulted ceiling holds more air and needs the next size up.

What does each fixture add in a large bathroom?

Once a room passes 100 square feet, HVI stops counting floor area and adds up what produces moisture or odour. Write down every fixture in the room, read its number from the table, and add them.

A combined tub and shower counts once as a shower. A steam shower is a separate case: HVI suggests its own fan, run after the steam session ends.

Why does a fan move less air than the box says?

The CFM printed on the box is the airflow when the fan pushes against very little resistance. Every metre of duct, every bend and every flap the air has to open takes some of that away, so a fan that meets the HVI number on paper can fall well short in the ceiling.

These are the usual places the airflow goes:

• A long run to a far wall, when the shortest way out was through the roof right above the fan.
• Several elbows where the duct had to dodge joists, pipes or a light fixture.
• Flexible duct that was pulled too short, crushed under insulation or left to sag between joists. A sag also collects water.
• A duct narrower than the fan's outlet, reduced with an adapter to fit an old hole.
• A damper at the outside hood that sticks shut in cold weather, or is painted or caulked closed.
• A door with no gap underneath, so the fan has nothing to pull replacement air through.

The fan's own instructions list the largest duct length and the number of elbows it was designed for. When the route is long, a fan with a higher rating or a smoother route with rigid duct is the answer, not a bigger hole.

Where should the bathroom fan duct end?

Outside the house, through a hood with a damper, every time. The attic is not outside. Air pushed into it is warm and wet, and in winter it meets roof sheathing and nails that are below freezing, where it turns to frost and then to water when the weather warms.

That leaves three real routes and one that should never be used. This is how they compare in a GTA house with a cold attic above the bathroom.

Any part of the duct that crosses an unheated attic should be insulated along its whole length and sloped slightly down toward the outside, so that water that condenses inside it drains out through the hood instead of running back into the fan and dripping from the grille.

How can you check the fan you have?

You do not need tools for most of it. Take a shower with the fan running, then walk through this list. Any one of these points to a fan that is too small, a duct that is blocked or a route that ends in the wrong place.

1. Hold a single square of toilet paper against the grille while the fan runs. If the fan cannot hold it there, it is moving very little air.
2. Time the mirror. With the fan on and the door shut, it should start clearing within a few minutes after you turn off the water. If it stays fogged until you open the door, the fan is not doing the work.
3. Look at the ceiling around the grille and in the corners above the shower: grey spots, peeling paint and a brown ring are moisture that stayed in the room.
4. Go outside and find the hood. With the fan running, the flap should open and you should feel air. A hood you cannot find usually means there is no duct to the outside.
5. In winter, look at the hood after a shower. A plume of vapour is good. Ice building up around it means the damper may freeze shut.
6. If you can reach the attic safely, follow the duct from the fan housing. It should run in one piece, insulated, to a cap in the roof or a hood in a wall, and not stop under the insulation or point at the roof boards.

If the duct turns out to end in the attic, fix the route before buying a bigger fan. A bigger fan on the same route only puts more water up there.

How long should the fan run, and what should switch it?

HVI recommends leaving the fan on for 20 minutes after you use the bathroom. Very few people remember to, so the control matters as much as the fan. The common choices are:

• A countdown timer switch, so one press runs the fan for a set time and turns it off.
• A humidity sensor, built into the fan or the switch, which starts the fan when the air gets damp and stops it when the room dries out.
• A plain switch, which works only if someone remembers to turn it off later.

Moving a fan, adding one where there was none or changing the switch to a timer usually means new wiring. In Ontario that work is done by a Licensed Electrical Contractor, who files the notification with the ESA (ESA, working with general contractors).

When does the fan get installed during a bathroom renovation?

The easiest time to get the duct right is when the ceiling is open, before any drywall goes up. In a full bathroom renovation the fan fits into the sequence like this:

1. Plan the fan with the layout: where the steam starts, how big the room is, and which way the duct can leave the house.
2. Frame and rough in. The fan housing is fixed to the joists and the duct is run, sloped and sealed to its hood or roof cap.
3. The electrician runs the wiring for the fan and its switch together with the lights.
4. Insulation and vapour barrier go in, sealed tightly around the fan housing so warm air does not leak into the attic around it.
5. Drywall, then the waterproofing and tile in the wet areas.
6. The grille goes on last, after paint, and the fan is run to check airflow at the hood.

Once the ceiling is closed and tiled, rerouting a duct means opening a finished ceiling, which is why the route is decided before framing. The same order of trades is why we treat bathroom waterproofing and the fan as two parts of one job: the membrane keeps water out of the walls, and the fan keeps it out of the air.

Where we fit in

Art Style Construction builds complete bathrooms across Toronto and the GTA as part of our bathroom renovations, with the fan, its duct and its controls planned together with the layout. If you are still choosing the layout itself, our guide to small bathroom layouts shows where the fixtures can go. For a stained or peeling ceiling around an old fan, our drywall repair work covers replacing the damaged section once the cause is fixed. To talk about your bathroom, send us a message; the first visit is free.