I get one call a month from someone who pulled their bathroom ceiling apart and found black on the back of the drywall. They are angry. They renovated 18 months ago. The bathroom looks great from the inside. The contractor is not returning calls.
The reason is almost always the fan. Not the brand of fan. Not the look of the grille. The duct behind it. Where it runs, what it is made of, and where it ends.
This is the boring part of a bathroom build that nobody talks about until it ruins the bathroom. Here is the part of the conversation that should happen before any tile goes up.
The CFM math, in plain English
CFM stands for cubic feet per minute. It is how much air the fan moves. Ontario Building Code requires a minimum of 50 CFM for an intermittent fan in a typical bathroom and 25 CFM continuous if the fan runs all the time. That is the legal floor.
The working rule we use in our shop is 1 CFM per square foot of bathroom area, with a minimum of 80 CFM. A 5 by 8 foot bath needs 80 CFM. A 10 by 12 foot bath needs 120 CFM. If the room has a separate water closet or a steam shower, add 50 CFM each.
The reason most builder-installed fans fail is that they were sized to clear a small powder room and got dropped into a master bath. A 50 CFM fan in a 96-square-foot bathroom is moving half the air it needs to. The mirror still fogs after a 10-minute shower. The drywall stays damp. Mould has a 6-month head start before anyone notices.
Quick CFM check on your current fan
Tape a single ply of toilet paper to the grille. Turn the fan on. If the paper hangs limp or falls off, your fan is undersized or the duct is blocked. A properly sized fan with a clean run will hold the paper flat against the grille for as long as the fan is on.
The duct has to go OUTSIDE
This is the single most-broken rule in older Ottawa houses. The Ontario Building Code says bathroom exhaust must terminate to the exterior. Not into the attic. Not into the soffit. Not into a crawl space. Outside the building envelope, through a roof cap or a wall cap, with a damper.
The "soffit dump" is the cheat I see most. The duct ends inside the soffit cavity. The contractor argues that the air is technically outside because the soffit is vented. The problem is that bathroom exhaust is warm and wet.
It rises. The soffit vents pull it right back into your attic. In a cold January, that wet air condenses on the underside of your roof sheathing and drips onto your insulation.
I tore out a Westboro attic last spring where the previous installer had taped the duct to the soffit baffle. The plywood roof deck above that bathroom had three feet of black mould in a perfect circle.
The repair meant new sheathing, new shingles in that section, and replaced insulation, a far bigger bill than the original installer "saved" by skipping the proper roof cap, which was 90 minutes of work.
Insulated duct, in any unconditioned space
If your duct runs through an unconditioned attic for any distance, it has to be insulated. R-6 minimum. R-12 is what we use on every job. Without insulation, the warm wet exhaust air hits the cold metal duct, condenses inside the duct, and runs back down toward your fan housing. You will hear it dripping in your ceiling on cold mornings.
Worse, that condensate pools at any low spot in a sagging flex duct. After two winters that puddle is a Petri dish. The first warm day in April releases everything that grew in there back into your bathroom every time the fan runs.
Rigid metal duct beats flex every time. Flex duct sags. Flex duct collects lint and dust on every internal ridge. Flex duct loses pressure faster per foot than smooth-wall metal. We use rigid 4-inch or 6-inch galvanized with sealed joints, slope the run downward toward the wall cap so any condensate drains outside, and wrap the entire run in R-12 sleeve.
Every "mystery moisture" problem in an Ottawa attic comes back to one of three things: bathroom exhaust dumped into the soffit, an uninsulated duct, or a fan undersized by half.
The 2-year mould timeline
People ask how fast bad venting causes a real problem. Here is the timeline I have seen on dozens of jobs:
- Months 0 to 6: No visible problems. Mirror fogs after showers. Paint near the ceiling looks slightly soft.
- Months 6 to 12: Caulk lines start to discolour. Tile grout near the showerhead looks darker than the rest. The fan grille has dust streaks.
- Months 12 to 24: Visible black spots appear at the top corners of the shower. Ceiling paint near the fan starts to stain. A musty smell shows up if the bathroom has been closed for a few hours.
- Years 2 to 4: The drywall behind the tile is compromised. Mould has reached the studs. The attic above shows visible damage on the underside of the sheathing. The repair is now a major structural and remediation bill.
The fan that would have prevented all of this is a small line item plus an extra hour of labour to do the duct run properly.
What a real fan install looks like
Here is what we spec on every full bathroom we build:
- A real-name fan unit (Panasonic WhisperGreen, Broan Sensonic, or similar) sized to 1 CFM per square foot, 80 CFM minimum
- Sone rating below 1.5 so the fan is quiet enough that the homeowner actually turns it on
- Humidistat or motion-and-humidity combo switch so the fan runs after the shower until the air actually dries
- Rigid galvanized duct, 6-inch where the fan body uses 6, 4-inch otherwise, with all joints sealed in foil tape
- R-12 insulated sleeve over any duct in the attic
- Slope of about 1/4 inch per foot toward the exterior cap
- A backdraft damper at the cap to keep cold winter air from running back in
On a fresh build, this is a modest materials line and three to four hours of labour. On a retrofit through an existing finished ceiling and attic, the labour climbs because of the access. Either way it is a small line item compared to a mould-and-sheathing repair three years down the road.
How to inspect your existing fan in 10 minutes
If you already have a bathroom that feels damp, you can check the basics yourself before calling anyone.
One, find the exterior cap. Stand outside your house. Look for a metal cap on a wall or on the roof above the bathroom. If there is no cap, the duct is dumping somewhere it should not be.
Two, climb into the attic if you have safe access. Find the duct from your bathroom fan. Trace it. Is it sloped toward the exterior, or does it sag? Is it insulated, or is it bare metal? Is the joint at the fan housing sealed, or is there a gap?
Three, test the airflow with the toilet-paper trick from the callout above.
If any of those three checks fail, you have a problem worth fixing now, before it becomes a structural one.
If you want a real fan install on your next bathroom, our Ottawa bathroom renovation page covers our full mechanical scope. Or call 613-862-4555 for a free 48-hour assessment.
The fan is boring. The mould bill is not.
Frequently asked questions
What size bathroom exhaust fan do I need?
A practical sizing rule for a bathroom exhaust fan is 1 CFM per square foot of floor area, with a minimum of 80 CFM. A 5 by 8 foot bathroom needs 80 CFM and a 10 by 12 foot bathroom needs 120 CFM.
Add 50 CFM each for a separate water closet or a steam shower. The Ontario Building Code floor is lower: 50 CFM for an intermittent fan in a typical bathroom, or 25 CFM if the fan runs continuously.
How can I test whether my bathroom fan works properly?
Tape a single ply of toilet paper to the fan grille and turn the fan on. A properly sized fan with a clean duct run holds the paper flat against the grille for as long as it runs. If the paper hangs limp or falls off, the fan is undersized or the duct is blocked.
Then look outside for a metal cap on a wall or on the roof above the bathroom. No cap means the duct is dumping somewhere it should not.
Can a bathroom fan be vented into the soffit?
No. The Ontario Building Code requires bathroom exhaust to terminate outside the building envelope, through a roof cap or wall cap with a damper, not into the attic, the soffit or a crawl space. Venting into a soffit fails because warm, wet exhaust rises and the soffit vents pull it back into the attic.
In a cold January that air condenses on the underside of the roof sheathing and drips onto the insulation.
Does a bathroom fan duct in the attic need insulation?
Yes. Any bathroom fan duct running through an unconditioned attic must be insulated to R-6 minimum, and R-12 is a better standard. Without insulation, warm wet exhaust condenses inside the cold duct and runs back toward the fan housing.
Rigid galvanized metal duct beats flex, which sags and lets condensate pool. Seal the joints with foil tape and slope the run about 1/4 inch per foot down toward the exterior cap so condensate drains outside.
How quickly does poor bathroom ventilation cause mould?
Poor bathroom ventilation typically produces visible mould within 12 to 24 months. In the first 6 months the mirror fogs and ceiling paint softens slightly. From 6 to 12 months, caulk lines discolour and grout near the showerhead darkens.
From 12 to 24 months, black spots appear in the top corners of the shower, ceiling paint near the fan stains, and a musty smell develops. From years 2 to 4, mould reaches the studs and the attic sheathing.
Official references
- Keeping the Heat In, Natural Resources Canada