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The Bathroom Fan That Vents Into the Attic: Why It Is the Most Common Cause of Attic Frost and Mould, and the $350 Fix
Roofing
The Bathroom Fan in the Attic: One Shower a Day, Twenty Years, and a Black Patch on the Roof Deck
Go into the attic of a house built between 1960 and about 2000 and there is a fair chance you will find a flexible duct from the bathroom fan lying on the insulation, ending nowhere, or pointed hopefully at a roof vent. Every shower sends a few litres of water vapour up it. In January that vapour hits sheathing at -15 C and becomes frost, and in March the frost becomes rain, and by the tenth year the sheathing above it is black. It is the single most common thing a roofer finds that is not a roofing problem, and it is the cheapest thing on this site to fix.
What it does
- Frost and drip. The moist air condenses on the coldest surface, the underside of the roof deck, as frost all winter. A mild day melts it and it rains onto the insulation and through the ceiling: a “leak” that only happens in a thaw, with no rain.
- Mould and rot. Wet sheathing grows black mould on the north side first, then delaminates. Roofers find it at tear-off and the deck replacement costs more than the fan fix did.
- Wet insulation. Cellulose and fibreglass that get wet lose most of their R-value and stay damp for months.
- Ice dams. The warm wet air also warms the deck, which melts the snow above it, which is how a dam starts. The ice dam page puts this fan at the top of the air-sealing list.
A dryer vent into the attic does all of the above faster and adds lint, which is a fire. It goes outside, with a smooth metal duct, no exceptions.
What the Code says
Exhaust fans must discharge to the outdoors, through a duct, with a hood or cap outside that keeps weather and birds out, and in a cold attic the duct must be insulated so the moist air does not condense inside it and run back to the fan. “Near a roof vent” is not outdoors. “Into the soffit” is not outdoors either, because the soffit is the attic’s air intake and the moist air is drawn straight back in. Every bathroom with a tub or shower needs a fan (or a window, in older code, which nobody opens in January), and a new house has them ducted and inspected before drywall. Older houses were often done by whoever installed the fan, with a box of flex duct and no plan.
Check yours this afternoon
- Outside, look for a vent hood on the roof (a low domed cap with a flap) or on a wall or gable end near the bathroom. One per fan. If you cannot find one, the fan is venting somewhere it should not.
- In the attic, follow the duct from above the bathroom. It should be a 4 in. insulated flexible duct (a silver or white padded tube) or a rigid pipe wrapped in insulation, running as short and straight as possible, slightly uphill, to a hood through the roof or out a gable wall. Not lying on the insulation, not ending at a vent, not taped to the side of a box vent.
- Run the fan and go outside: you should feel air at the hood and see the flap lift. If the flap is stuck or painted shut, the fan is pushing against a closed door and most of the moisture stays in the bathroom.
- While up there, look at the sheathing above the bathroom. Black staining or frost means it has been doing this for a while.
The right way
- Through the roof: a roof vent hood made for exhaust (with a damper and a screen, not an attic vent), flashed like any other penetration, with the insulated duct running up to it with a slight rise so condensation drains back toward the fan rather than pooling. Shortest run, best airflow. The preferred route when the roof is being done anyway.
- Through a gable wall: a wall hood with a damper, duct running slightly up to it. Good when the bathroom is near a gable end; keeps a hole out of the roof.
- Not through the soffit unless with a purpose-made soffit exhaust vent that discharges outward and down, and even then only when nothing else is possible, because some of it comes back in.
- Rigid or insulated flex, 4 in. (6 in. for a big fan), no more than one or two gentle bends, joints taped with foil tape, the duct sealed to the fan housing, and the fan housing itself sealed to the ceiling drywall so it is not also an air leak into the attic.
- A fan with a timer or humidistat so it runs 20 minutes after the shower, which is when the moisture actually leaves.
What it costs
| Work | Cost |
|---|---|
| Duct the fan through the roof during a re-roof | $350 per fan (the roofer is already up there) |
| Duct through the roof as a standalone job (roofer or HVAC) | $500 to $900 |
| Duct through a gable wall (handyman or HVAC) | $350 to $700 |
| Replace the fan with a quieter one with a timer, while at it | $150 to $400 plus electrician if the wiring changes |
| Replace mouldy sheathing found above it, per 4×8 sheet | $120 with a roof; much more alone |
On the roof replacement calculator it is one checkbox. If you are not doing the roof, it is the cheapest call to an HVAC company you will make this year.
Questions people ask
Can a bathroom fan vent into the attic?
No. The Ontario Building Code requires exhaust fans to discharge outdoors through a duct with an exterior hood. Into the attic, at a roof vent or into the soffit is not outdoors, and it causes frost, mould and ice dams.
Should a bathroom fan vent through the roof or the wall?
Either, properly: a damper hood, an insulated duct, a short run with a slight rise. Through the roof is shortest for most bathrooms and best done with a re-roof; through a gable wall avoids a roof penetration.
Why is there frost in my attic?
Warm moist air from the house reaching cold sheathing: a bathroom or dryer fan venting into the attic, or air leaks around the attic hatch, pot lights and plumbing stacks. Find the fan duct first; it is the biggest single source.
How much does it cost to vent a bathroom fan outside?
$350 as part of a roof replacement; $500 to $900 as a standalone job through the roof; $350 to $700 through a wall. Before HST.
Does the duct need to be insulated?
In a cold attic, yes. An uninsulated duct lets the moist air condense inside it, and the water runs back down and drips out of the fan onto the bathroom floor, which is the other symptom people call about.
Follow the duct
If it ends in the attic, it ends in your roof deck. Three hundred and fifty dollars with the roof, a few hundred more without, and the black patch stops growing.
Sources and further reading
- Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
- Addressing moisture and mould in your home: Health Canada. What attic mould means for the house and when to call someone.
- Keeping the Heat In, Chapter 5: Roofs and attics: Natural Resources Canada. The best free guide to attic insulation, air sealing and ventilation written for Canadian houses.
