Bathroom and kitchen exhaust fans are meant to push warm, moist air outside through a duct to a roof or wall cap. A common shortcut — or a duct that has simply fallen off — lets them dump that air into the attic (or just recirculate it into the room), trapping moisture where no one looks.
$150–600 per fan to duct properly to the exteriorAddress soonInsulationInterior
Why it matters
In a cold Canadian attic, all that bathroom humidity condenses on the underside of the roof sheathing and soaks the insulation, growing mould and rot over time and quietly degrading both the roof structure and indoor air quality. Building codes require these fans to vent to the exterior for exactly this reason, and Natural Resources Canada and CMHC both flag attic moisture from interior air leakage as a problem to control. It's cheap to fix and easy to verify, yet almost never noticed by buyers.
How to spot it
Run the fans and check where they actually exhaust: no cap visible on the roof or exterior wall; a flex duct lying loose or disconnected in the attic; a range hood that just blows back into the kitchen (recirculating); or damp, matted, or mouldy insulation and dark, stained sheathing above a bathroom.
What it costs
Roughly $150–600 per fan to duct it properly to the exterior with insulated duct and a proper exterior cap.
What to do
Address soon. Have each fan vented to the outside with insulated duct and a backdraft-damper cap, and check the attic for moisture damage that may already have started.
Education and triage, not a home inspection. Casaroo flags improper venting from your fan and attic photos — easy and inexpensive to correct, and a contractor confirms.
Common questions
What is Exhaust fans venting into the attic?
Bathroom and kitchen exhaust fans are meant to push warm, moist air outside through a duct to a roof or wall cap. A common shortcut — or a duct that has simply fallen off — lets them dump that air into the attic (or just recirculate it into the room), trapping moisture where no one looks.
Why does it matter for home buyers?
In a cold Canadian attic, all that bathroom humidity condenses on the underside of the roof sheathing and soaks the insulation, growing mould and rot over time and quietly degrading both the roof structure and indoor air quality. Building codes require these fans to vent to the exterior for exactly this reason, and Natural Resources Canada and CMHC both flag attic moisture from interior air leakage as a problem to control. It's cheap to fix and easy to verify, yet almost never noticed by buyers.
How can I spot it?
Run the fans and check where they actually exhaust: no cap visible on the roof or exterior wall; a flex duct lying loose or disconnected in the attic; a range hood that just blows back into the kitchen (recirculating); or damp, matted, or mouldy insulation and dark, stained sheathing above a bathroom.
How much does it cost to fix?
Roughly $150–600 per fan to duct it properly to the exterior with insulated duct and a proper exterior cap.
Venting to the exterior is required by building code; a contractor confirms. Casaroo flags, it does not inspect.
Last reviewed 2026-06-27.
Casaroo reviews each guide against current pricing, code, and insurer practice. Cost ranges are 2026
estimates that vary by region, size, and access — confirm specifics with a licensed professional.