Bathroom fan not vented outside in my Calgary house, mold risk?

Bathroom Fan Venting and Mould Risk

A bathroom fan that exhausts into the attic, a wall cavity or a soffit instead of outside is one of the conditions that can allow mould to begin growing within 24 to 48 hours on a damp surface, according to an Alberta OHS bulletin. The warm, humid air condenses on cooler surfaces, and if it has been doing so for months or years, you may already have mould in the attic or cavity even if you cannot see it yet.

The first step is to find where the duct actually ends. In a home built before 1990, the attic insulation may be vermiculite, which the Alberta Asbestos Abatement Manual says may be contaminated with asbestos, so have it tested before anyone moves it to trace the duct. If the duct ends inside the attic, you may see staining or mould on the sheathing or rafters nearby. Health Canada classifies mould by area: small is one to three patches each under one square metre, medium is more than three patches or a patch between one and three square metres, and extensive is any patch over three square metres. A person in good health can clean a small area using water and dish detergent, a disposable N95 respirator, safety glasses and disposable gloves, without bleach. Alberta Health Services notes that porous materials such as drywall, carpet and ceiling tiles typically require removal rather than cleaning. For medium or extensive growth, the Canada Mortgage and Housing Corporation says to bring in qualified and trained mould remediation contractors.

Before you do anything else, fix the source. Venting the fan to the outside is how that moisture source gets fixed. Without that repair, any remediation is temporary. Keep indoor relative humidity between 30 and 50 per cent year-round.

A few practical points worth knowing before you hire anyone:

• Alberta Health Services advises that air testing for mould is usually not recommended, especially when you can already see it. Paying for air sampling before remediation in a visible-mould situation adds cost without changing the plan.
• If there is vermiculite in the attic, Health Canada advises not to disturb it or attempt to remove it yourself.
• We found no City of Calgary permit for mould remediation itself, and no Alberta licence or certificate exists for mould remediation work. Ask any contractor what training their crew holds and what industry standard they follow.
• A prepaid contract of $200 or more must be in writing under Alberta consumer protection rules, and you can request a WCB clearance letter online to confirm the contractor has coverage in good standing.

Published Calgary-area prices for mould remediation read in September 2026 run from roughly $500 to $1,500 for a small area, $1,800 to $6,000 for a medium area, and $3,000 to $15,000 or more for an extensive job such as a full attic.

If you want to hear from local contractors, the request form on this site sends your details to Calgary-area remediation contractors listed in the Calgary Construction Network directory.