Window sills grow mold every Calgary winter, how to stop it?

Window sill mould in a Calgary winter is usually a condensation problem. Warm indoor air meets the cold glass and frame, water forms, and mould can begin growing on that damp surface within 24 to 48 hours, according to an Alberta OHS bulletin.

Why It Keeps Coming Back

Possible reasons to check are indoor humidity above the range Health Canada recommends, air that cannot reach the glass, or a window that runs cold. Cleaning the mould without dealing with the moisture means it can return.

Start with the moisture, not the mould. Keep indoor relative humidity between 30 and 50 per cent, which Health Canada identifies as the target range. A basic hygrometer from any hardware store tells you where you stand. Run bathroom and kitchen exhaust fans longer after showers and cooking. If you have a humidifier on the furnace, turn it down through the coldest months. Move furniture or curtains that block air from reaching the glass, because still air at the sill stays colder and wetter.

For the mould itself, Health Canada classifies patches that are each under one square metre and number three or fewer as small, and says a person in good health can clean those areas. Use water and dish detergent, wear safety glasses or goggles, a disposable N95 respirator and household disposable gloves. Alberta Health Services notes that porous materials such as drywall and ceiling tiles typically need removal rather than cleaning, so a sill that is soft, stained through or crumbling is a sign the material itself is affected. Dry the sill completely after cleaning and keep the area ventilated.

A few things worth checking once the surface is clean:

• Inspect the window seal and weatherstripping; a failed seal lets cold air in and raises condensation on the frame
• Look at the wall below the sill for soft drywall or paint bubbling, which can mean moisture has already moved into the wall cavity

If you find more than three patches, or any single patch larger than one square metre, Health Canada calls that medium, and the CMHC guide says qualified and trained maintenance staff should handle it; above three square metres, CMHC says a qualified and trained mould remediation contractor. If the mould has spread behind the drywall or into framing, that is a larger job. Alberta Health Services advises that air testing for mould is usually not recommended when you can already see it; the priority is finding and stopping the water source.

We found no City of Calgary permit for mould remediation, and no Alberta licence or notification requirement for mould work the way there is for asbestos. If you do hire someone, published Calgary-area prices read in September 2026 put small-area remediation at $500 to $1,500 and medium-area work at $1,800 to $6,000, with the spread driven by how far the moisture has travelled and whether any drywall or framing needs replacing.

If the problem is recurring across multiple windows or you suspect it has reached the wall structure, it is worth having a contractor look at it. Before agreeing to any scope of work, ask the contractor exactly where they traced the moisture source and what they are doing to prevent it from returning, because remediation without that answer is just cleaning the same spot twice.