The pattern behind most attic mould
Attic mould is the most common mould problem in GTA houses, and it follows the same story nearly every time. In winter the air inside your home is warm and carries moisture from cooking, showers and breathing. Some of it gets into the attic. When it touches the underside of the roof sheathing, which is close to outdoor temperature, it condenses. The wood stays damp for weeks, and mould grows.
Toronto’s 1950s to 1970s bungalows and two-storeys, common across Etobicoke, are the classic case: modest attic ventilation to begin with, bathroom fans added later and vented into the roof space, and decades of small air leaks through the ceiling.
Where the moisture gets in
- Bathroom fans that end in the attic. The single biggest cause we see. Every shower pushes humid air straight onto the sheathing.
- Air leaks through the ceiling. Pot lights, the attic hatch, plumbing stacks, wiring holes and the tops of interior walls.
- Blocked soffit vents. Insulation pushed into the eaves, or soffits painted or covered during a renovation, stops the outside air that should flush moisture out.
- Kitchen range hoods and dryer vents that were never ducted all the way outside.
Warning signs to check this winter
- Black or grey staining on the underside of the roof sheathing, often heaviest on the north slope and near the eaves
- Frost on the nail tips poking through the sheathing on cold mornings, then drips on the insulation when it thaws
- Ice dams along the eaves
- A musty smell on the top floor or around the attic hatch
- A bathroom fan you can hear but cannot find a vent for outside
If you see these, a mould inspection confirms whether it is mould, maps how much sheathing is affected and identifies which of the sources above is responsible.
How ventilation and air sealing fix it
A healthy attic stays cold and dry in winter. Outside air comes in low at the soffits, moves up under the roof and leaves high through roof or ridge vents, carrying moisture with it. The Ontario Building Code sets minimum attic vent area for new construction, but older houses often fall short, and renovations frequently block what was there.
The fix that lasts has four parts:
- Duct every fan outside. Bathroom fans and range hoods get insulated ducting to a roof or wall cap.
- Open the soffits. Baffles are installed between the rafters so insulation cannot block the airflow at the eaves.
- Seal the ceiling below. Gaps at fixtures, the hatch, stacks and top plates are air-sealed so less house air reaches the attic.
- Top up insulation once the ceiling is sealed, so the attic floor stays warm and the roof deck stays cold.
Why cleaning alone fails
Spraying or painting stained sheathing without changing the airflow leaves the conditions that grew the mould in place. By the next cold snap the wood is wet again. That is why every YAREX attic mould remediation pairs the cleaning and sealing of the sheathing with the ventilation and air-sealing work, done by the same team.
Attic ventilation in Etobicoke, Mississauga and Oakville
We work on attics across Etobicoke, Mississauga, Oakville and the rest of the GTA from our Etobicoke base. Most attic remediations take one to three days and typically cost $4,000 to $10,000 including the ventilation corrections. The assessment is free. Call (343) 947-2298.
Published October 13, 2026.
The service behind this guide
YAREX Renovation & Restoration, a licensed and insured Etobicoke contractor, removes attic mould (mold) across Toronto and the GTA and fixes the ventilation problem that caused it.
Attic Mould Remediation in Toronto & Etobicoke
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