Mould is a significant health and structural concern in Ontario homes. High summer humidity combined with cold basement surfaces in winter creates ideal mould conditions. This guide covers identification, professional remediation thresholds, and prevention through building science.
When Professional Remediation is Required
Health Canada guidance recommends professional remediation when: visible mould exceeds 1 square metre, mould is in HVAC systems, occupant has respiratory conditions, or mould is suspected behind walls from prior water damage.
Common Mould Types in Ontario Homes
Cladosporium: Most common. Green/black, typically on windowsills and bathroom surfaces. Grows when indoor RH exceeds 55% on cold surfaces.
Stachybotrys (black mould): Requires prolonged wet conditions (water-damaged drywall, wood). Requires professional remediation plus source moisture correction.
Penicillium/Aspergillus: Common in water-damaged carpets and insulation. Blue-green, powdery appearance.
Control humidity: Maintain RH below 50% in summer with AC/dehumidifier. Use HRV (Heat Recovery Ventilator) in winter.
Air sealing: Summer humid air infiltrating into cool basement condenses on cold surfaces. Air sealing at rim joist and foundation penetrations is the most effective long-term prevention.
This guide is solid - want to add a couple of GTA-specific notes from the waterproofing side, since that is what most calls turn into.
The pattern we see this time of year is almost always the same: snowmelt soaks the soil right against the foundation, then a cold spring rain pushes hydrostatic pressure up through the slab or the cove joint, and three weeks later the homeowner notices a musty smell or a fuzzy patch behind the basement couch. By the time it is visible on drywall, the cavity behind has usually been wet for a month.
A few practical things that come up over and over:
Test before you tear out. A $40 swab kit or an actual indoor air quality test will tell you whether you are dealing with surface Cladosporium or actual Stachybotrys behind the wall. The remediation plan is very different.
Run a dehumidifier in the basement at 45-50% RH year-round if you have a finished basement in the GTA. Cheaper than any remediation invoice.
Fix the source first. Repainting over a stain, or even removing the affected drywall, is wasted money if the grading, downspout extensions, weeping tile, or window-well drainage are still pushing water in. We have lost count of the second-opinion calls where someone paid for remediation twice in 18 months because nobody traced the water.
Insurance: most GTA policies will cover sudden water damage but explicitly exclude long-term moisture and mould caused by maintenance issues. Document the source and the timeline carefully.
If anyone reading this has visible mould plus a finished basement, do not start ripping out drywall yourself - the spore release during demolition is the worst part of the problem. Containment plus negative-air machines is what makes pro remediation worth the money.
Spring is the busiest mould-call month for us. If you are seeing efflorescence (white chalky stuff) on basement walls, that is your early warning - water is coming through, mould is the next chapter.