Find the water before counting the colonies
Mould is a moisture symptom: treating stains without fixing the water guarantees the colonies return in the same spot. The sources we meet most in Montreal are the spring sewer backup, a slow washer-hose leak, condensation behind an exterior-wall closet, and capillary rise in a poorly drained concrete-block foundation.
The visit starts with a moisture meter on suspect walls and a thermal camera to track the water path inside the wall. We open where the reading justifies it, not everywhere: the cut reveals the real extent behind the gypsum, and that extent, not the visible stain, sets the removal area and the containment class.
Removal under negative pressure
As soon as the contaminated surface exceeds about one square metre, the zone is worked under containment: polyethylene over thresholds and vents, negative pressure held by a HEPA-filtered air scrubber, and circulation through an airlock. The goal is simple: cutting gypsum releases millions of spores, and negative pressure keeps them in the zone instead of the stairwell.
Contaminated drywall is cut out and bagged sealed. Structural lumber — plates, joists, plywood — is treated on site by brushing under HEPA suction and local sanding, then dried below 16 percent moisture before any rebuild. The attic, crawlspace and basement are checked too, because spores travel with air currents.
Rebuild so it never comes back
The rebuild follows the removal, never the reverse. Basement walls go back in moisture-resistant gypsum, with the vapour barrier on the right side and acoustic sealant at the plates. A bathroom redone properly gets a waterproofing mat under the shower tile, not just fresh grout.
Ventilation is corrected at the same time: a range hood discharging into the attic or an undersized dehumidifier sets up the next invasion. We target a relative humidity under 50 percent in summer, with an exchanger or dehumidifier sized to the basement volume, and the guidelines go into the final report.
The air test that closes the file
After removal and drying, we sample air in the treated zone and in a reference room outside it. The lab counts and identifies the spores: the report compares both samples, and the file closes when the treated zone returns to reference levels, without dominance of the species found at the start.
That document works for an insurer after a backup, for a buyer in a transaction, or for a tenant and landlord who want proof instead of a promise. It is delivered with before-during-after photos and the disposal certificates for contaminated materials.
Frequently asked questions
How do I know if a stain is mould?
Mould forms clusters that return after washing and spread with humidity; a water stain stays put and pales as it dries. In doubt, tape lifted from the surface and analysed in a lab identifies the species within days.
What area justifies containment?
North American guidelines set the limit around 1 square metre, about 10 square feet: below it, careful cleaning is enough; above it, we contain with HEPA negative pressure to avoid seeding spores through the house.
Can I clean a small area myself?
On less than a square metre of non-porous material, detergent washing and full drying does the job. On gypsum or textured ceiling, the material gets replaced instead of washed: the colony anchors in the paper core and returns under the paint.
Is a sewer backup always contaminated?
Sewage water carries bacteria on top of spores, and affected drywall gets replaced rather than dried: the gypsum paper holds contaminants even after surface drying. Insurance often covers preventive cutting; we document the extent for the claim file.
Related articles
That basement smell: spotting mould before it climbs
The nose detects a mould colony before the eyes. The zones to open first, the test that settles it, and the…
The crawlspace feeding your mould — and how to dry it out
Crawlspace humidity climbs through floors and feeds mould in living rooms. The drainage, barrier and…
A contaminated material spotted in your building?
Describe what you see — we tell you which procedure applies and how it will be handled.