Mould is fungal growth that appears wherever moisture lingers, basements, bathrooms, around windows, in attics, and behind finishes. The mould itself is really a symptom: the thing that matters is the water source feeding it, whether that's a leak, poor grading, a roof or plumbing problem, or a room that can't dry out because it isn't ventilated.
Cleanup from a few hundred $; fixing the source $500 to $15,000+Address soonInteriorExterior
The quick answer
Visible mould is a symptom, what matters is the water source feeding it (a leak, poor grading, or a room that can't dry out). Cleaning a small patch can be cheap, but fixing the source runs $500 to $15,000+, and sellers often paint over the signs. Treat the water, not just the stain, and don't accept “it was just cleaned”, ask what caused it and whether the cause was fixed.
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Why it matters
Two reasons. Health, mould can trigger allergic and respiratory reactions, more so for sensitive people (Health Canada). And condition, persistent moisture quietly rots framing, subfloors, and finishes, so visible mould often points to a bigger, hidden repair. Sellers frequently paint over or clean up the signs before listing, which is exactly why it's worth knowing what to look for.
How to spot it
A musty smell; staining on ceilings, walls, or baseboards; peeling or bubbling paint; condensation fogging the windows; white powdery efflorescence on basement walls; and fresh paint in one odd spot (a common cover-up). Bathrooms and kitchens with no exhaust fan, and attics where fans vent into the space instead of outside, are repeat offenders. Health Canada's guidance: the size of the mould matters less than finding and fixing what's keeping things wet.
What it costs
Cleaning a small surface patch can be cheap, under a few hundred dollars, sometimes DIY. The real cost is fixing the moisture source: re-grading, basement waterproofing, a roof or plumbing repair, or adding ventilation, which ranges from a few hundred to $15,000+. Professional remediation of a larger affected area typically runs $1,500 to $6,000+.
What to do
Address soon. Treat the source, not just the stain, fixing the water is what prevents recurrence. Have a home inspector assess it, and for anything extensive, an indoor-air or mould remediation professional. Don't accept "it was just cleaned" as the end of the story; ask what caused it and whether the cause was repaired.
Education and triage, not a home inspection. Casaroo flags visible staining, condensation, and moisture clues in listing photos, it can't see behind walls or test the air. A professional confirms the source and extent. We flag; we don't certify.
Common questions
The seller says the mould was cleaned. Is that enough?
Not on its own. Cleaning the stain does nothing about the water that made it. Ask what the source was and whether the source was repaired, because fixing the moisture is what ranges from a few hundred dollars up to $500 to $15,000+.
Can an inspector or a photo tell me how bad it is?
No. Neither can see behind walls or test the air. What they can do is point at the staining, the condensation, the efflorescence, and the fresh paint in one odd spot, and tell you it is worth bringing in someone who can.
Does mould affect insurance?
Insurers care about the water damage that mould implies, and long-term seepage is commonly excluded as maintenance. That is another reason to identify the source rather than accept that it was cleaned up.
How much mould is too much?
Health Canada's framing is useful here: the size of the mould matters less than finding and fixing what is keeping things wet. A small patch with an unexplained water source is a bigger finding than a large patch with an obvious, repaired one.
Health effects vary by person and exposure; remediation cost depends on the source and area. Confirm with a qualified professional. Casaroo flags, it does not inspect.
Last reviewed 2026-06-29.
This guide is general education, not a home inspection and not advice for your specific property. Always
consult the appropriate licensed professional, and get a licensed home inspection before you remove conditions
or buy. Cost ranges are 2026 estimates that vary by region, size, and access; confirm specifics with a
qualified professional.