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 $500–15,000+Address soonInteriorExterior
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 $1,500–6,000+.
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
What is Mould & hidden moisture?
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.
Why does it matter for home buyers?
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 can I 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.
How much does it cost to fix?
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 $1,500–6,000+.
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.