Humidity and mould in a house: when should you worry?

Health Canada recommends keeping a home’s relative humidity between 30 and 50%. Above that, water condenses on cold surfaces (windows, wall corners, basement) and mould sets in within days.

A small patch on a shower joint cleans off. A stain that comes back, a cellar smell or a spreading surface signal a water source to find, not a cleaning problem. The inspector reports what they see and sometimes measures humidity, but does not test for mould.

Stains and rings on ceilings and at the base of walls, condensation on windows, smell, bathroom and range-hood ventilation, the dryer exhaust, attic insulation and ventilation, basement dampness. They may use a hygrometer or a moisture meter on a suspect surface.

Visible mould over a large area, or a smell with no visible source, warrants an evaluation by a specialist before buying: the origin (infiltration, condensation, leak) determines the cost.

Mould is a symptom. What you buy is the cause: gutters, grading, drain, roof, insufficient ventilation, plumbing leak. A well-ventilated house with a dry basement in winter is the best sign.

For someone with asthma or allergies, a chronically damp house is paid for in health. It is a decision criterion, not only a price one.

A hygrometer costs a few tens of dollars. An installed air exchanger, a few thousand. Professional cleaning of a limited mouldy surface, hundreds to a few thousand. Correcting the cause follows its own orders of magnitude (gutters, drain, roof).

Confirm all of this with quotes.

Here is what I would do.

1. Ask the seller for the history of water damage and insurance claims.
2. Check that every bathroom and the kitchen have ventilation exhausting outdoors.
3. Measure basement humidity in winter and summer after moving in, and aim for 30 to 50%.
4. Treat the cause before redoing finishes.

I gather the moisture signs noted throughout your report (roof, walls, basement, ventilation) to show whether they point to a single cause, and I say so plainly in the overall opinion.

Does your report mention this? Send it to me. In five minutes, I tell you what is priority, what to have checked and what to ask the seller.

Effects depend on the area, the duration of exposure and the occupants’ sensitivity. Health Canada recommends removing any mould and correcting the source, whatever the type.

An evaluation by a specialist is, when there are signs. An air test alone, without inspecting the source, is inconclusive.

A little, in deep cold. Abundant, frequent condensation indicates humidity that is too high or ventilation that is insufficient.