How to Test for Mould in Your House (A Rain-City Guide for Vancouver)

Search this question and the first thing you are offered is a petri dish, a swab kit or an air pump. Health Canada takes the opposite position, in one flat sentence: it does not recommend testing the air for mould. The reason is not squeamishness about cost. An air test tells you nothing about your health, and it does nothing about the water that grew the mould in the first place.

The Canadian Centre for Occupational Health and Safety goes further and calls a visual inspection the most reliable method of identifying mould problems. That is a strong statement from a body whose whole job is measuring workplace hazards, and it exists for a practical reason. There is no numerical exposure limit for mould in Canadian indoor air. No regulator publishes a spore count that separates a safe home from an unsafe one, so a lab result arrives with nothing to compare it against.

The species question dissolves for the same reason. Health Canada recommends controlling dampness and cleaning up any visible mould regardless of the type of mould present. Whether the lab comes back with Cladosporium or the black-mould name everyone fears, the response is identical: find the water, stop it, clean the surface, and get a qualified professional in if the patch is large. Paying to learn the name changes none of those steps.

This is the part national guides cannot write for you. Warm indoor air holds moisture; cold surfaces make it let go. Through our wet season the coldest surfaces in a Metro Vancouver home are almost always at the perimeter of a room. That is why growth shows up as a band along an outside corner while the middle of the same wall stays perfectly clean. Once you know that, you stop searching randomly and start searching the envelope.

Older low-rise stock across East Vancouver and North Vancouver makes this easy to see. Aluminium window frames and single glazing conduct cold straight through to the indoor surface, so the reveal and the sill sit several degrees below the rest of the room and collect water every cold night. That is the same mechanism behind mould on window frames and sills, and it is why wiping the glass never fixes it.

Then add the things that block air movement. A wardrobe pushed flat against an exterior wall, a headboard against the same, a stack of storage boxes in a garden-level suite. The wall behind them stays colder and never dries, and because you cannot see it, growth gets months of undisturbed time. Most of the genuinely bad patches we find are not hidden inside a wall cavity at all. They are on the visible face of a wall that nobody has looked at since they moved in.

Mould that is actively growing gives off a musty, earthy, faintly sweet smell, and most people pick it up long before there is anything to point at. Do not dismiss it because a surface inspection came back clean. Dismissing it is how a small patch behind a wardrobe becomes a wall.

Use the smell as an instrument. Close the suspect room, with the window shut, for three or four hours. Then walk in and pay attention to the first two seconds before your nose adapts, because adaptation is fast and you get one clean read. Move around the room at floor level, then at head height, then inside the closet. The smell will be noticeably stronger within a metre or so of the source.

In apartments and condos, work out whether the smell belongs to your unit at all. If it arrives through a bathroom or laundry vent, or is strongest at a shared wall, the source may be in a neighbouring unit or in the building's own assembly. That makes it a strata matter rather than a cleaning one. Our guide to a musty smell in an apartment walks through separating those cases.

Health Canada's position is about routine testing, not an absolute ban on ever sampling. Sampling earns its cost when a visual inspection has genuinely run out of road, or when someone other than you needs to be convinced.

The first case is a smell you cannot source. You have done the closed-room test, emptied the closets, checked behind the furniture, and if you own a meter, compared readings on the exterior walls against a known-dry interior wall. There is still a clear musty smell. At that point an inspection with thermal imaging and moisture mapping can find water inside an assembly that no amount of looking will reveal, and that is worth paying for.

The second is documentation. If you are in a tenancy or strata dispute, or buying a home, a dated report from an independent inspector carries weight that your own photographs do not. The third is verification after a large cleanup, where you want evidence that the area is genuinely clear before the wall goes back together. In all three, hire an inspector who does not also sell you the remediation, so the finding and the invoice are not written by the same hand.

Instead of a spore count, use area. The federal guidance sorts mould into three sizes and gives a different answer for each. Small is three or fewer patches with a total area under 1 square metre. Medium is more than three patches, or patches totalling between 1 and 3 square metres. Large is a single patch bigger than 3 square metres, and for that Health Canada recommends a qualified professional assess and clean it.

Those numbers are easier to judge than they sound. One square metre is about a bath towel laid out flat. Three square metres is closer to two interior doors set side by side. So if the growth on your bathroom ceiling would sit inside a towel outline, you are in small territory. If it runs the length of a bedroom wall, you are not. The reason for the professional threshold is physical: a patch that size means water has been sitting in the assembly long enough that you are looking at a repair and a containment job, not a surface you can wipe.

There is a second, quieter test that matters as much as size: whether you have found and stopped the water. Clean growth off a cold corner while the humidity that fed it is still sitting above the 30 to 50 percent band and it simply comes back, because nothing about the conditions has changed.

Once the moisture is genuinely fixed, the room is worth a proper reset. A deep clean goes after exactly the surfaces this walkthrough sends you to and ordinary cleaning skips: grout lines, shower doors and tracks, exhaust fans and vents. For the small patches you are handling yourself, our mould and mildew removal guide has the surface-by-surface method, and we have written separately on what baking soda actually does to mould.

A large share of Metro Vancouver households rent, and mould is one of the most common repair disputes here. The BC Residential Tenancy Branch sets out that landlords are responsible for most repairs to a rental unit and must comply with the health, housing and safety standards required by law, including local maintenance bylaws. Tenants handle minor upkeep, like changing a light bulb.

The mechanism matters. The Branch is explicit that tenants must request repairs in writing, describing the problem and allowing a reasonable amount of time for the repair. A text or an email counts, and a verbal mention in the hallway does not. If the landlord does not carry out the repair, a tenant may apply for dispute resolution and ask for an order that the repairs be made, or for money to cover the inconvenience.

So the useful move is a dated written record, not a petri dish. Photograph the patch with something in frame for scale, note the date, write down what you saw and where the water seems to be coming from, and send it. Keep doing the ordinary things that reduce the moisture load in the meantime: run the bathroom fan, and do not dry laundry indoors through the wet season. That early dated photo earns its keep again at the end of the tenancy, because the move-out condition inspection is where the state of the unit gets recorded and compared. Our move-out cleaning checklist covers the surfaces that inspection scores.

Health Canada recommends maintaining indoor relative humidity between 30 and 50 percent, using a dehumidifier as necessary. That is the measurement worth taking, because unlike a spore count it comes with a target and an action. Too high, and you dehumidify or ventilate. A basic hygrometer is an inexpensive one-off purchase and keeps working for years, which is more than a single-use kit can say.

Place it where the problem is rather than where it is convenient. A hygrometer in the middle of a warm living room will read comfortably while the bedroom you keep cool and closed sits far higher. Put one in the coolest bedroom and one in the basement or garden level, and read them on a cold, wet evening rather than a dry afternoon, because that is when condensation actually happens.

Then work on the two moisture loads most Metro Vancouver homes underestimate. The first is the bathroom. Run the fan during the shower and for a good twenty minutes after, and check once that the duct actually terminates outdoors rather than into the attic space, which in older houses it often does not. You can usually confirm that from outside by finding the vent hood on the wall or roof, or with a look in from the attic hatch. Do not crawl across the joists or disturb old loose-fill insulation to chase it; that is a job to hand to a contractor. The second is laundry dried indoors, which puts litres of water into the air across the wet season. Fix those two and a cool, closed room usually settles back inside the 30 to 50 percent band, at which point mould has no reason to grow.