Why Do Fraser Valley Homes Get Mould So Easily?

Is it really wetter here, or does it just feel that way?

It is really wetter, and the specific way it is wetter is what causes the problem. Abbotsford is reported to have 174 rainy days a year, the most frequent rain of any Canadian city (Current Results). Annual precipitation sits around 1,337 mm, against roughly 1,189 mm at Vancouver airport.

Note which number matters. It is not the volume, it is the frequency. A city that receives its rain in a few heavy events dries out in between. A city that gets rain on close to half the days of the year never fully does. That means more wet-shoe days, more damp coats hanging in entryways, more laundry that cannot go on a line, and indoor humidity that stays elevated for months rather than spiking and falling.

Mild winters compound it. Surfaces in a Fraser Valley home stay cool without freezing, which is the condition in which condensation forms and keeps forming: window reveals, the outside corner of a bedroom, the back of a wardrobe against an exterior wall, the cold spot behind a sofa. Cold air holds less moisture, so warm humid indoor air meeting a cool wall gives up its water right there.

Health Canada’s timeline makes the consequence concrete. “Mould will begin to grow within 48 hours” of water damage, and “unless the cause of the moisture problem has been identified and solved, mould will reappear” (Health Canada). Forty-eight hours is not long in a climate where nothing dries out for a week at a time.

What is it about the housing stock?

Three features of how homes here are built and used turn a damp climate into a mould climate.

Basement suites. The Fraser Valley has a very large number of secondary suites, and a basement suite is a difficult building science problem wearing a friendly face. It is below grade, so it is cool and surrounded by soil that holds water. It often has smaller windows and less cross-ventilation. It frequently has a bathroom fan that was added later, and a kitchen without proper extraction. And it usually houses cooking, showering and laundry, which are the three biggest moisture generators in any home, in the part of the building least able to disperse them.

Crawlspaces. A vented or dirt-floored crawlspace under a Fraser Valley home stays cool and damp for most of the year. Air moves upward through a house, so crawlspace air ends up in the living space. A persistent musty smell in a main floor hallway very often begins under the floor rather than in the room.

Building envelopes that were designed for a drier idea of weather. Older stock in particular tends to have less insulation, more thermal bridging at corners and window frames, and single-point extraction rather than continuous ventilation. That produces cold interior surfaces exactly where furniture blocks air movement. This is a general description of how buildings behave rather than a survey of local homes, but it matches what we find when we get called about a black corner behind a bed.

Add the ordinary habits that push humidity up in a wet region: hanging laundry indoors because outside is not an option, running a dryer that vents into a crawlspace, keeping windows shut from October to April, and shutting the bathroom door after a shower to keep the heat in.

What actually keeps mould out of a Fraser Valley house?

Moisture control, in a specific order. These are Health Canada’s numbers, and they are the ones worth putting on the fridge.

• Keep relative humidity between 30% and 50%. Health Canada’s Healthy Home Guide: “Keep humidity levels between 30 and 50%, using a humidifier or dehumidifier if necessary. Low humidity may aggravate skin allergies and cause respiratory infections. High humidity can lead to mould.” A ten dollar hygrometer tells you where you actually are.
• Run the bathroom fan for at least 30 minutes after your shower, and check it vents outside, not into the attic. A fan discharging into an attic moves your bathroom’s moisture into your roof structure.
• Squeegee and dry the walls around the tub and shower.
• Repair cracked tile, grout and caulking, which is where water gets behind the finish.
• Stop hanging wet laundry indoors where you can. Health Canada is explicit that it raises indoor humidity.
• Vent the clothes dryer outside.
• Pull furniture a few centimetres off exterior walls so air can move behind it.
• Deal with water within 48 hours. A leaking trap, an overflowing washer, a wet carpet after a storm. Two days is the whole budget.

BC Housing publishes a resident-facing Mould Management Guide specifically because this is a provincial reality rather than an occasional accident. If you rent, that document is a useful thing to have read before the conversation with a landlord.

When is mould a cleaning problem, and when is it a building problem?

This is the distinction that decides whether anyone should be scrubbing at all. Health Canada sorts mould by area, and CMHC uses the same thresholds.

“Health Canada does not recommend that an unqualified person clean up large areas of mould.” And if you are comparing against American advice, the US EPA’s 10 square feet figure is a different agency’s threshold in a different country. For a BC home, use the Health Canada and CMHC square-metre numbers.

Beyond area, the practical test is behaviour. It is a cleaning problem if it wipes off a hard surface, stays gone, and the room dries out between uses. It is a moisture problem if it comes back within weeks in the same spot, if it is under or behind a finish rather than on it, if the drywall is soft, or if there is a smell without a visible patch. Health Canada: “Mould that comes back after cleaning is usually an indication that the source of moisture has not been removed. Seek professional assistance.”

Two more things that save people money here. Health Canada states plainly that “HEALTH CANADA DOES NOT RECOMMEND TESTING THE AIR FOR MOULD”, because an air test gives no information about health, does not find the cause, and species identification is not needed in order to remove it. Spores are in every indoor air sample. And porous materials that got wet do not get cleaned, they get discarded: “It is not possible to clean a mouldy mattress”, and mouldy carpets and underpads “should be cut out and discarded.”

What should you clean it with, and what do we not do?

Unscented soap and water, or baking soda on walls. Health Canada is direct about this, in capitals: “DO NOT USE BLEACH TO CLEAN UP MOULD.” The Healthy Home Guide repeats it in plain language: “If you find small amounts of mould, clean them up with dish soap and water. You don’t need to use bleach.”

For walls, the instruction is “clean the surface of the wall with a damp cloth using baking soda or a small amount of unscented soap solution”, keeping the drywall from getting too wet. For washable surfaces it is a cloth with unscented soap solution, sponged clean and dried quickly. The word unscented appears every time, which is a federal document making the case for fragrance-free products more clearly than we could.

The precautions matter in an occupied house. Minimum PPE is safety glasses or goggles, a disposable N95 mask and disposable gloves. Use a HEPA-filtered or externally exhausted vacuum, never an ordinary one. And keep susceptible people out of the area while it is happening: Health Canada names pregnant women, infants, children, the elderly, pets, and anyone with asthma, allergies or other health problems.

We should be equally clear about our own limits. We are a cleaning company. We handle small-category surface mould as part of a normal clean, and we tell you when what you are looking at is past that line rather than taking the job anyway. That boundary is written down on what we don’t do, along with the other work we send elsewhere. For the step-by-step on the part that is genuinely DIY, see how to clean mould naturally in a bathroom. If you are looking for regular help keeping a damp house on top of it, here is what we do in Abbotsford.