Mould in a Winnipeg Crawlspace: Why It Grows in a Dry City
Low annual rainfall does not protect a crawlspace. Condensation does the work instead — in both directions, in different seasons.
The question we hear most
“How can there be mould down there? This is one of the driest cities in Canada.”
It is a fair question, and the answer is that crawlspace mould in Winnipeg rarely comes from rain. It comes from condensation — moisture that was already in the air, deposited onto a surface that was colder than the air’s dew point.
Our climate produces those conditions twice a year, in opposite directions.
Winter: moisture from inside the house
In January an unconditioned, vented crawlspace runs close to outdoor temperature. Meanwhile the house above it is warm and carries indoor humidity from cooking, showers, laundry and breathing.
That warm, moist air moves down through the floor assembly — around plumbing penetrations, through gaps in the subfloor, along duct chases. When it reaches the cold underside of the subfloor and the cold rim joists, it condenses. Below freezing, it frosts.
Come the first thaw, that frost melts into the wood. We have photographed sheathing with visible ice crystals in February and active mould growth in the same spot in June.
Summer: moisture from outside
By July the direction reverses. Outdoor air is warm and humid. It enters through the open foundation vents and meets soil and framing that are still cool, because the ground warms slowly.
The air condenses on those cool surfaces. The vents that were supposed to dry the crawlspace are now the delivery mechanism for the moisture.
This is why the old vented-crawlspace strategy has been largely abandoned in building science. In a cold climate with humid summers, vents work against you in both seasons.
What mould needs, and what you can remove
Mould requires an organic food source, moisture and moderate temperature. A crawlspace supplies the food in abundance: joists, subfloor sheathing, beams, paper-faced batt insulation, stored cardboard.
You cannot remove the food. Temperature is not practical to control downward. So control comes down to moisture — which is why every legitimate remediation starts with finding the water, not with spraying something.
The threshold that matters: wood holding moisture content above roughly 19 percent supports decay organisms and mould. Below that, it does not. That is the number we measure.
Why it matters upstairs
A crawlspace is not sealed off from your living space. Air moves between the two continuously, and during the heating season the stack effect gives that movement a consistent direction: in at the bottom, out at the top. Crawlspace air is being drawn into the house for months at a time.
We are not qualified to tell you what that does to your health — that is a conversation for your physician. What we can tell you plainly is that the air is connected, which is why a musty smell on the main floor so often traces to a space nobody has looked at in a decade.
What proper remediation involves
Containment and protection. So spores are not distributed through the house during the work, with negative air and filtration where the situation warrants it.
Removal of porous materials that cannot be cleaned. Contaminated fibreglass or cellulose insulation, cardboard, stored soft goods. These get bagged and removed, not treated in place.
Mechanical cleaning of framing. Surface growth on structural wood is abraded or HEPA-vacuumed and wiped. A coating sprayed over a living colony does not remove it.
Structural assessment. Prolonged moisture causes section loss, not just staining. Sill plate ends, post bases, beam ends and sheathing get probed to find where decay has gone structural. In Winnipeg we find this most often on the north and east sides, and under bathrooms and entries.
Moisture correction. Drainage, sealing, perimeter insulation, humidity control — whichever combination the diagnosis calls for.
What to be sceptical of
Fogging as a solution. Antimicrobial fogging has a limited role, but a contractor who tells you fogging a crawlspace solves mould is selling a cosmetic result.
Species testing by default. For a visible colony, identification rarely changes the work: remove the growth, fix the moisture. Testing is appropriate for disputes, insurance claims, real estate transactions, or when a clinician specifically asks. We will arrange sampling when it serves a purpose.
Bleach. It is mostly water, which adds moisture to wood, and it does not penetrate porous material.
After the work
Framing moisture content should be measured again, and it should be trending down. Relative humidity in a sealed, conditioned crawlspace should sit around 50 to 55 percent year-round.
If the smell upstairs persists after remediation and sealing, that points to a remaining moisture path or a second source — and it means somebody should come back and find it.