It is not that it rains a lot. It is that it never stops.
Vancouver gets about 42 inches of rain a year. That is only four inches above the national average, and considerably less than Miami, Nashville, or New Orleans. What makes it different is the distribution. Our rain falls on roughly 160 days, almost all of them between October and May, mostly as persistent light drizzle rather than heavy downpours. December alone averages around five inches across nearly twenty rainy days. A city that gets 60 inches in violent summer thunderstorms and then dries out in the sun has a fundamentally easier time than we do.
The second half of the equation is temperature. Vancouver winters are mild — typically in the 40s, with less than a couple of inches of snow a year and hard freezes that last a night or two at most. Mold needs moisture, an organic food source, and a temperature range that our winters sit squarely inside. In a Minnesota January, growth pauses because everything is frozen. Here it does not pause at all. It just keeps going, from the first October rain until things finally dry out in June.
Add relative humidity that averages in the mid-80s through the winter months, and you have a climate where a wet surface simply stays wet. That is the whole story, and everything below is a variation on it.
The crawl space problem
Pacific Northwest builders have raised houses off the ground for a century, and it was a smart response to wet soil. The standard detail was a shallow crawl space with foundation vents around the perimeter, on the reasonable theory that airflow carries moisture away.
It works beautifully in July. From October to May it works against you. When you pull 85-percent-humidity outdoor air into a space where the joists, ductwork, and pipes are cooler than that air, moisture condenses out onto them. Meanwhile the bare or thinly covered soil below evaporates groundwater upward continuously. Wood moisture content climbs past roughly 20 percent, which is the threshold where mold takes hold, and growth starts on the joists and the underside of the subfloor.
Then the stack effect distributes it. Warm air rising through the house pulls replacement air from the lowest available space, which means a meaningful fraction of the air you breathe upstairs passed through the crawl space first. This is why the most common symptom of a Clark County crawl space problem is a musty smell in the living room.
The attic problem
The same physics, running the other direction. Warm, humid indoor air rises. If it reaches the attic — through an unsealed hatch, can lights, plumbing penetrations, or a bath fan that terminates there instead of outside — it meets a roof deck that is cold from November through April and condenses on it. Do that nightly for five months and you get the grey-black staining on the sheathing that Vancouver home inspectors write up constantly.
Two local factors make it worse. Blown-in insulation retrofits from the 1980s and 1990s frequently buried the soffit vents, which cuts off the intake side of the ventilation path and leaves the exhaust vents with nothing to pull. And our tree cover — especially the mature Douglas firs across Cascade Park, Felida, and Camas — keeps north roof planes in shade for months, so they stay colder and condense more. Ice dams, the northern-climate version of this problem, are essentially a non-issue here; moss is our equivalent.
The building-tightness problem
Newer homes have the opposite issue from old ones, and it catches people off guard. A 1920s bungalow leaked air constantly, which was expensive to heat but did dispose of indoor moisture. A 1998 Orchards two-story or a 2021 Ridgefield build does not leak. A family of four still puts several gallons of water a day into the air through showering, cooking, laundry, and breathing, and if the bath fans do not run and the windows stay shut — which they do, from October to May — indoor humidity climbs all winter and condenses on the coldest surface in the building. That is why brand-new houses in Clark County get mold in closet corners and behind furniture.
Water from outside
Beyond the seasonal humidity, the recurring intrusion sources here are: gutters and downspouts discharging at the foundation; grade that has settled and now slopes toward the house; footing drains that were never installed or have silted up; wind-driven rain, which is a serious factor at the mouth of the Gorge in Camas and Washougal; and, on the low ground, actual flooding. Clark County's last major flood was February 1996, when extended warm rain on early snowmelt damaged roughly 300 homes and destroyed 82. The 1948 Columbia flood remains the benchmark at a 31-foot crest.
What actually helps
The interventions that work here are unglamorous and mostly cheap. Get a hygrometer and keep indoor humidity under 50 percent in winter. Run bath fans during the shower and twenty minutes after, and make sure they exhaust to the exterior. Extend your downspouts. Clear your gutters, twice, because fir needles do not fall on a schedule. Keep furniture a few inches off exterior walls. Get a proper vapor barrier over your crawl space soil. And look under your house once a year — more Clark County mold goes undiscovered for lack of a flashlight than for any other reason.