Vancouver, WA and Portland, OR see over 40 inches of rain a year and relative humidity that sits between 63% and 83% for most of the fall, winter, and spring. That combination is exactly what mold needs to take hold in a bathroom, and an undersized or missing exhaust fan is the single most common cause we find during remodels.
In this guide, we'll break down exactly how bathroom ventilation should be sized and installed for our climate, the code minimums Clark County and Multnomah County inspectors actually check, and the everyday habits that keep mold from coming back after a remodel.
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These are HVI and ASHRAE 62.2 sizing guidelines, plus the code-minimum intermittent and continuous exhaust rates that Washington and Oregon's IRC-based residential codes require. Always confirm final requirements with your local building department.
Average relative humidity of 63%-83%, peaking December through February (source: regional climate data)
Over 40 inches of rain a year, concentrated October through April
Sustained indoor humidity above roughly 60% is enough to support mold growth on drywall paper and grout within days
A moisture source: shower steam and condensation on cold tile, mirrors, and windows
An organic surface: drywall paper, grout haze, wood trim, and unsealed caulk
Time without airflow: a fan that never runs, runs too briefly, or was never installed
We size every fan to the room using the HVI method above, not a generic one-size-fits-all unit, and mount it near the shower or tub where moisture actually originates rather than tucked by the door where it moves the least air.
Every fan we install vents to the exterior through a roof or wall cap, never into an attic or crawlspace, and includes a damper that closes when the fan is off to stop outside air and moisture from creeping back in.
A basic timer switch that keeps the fan running 20 minutes after you leave the bathroom is the minimum; on larger or higher-humidity bathrooms, we recommend a humidity-sensing switch that turns the fan on automatically once moisture rises, so it works even when you forget.
A fan is only as good as how often it actually runs, and a loud fan gets switched off. We spec continuous-duty fans at 1.0 sone or quieter and standard fans at 3.0 sones or below, the same thresholds ENERGY STAR and ASHRAE 62.2 use to define acceptable noise.
Fresh air alone can't fix a leak. We pair every ventilation upgrade with a vapor-permeable moisture barrier behind tile, proper shower-pan flashing, and sealed grout, so the wall assembly can dry out even during the wettest stretch of a Vancouver winter.
Run the fan during every shower and for 20 minutes after, every time
Squeegee glass and tile after showering to cut standing moisture
Fix a dripping faucet or a running toilet the day you notice it, not the month you get around to it
Schedule an annual check of the fan, duct, and grout, an out-of-warranty motor or a cracked duct connector is an easy thing to miss until the ceiling stains
A 50 CFM fan installed for a small original bathroom often gets carried over into a remodel that adds square footage or a second shower. The fixture count and floor area both went up, but the airflow didn't, so the new bathroom is effectively underventilated from day one.
We still find bathroom fans, especially in older or DIY-remodeled homes, that dump straight into the attic instead of running a duct out through the roof or a sidewall. That moisture then condenses on the roof sheathing, feeding attic mold and rotting framing that isn't visible from inside the house.
Many homes built before local codes required mechanical exhaust have only an operable window, and in Vancouver and Portland's wet months that window stays shut for weeks at a time. A window alone can't move air on a rainy February night, so moisture keeps building until we install a properly sized exhaust fan during the remodel.
In most cases, yes. Clark County and Multnomah County both require a permit for new or upsized bathroom exhaust ducting that penetrates the roof or wall, even during a remodel. We pull the permit and schedule the inspection as part of the job.
No. Current residential codes require mechanical exhaust ventilation in bathrooms regardless of window size, and in our climate a window stays closed through most of the wet season anyway. A fan is required even in a bathroom that has a window.
If your bathroom still fogs up or smells musty more than a few minutes after a shower with the fan running, it's undersized for the room. We measure your square footage and fixture count against HVI sizing guidelines during every bathroom remodel.
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