Clean, dry crawl space with vapor barrier, dehumidifier, and humidity gauge
The target state: sealed soil, controlled humidity, and a number you can check.

Start with a number

Buy a hygrometer. They cost about fifteen dollars, and knowing your actual indoor and crawl space humidity turns mold prevention from guesswork into maintenance. Targets for a Clark County home: keep living-space relative humidity between 40 and 50 percent through the wet season, and keep crawl space humidity under about 60 percent. Above 60 percent sustained on a cool surface, mold grows. Below 50 percent, it generally does not. Everything else in this article is in service of those two numbers.

Put one hygrometer in the main living area and, if you can, a wireless one in the crawl space so you can read it from inside. Watching it climb through November tells you more about your house than any single inspection.

The autumn checklist — September and October

This is the highest-value month of the year for moisture work in this region, because everything you do before the rain starts pays off for the following eight months.

  • Clean the gutters, and then plan to clean them again. Fir and cedar drop needles continuously, not in one autumn burst. Overflowing gutters send water down the wall and behind the fascia.
  • Extend every downspout. Get discharge at least six feet from the foundation, further if the ground slopes back toward the house. This is the single most cost-effective thing most Vancouver homeowners can do.
  • Walk the grade. Soil should fall away from the house for the first ten feet. Decades of settling reverses this quietly. Adding soil to correct it is a weekend and a few yards of dirt.
  • Look in the crawl space. Once a year, with a flashlight and a mask. You are looking for torn or missing plastic, damp soil, fallen insulation, disconnected ducts, and dark staining on the joists.
  • Check the roof and flashing. Moss on the shingles, debris in the valleys, and flashing at chimneys and skylights. Wind-driven rain finds every weakness once the storms start.
  • Test the bath fans. Hold a square of toilet paper to the grille; if it does not hold, the fan is not moving air. Then go into the attic and confirm the duct actually reaches an exterior cap.

The winter routine — November through March

Once the wet season is running, prevention is behavioral rather than structural.

  • Run bath fans during every shower and for twenty minutes after. A humidity-sensing switch or a simple timer makes this automatic and is the best few-hundred-dollar upgrade in a Pacific Northwest house.
  • Use the kitchen range hood, and confirm it exhausts outside rather than recirculating through a charcoal filter.
  • Keep furniture a few inches off exterior walls so air can circulate against the coldest surfaces.
  • Leave closet doors on exterior walls open, or at least ajar. Closed closets on cold outside corners are where a surprising amount of Clark County mold starts.
  • Leave the front-load washer door open between loads.
  • Crack a window for a few minutes after showering or cooking, even in December. Cold outside air holds little total moisture; once it warms indoors its relative humidity drops sharply and it absorbs more. Winter ventilation genuinely works here, and the belief that it does not is one of the more expensive local myths.
  • Do not turn the heat off in unused rooms. Cold rooms in a humid house are condensation surfaces.
  • Watch the hygrometer. If living-space humidity is holding above 55 percent, something needs attention.

The structural fixes, in order of value

  1. A proper crawl space vapor barrier. Six-mil or heavier poly across the entire soil surface, overlapped and taped at the seams, running up the stem walls and mechanically attached. Roughly $1,500–4,000 installed for a typical Clark County home, and it eliminates the largest single moisture source most houses have.
  2. Exterior water management. Downspout extensions, grading, and where necessary a French drain or curtain drain upslope. Cheap relative to what it prevents.
  3. Ventilation corrections. Correctly sized and correctly ducted bath fans, soffit baffles to reopen blocked attic intake, and adequate ridge or roof venting.
  4. A crawl space sump. Only where water genuinely collects rather than merely dampens the soil.
  5. Full encapsulation with a dehumidifier. The premium option, warranted when the space still runs high after a good barrier, when HVAC ductwork is down there, or when there is a history of growth. $3,000–9,000 and up. See our vapor barrier and vents guide for whether you need it.

One warning about closing crawl space vents. Sealing the vents without also sealing the soil and adding humidity control is how a damp crawl space becomes a wet one. Vents are part of a system. Change the system, not one component of it.

What does not work

Sealing or painting over active growth without fixing the water. Bleaching porous materials, which adds moisture and leaves the growth structure intact. Ozone generators, which are ineffective against mold in occupied buildings and produce a lung irritant. Fogging treatments sold as a standalone solution — they can be a legitimate supplement inside a real remediation, and are worth nothing on their own. And a small consumer dehumidifier fighting an open crawl space, which will run continuously, cost you money, and lose.