Why this question is genuinely contested
Ask three Pacific Northwest building professionals whether crawl space vents should be open or sealed and you will get three answers, all defensible. This is not because the field is confused — it is because both approaches work, under different conditions, and the marketing around encapsulation has flattened a nuanced question into a sales pitch. Here is the actual state of things.
The traditional Pacific Northwest crawl space is vented: openings around the foundation perimeter let outside air move through. Home inspectors in western Washington who look at hundreds of these spaces will tell you that most vented crawl spaces in this region are in acceptable condition — not moldy, not elevated in moisture content — provided the soil is covered and the drainage works. The region's breezy conditions and generally well-draining soil help. A well-vented crawl space with a good barrier can run for decades without incident.
What vents do poorly is the winter condensation problem. From October through May, the outdoor air being drawn in averages around 85 percent relative humidity, and when it hits joists and ductwork that are cooler than the air, moisture condenses on them. That is the argument for sealing. And it is a real argument — but it applies most strongly to specific conditions, not universally.
What the ground is doing matters more than the vents
Here is the part that gets lost. In most Clark County crawl spaces, the dominant moisture source is not the vented air. It is evaporation from the soil below. Bare or partially covered dirt in this climate releases moisture continuously, and it dwarfs what comes in through the vents. Which means the first and highest-value intervention, by a wide margin, is covering the ground properly — and that is true whether you eventually seal the vents or not.
A properly installed vapor barrier means six-mil polyethylene or heavier, covering the entire soil surface, seams overlapped by at least a foot and taped rather than merely laid adjacent, running up the stem walls and mechanically fastened, wrapped around piers, and continuous around obstructions. The failure mode we see constantly in Vancouver is a barrier that technically exists: a sheet laid down twenty years ago, never taped, pushed aside by everyone who has crawled under the house since, stopping two feet short of the walls. That is not a vapor barrier. That is a tarp.
The decision, plainly
A vapor barrier alone is likely sufficient if: the crawl space has no history of mold, drainage around the house is working, no HVAC ductwork runs through the space, and measured humidity down there stays under about 60 percent through the winter with the barrier in place. Cost: roughly $1,500–4,000 for a typical Clark County home.
Full encapsulation is worth the money if: humidity stays high through the wet season despite a properly installed barrier; there is HVAC ductwork or plumbing down there you want in a semi-conditioned space; there has been mold growth before; the floors above are uncomfortably cold; or the home has significant asthma or allergy concerns. Cost: roughly $3,000–9,000 and up, including a dedicated dehumidifier.
The one rule with no exceptions: never seal the vents without adding humidity control. A sealed crawl space with no dehumidifier has no way to remove the moisture that gets in from the soil, from plumbing, or from air leakage — and it has lost the airflow that used to carry some of it away. That is how a damp crawl space becomes a wet one. If a contractor proposes closing your vents and does not include a dehumidifier and a way to drain it, get another quote.
What a complete encapsulation includes
- Debris, old plastic, and contaminated or fallen insulation removed.
- Any existing mold on joists and subfloor remediated before anything is sealed. Sealing over growth traps it.
- Drainage corrected: downspouts extended, grade fixed, a crawl space sump added where water genuinely collects.
- Heavier liner — commonly 10 to 20 mil, reinforced — across the floor and up the stem walls, seams taped, mechanically attached at the top, piers wrapped.
- Foundation vents sealed and insulated.
- Rim joist air sealing and insulation.
- A dedicated crawl space dehumidifier with a drain to a condensate pump or sump, ideally with a remote humidity readout you can check from inside the house.
Radon, briefly
One consideration people rarely raise: sealed crawl spaces can concentrate radon that a vented space would otherwise disperse. Monitoring work in Washington homes has found meaningfully higher radon readings in sealed crawl spaces than in vented ones at the same sites. This is not an argument against encapsulation — it is an argument for testing for radon after sealing, which costs very little and is worth doing regardless. If levels come back elevated, mitigation systems are well established and not expensive relative to the encapsulation itself.
Checking the work
Most homeowners will never crawl under their own house, which makes it easy for a mediocre installation to pass. Before final payment, ask for photographs of the finished space and look for: taped seams rather than loose overlaps, the liner running up and attached to the stem walls rather than dying in the dirt, piers wrapped, no debris left underneath the plastic, and a dehumidifier whose condensate line actually terminates somewhere sensible. Then ask what humidity the space should hold, put a wireless hygrometer down there, and check it yourself in February. Under 60 percent through the wet season means the job worked. For the remediation side of this work, see crawl space mold removal and encapsulation.