Vented crawl spaces were standard practice for good reasons, and in a climate where outside air is drier than the crawl space they work. Western Washington is frequently not that climate.
Why venting underperforms here
Through much of the year, air arriving through crawl space vents carries as much moisture as the air it replaces. In summer, warm humid air entering a cool crawl space can condense on the cool surfaces — actively adding moisture rather than removing it.
The result is a space that stays damp, and because air moves upward through a house, that dampness becomes the humidity of the living space above.
What encapsulation involves
A heavy vapour barrier across the entire ground surface and continuing up the foundation walls, with seams sealed and the barrier mechanically attached at the top.
Vents are closed. Insulation typically moves from between the floor joists to the perimeter walls, bringing the crawl space inside the building’s conditioned envelope rather than leaving it outside.
Humidity is then controlled mechanically, usually with a dedicated dehumidifier sized for the space.
Drainage comes first, always
This is the point most worth insisting on. Encapsulating over standing water or an active groundwater path seals in exactly what you are trying to remove.
Before any encapsulation, the exterior water management should be addressed: gutters clear, downspouts discharging well away from the foundation, ground sloping away from the house.
In a meaningful number of cases, sorting the downspouts is the entire fix and the crawl space dries out on its own. A contractor who proposes encapsulation without discussing drainage is skipping the cheap answer.
Where drainage has a genuine ceiling
In the Skagit valley floor and parts of Thurston County, high water tables mean groundwater arrives from below regardless of surface management.
There, drainage helps but cannot fully resolve it, and encapsulation with active humidity control is often the realistic answer rather than an upsell. A contractor promising drainage alone will solve it in those locations may be overpromising in the opposite direction.
What good work looks like
Full coverage including up the walls, not just a sheet laid loosely on the ground. Sealed seams. Proper mechanical attachment. A sump or drainage provision where water can arrive. Dehumidification sized for the actual volume. Any plumbing insulated while access is open.
Ask what happens to the existing floor joist insulation, and confirm perimeter insulation is part of the scope rather than an afterthought.
Getting an honest assessment
This field has real variation in quality, and the homeowner is at a disadvantage because they will not go down there to verify anything.
Ask for photographs of your actual crawl space rather than examples. Ask for humidity readings, which are a measurement rather than an opinion. Ask specifically what was found regarding drainage and grading.
Be cautious of assessments that arrive unsolicited and move quickly from a free inspection to a large quote. That sequence is a recognisable sales pattern.
When it is not necessary
A dry crawl space with sound drainage, an intact ground barrier and no symptoms upstairs may be performing perfectly well. The test is evidence — damp ground, condensation on pipes, musty smell, symptoms in the house — rather than the age of the construction.
East of the Cascades
Largely a different question. Drier air means vented crawl spaces perform much closer to their design intent in Spokane and Yakima, and the dominant crawl space concern there is insulating plumbing against sustained winter cold rather than controlling humidity.