Most older Washington homes sit above a vented crawl space, and a large proportion of them have a moisture problem they have had for decades.
The theory behind venting
Vents allow outside air to circulate beneath the house, carrying moisture away before it accumulates. It is sound reasoning in a climate where outside air is drier than the crawl space.
Western Washington is frequently not that climate. Through much of the year the outside air arriving through those vents carries as much moisture as the air it replaces, and in summer warm humid air entering a cool crawl space can condense on the cool surfaces — actively adding moisture rather than removing it.
How the problem shows up
Rarely in the crawl space, because nobody goes down there. It shows up in the house above.
Musty smells that seem to come from nowhere. Floors that are cold regardless of the heating. Windows fogging more than they should. Indoor humidity the heating system cannot bring down. Over longer periods, mold on the underside of the subfloor, degraded insulation, and eventually damage to framing.
Air moves upward through a house, so crawl space air becomes house air. That is why a problem hidden under the floor presents as a problem in the living space.
What encapsulation involves
Sealing the crawl space rather than ventilating it. A heavy vapour barrier over the ground and up the foundation walls, sealed at the seams. Vents closed. Insulation typically moved from between the floor joists to the perimeter walls, bringing the crawl space inside the conditioned envelope. Humidity controlled mechanically, usually with a dedicated dehumidifier.
Drainage issues must be addressed first. Encapsulating over standing water or an active groundwater problem seals in what you are trying to remove.
Where the water table sets a limit
Worth being honest about. In parts of Washington — the Skagit valley floor especially, and low-lying areas of Thurston County — high groundwater means drainage improvements have a genuine ceiling on what they can achieve.
In those locations encapsulation with active humidity control is often the realistic answer rather than drainage alone, and a contractor who claims drainage will fully solve it may be overpromising.
When venting is still adequate
Not every crawl space needs encapsulating. A dry crawl space with good drainage, an intact ground vapour barrier and no moisture symptoms upstairs may be performing acceptably.
The test is evidence rather than theory. Is there condensation on ducts or pipes? Is the ground damp? Is there any smell? Are there symptoms in the house above? If not, the existing arrangement may be working.
Getting an honest assessment
This is a field with genuine variation in quality, and the homeowner cannot easily verify the diagnosis because they will not go down there.
Ask for photographs of the actual conditions. Ask for humidity readings rather than a description. Ask specifically whether drainage has been assessed and what was found. And be cautious of an assessment that arrives unsolicited, particularly one that moves quickly from inspection to a large quote.
Eastern Washington
Largely a different question. Drier air means vented crawl spaces perform closer to their design intent, and the dominant crawl space concern in Spokane and Yakima is freeze protection for plumbing rather than chronic humidity.