Air moves upward through a house. That means crawl space air becomes house air, which is why a problem nobody has looked at for a decade shows up as a smell in the living room.
The signs from upstairs
Before going down, notice what the house is telling you. Musty smells that seem to come from nowhere. Floors cold regardless of heating. Windows fogging more than they should. Indoor humidity the heating cannot bring down.
These frequently point downward rather than to anything in the living space, and in Western Washington that is the most common explanation.
What to check
Standing water or damp ground. The clearest signal. Ground should be dry, not merely not-puddled.
The vapour barrier. Plastic sheeting over the ground should be present, intact, and covering everything. Torn, displaced or absent barriers let ground moisture evaporate straight into the space continuously.
Condensation on ducts and pipes. Water beading on cold surfaces means humidity is high enough to condense, which means it is high enough to cause problems.
The underside of the subfloor. Dark staining, white growth or soft spots all indicate sustained moisture.
Insulation. Insulation between floor joists that has fallen, sagged or become damp is doing nothing. Damp insulation is worse than none, because it holds moisture against the structure.
Ductwork. Disconnected sections, crushed flexible duct and obvious leaks. Ducts running through a crawl space lose conditioned air into a space you are not trying to heat.
Pests and entry points. Droppings, nesting material, gaps in the perimeter.
Smell. A properly dry crawl space smells of nothing in particular. Musty means moisture.
Where the water comes from
Usually not from a leak. In Western Washington it is normally exterior water management — gutters overflowing, downspouts discharging at the wall, ground sloping toward the house, or a high water table.
Fixing the crawl space without fixing the source is temporary. Where a contractor proposes encapsulation without discussing drainage and grading, ask why.
Eastern Washington: different priority
Spokane, Yakima and the Tri-Cities have drier air, so vented crawl spaces perform much closer to their design intent and chronic humidity is less common.
The dominant concern there is freeze protection. Plumbing running through a crawl space in sustained cold is genuinely at risk, and insulation on those runs matters more than humidity control.
The Skagit and Thurston caveat
In the Skagit valley floor and parts of Thurston County, high water tables mean groundwater arrives from below regardless of drainage improvements.
Drainage helps but has a real ceiling in those locations, and encapsulation with active humidity control is often the realistic answer. A contractor promising that drainage alone will fully solve it may be overpromising.
Getting an assessment you can trust
This is a field with genuine variation in quality, and homeowners cannot easily verify a diagnosis because they will not go down there themselves.
Ask for photographs of the actual conditions in your crawl space, not stock images. Ask for humidity readings rather than a description. Ask specifically what was found regarding drainage. Be cautious of assessments that arrive unsolicited and move quickly to a large quote.
Safety
Wear a mask and gloves. Watch for exposed wiring and standing water together. If access is genuinely difficult or the space is very tight, having someone else do it is entirely reasonable.