Why it matters here
Pierce County's glacial soils hold water, winters are long and wet, and crawl spaces and slabs carry that moisture up to the floor above. Laminate swells, vinyl plank can lift or trap mold beneath it, and adhesives release. Every warranty requires the subfloor to be within the product's moisture limits at installation, and every failed floor investigation starts with a meter.
The tests
Wood subfloors. A pinless meter scans quickly; a pin meter confirms. Readings are taken in several places per room, especially near exterior walls, bathrooms and the kitchen, and compared to a dry reference and the product's maximum.
Slabs. Calcium chloride tests measure vapor emission over a sealed dish for about three days; in-slab relative humidity probes read the slab's internal humidity after equilibrating. The product's instructions say which test and what limit. The space should be at normal living temperature for a meaningful result.
Crawl spaces. A visual check through the access hatch for standing water, wet soil, sagging insulation and vapor barrier condition tells a lot before any meter does.
When the numbers are high
The floor waits. A damp crawl space needs its moisture controlled, with a ground vapor barrier at minimum and often drainage or encapsulation; a slab with high emissions needs a mitigation coating or the water source addressed. The installer is not a crawl space contractor, but the test tells you the problem exists before you spend money on a floor over it, and the floor goes in once a retest passes.
When the numbers are fine
A vapor-barrier underlayment still goes down over every crawl space and slab, seams taped, because normal vapor transmission continues year round. The readings go in your file with the quote.
The timing
Test at the measure for planning and again after tear-out, when the subfloor is exposed and the house is at normal temperature. Readings in an unheated basement in January mislead; test when the space is conditioned.
