What the meters show
Dry to the Touch Is Not Dry
Here is a call the dispatch desk gets every week: a leak was fixed, the wall looks fine, but a tech puts a meter on it and it reads high. Painted drywall dries at the face while the gypsum core and the stud behind it stay wet. Close that up or hang a picture on it and you can end up with mold you will not see for weeks.
The drying target comes from what techs call the dry standard: readings taken on unaffected drywall, framing or flooring elsewhere in the house. When the wet wall reads within range of that dry standard, it is dry; until then, the equipment keeps running.
Hardwood floors show the same problem in a different way. With a slow leak under a dishwasher, the underside of the boards swells and the edges lift, which is called cupping. Sanding it flat too soon leaves the floor crowned once it finishes drying. Drying mats and steady monitoring can often bring the boards back flat.
The physics in plain words: warm, dry air pulls moisture out of wet materials, and dehumidifiers pull it out of the air. Air movers keep the surface boundary layer moving so evaporation does not stall.
- Relative humidity is how full the air is relative to what it can hold at that temperature
- GPP is the actual weight of water vapor in the air, which is the number techs track day to day
- Moisture moves from high vapor pressure in wet materials toward lower pressure in drier air
- Dew point is where the air would condense on a cold surface, which matters on slabs and exterior walls