What the meters show
Why Dry to the Touch Does Not Mean 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. Drywall paint dries on the surface first and seals moisture inside the gypsum core and the stud behind it. Seal it up or push furniture against it, and mold can grow out of sight 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 short version: dry, warm air draws water out of materials, dehumidifiers remove it from the air, and air movers sweep the surface so evaporation keeps going.
- Relative humidity tells you how saturated the air is at its current temperature
- Grains per pound (GPP) measures how much water is actually in the air, regardless of temperature
- 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