Psychrometry: Reading the Air You're Trying to Dry
Drying is the controlled movement of water from wet materials into the air, then out of the structure. To control it you must read the air. Five readings matter. Dry-bulb temperature (Td) is plain air temperature. Relative humidity (RH) is the percent of moisture the air holds versus what it could hold at that temperature, but RH alone is misleading because warmer air holds far more water. Specific humidity, or grains per pound (GPP), measures the actual water in the air and is the number that tells you whether you are gaining or losing moisture. Dew point is the temperature at which air becomes saturated. Vapor pressure differential is the engine of drying: water leaves a wet material only when vapor pressure in the material is higher than in the surrounding air.
The practical workflow: carry a calibrated thermo-hygrometer and read Td and RH at four points every visit. Convert RH and temperature to GPP using a psychrometric chart or a calculator. Record the affected-area GPP, the dehumidifier outlet GPP, the unaffected (control) area GPP, and the exterior GPP. The dehumidifier is working when its outlet GPP is meaningfully lower than its intake. The room is drying when affected-area GPP trends downward toward the unaffected control. If outdoor GPP is low, ventilation may help; if high, never open up. Warmer air speeds evaporation but you must remove the resulting vapor, or you simply relocate the problem.