Relative humidity is the amount of water vapor in the air as a percentage of the most it could hold at that temperature; warm air holds more, so the same vapor load reads as a different RH as temperature changes.
What is relative humidity?
RH is temperature-dependent, which is the part inspectors have to keep front of mind: cool a parcel of air and its RH rises even though no water was added, until it hits 100% at the dew point and condenses. The EPA recommends keeping indoor relative humidity below about 60%, and ideally between 30% and 50%, to limit mold and dust-mite growth (EPA, A Brief Guide to Mold, Moisture and Your Home). High RH alone, with no leak, can push surface and material water activity into the growth range, especially on the cooler surfaces of a room.
Why it matters to a mold inspection
A humidity reading is only meaningful with the temperature it was taken at, and an inspector who logs RH without temperature has logged half a data point. Persistent RH above 60% is itself a finding, because it explains diffuse surface mold that no single leak accounts for, and it points the fix toward ventilation, dehumidification, or source control rather than demolition. ASHRAE 160 treats interior humidity as a controlled input in moisture-design analysis (ASHRAE 160). See HVAC mold for homeowners and condensation vs leak.
MoldMind records temperature, RH, and dew point together as environmental fields, so a humidity finding is never reported without the temperature that gives it meaning.
Sources
- EPA, A Brief Guide to Mold, Moisture and Your Home: keep indoor RH below 60%, ideally 30 to 50%.
- ASHRAE 160: interior humidity is a controlled input in moisture-control design analysis.