Dew point & condensation risk calculator
Temperature and relative humidity give the dew point — the surface temperature below which water condenses out of the air and a building assembly starts feeding mold.
Ambient air temperature near the surface of concern.
Relative humidity of the surrounding air (1–100%).
Dew point, condensation, and mold-growth risk
The dew point is the temperature to which air must cool, at constant pressure, for its water vapor to condense into liquid. This calculator uses the Magnus approximation (γ = ln(RH/100) + 17.27·T/(237.7 + T); dew point = 237.7·γ / (17.27 − γ), in °C). It is not a comfort metric — for a mold inspector it is a condensation-risk reading.
Any building surface at or below the dew point of the surrounding air will accumulate liquid water: single-pane windows, uninsulated exterior-wall corners, cold-water pipes, attic sheathing, and ductwork are the usual cold spots. And condensation does not have to puddle to grow mold — sustained surface relative humidity at or above about 80% is the threshold most moisture-control guidance (EPA Moisture Control; the ASHRAE 160 design framework) builds around, and surface RH climbs toward 100% as the surface approaches the dew point. That is why a cold wall in a humid room grows mold without any visible drip.
The fix follows the cause. Condensation is solved by lowering indoor humidity, improving ventilation, and warming surfaces with insulation — not by patching a leak. Telling condensation apart from a hidden leak is the entire diagnosis: see condensation vs. a leak.
Related reading
- Condensation vs. a leak: what's causing your mold?
- Attic & HVAC condensation mold
- Thermal imaging in mold inspection
Sources
Write the report in minutes, not hours.
MoldMind turns your field notes, photos, and lab results into a standards-compliant report you review and approve. Try MoldMind free — 3 jobs, no card.