Where the line sits
dew point at this temperature
mold risk begins
your surface
How it works: warm air holds more water than cold air, so cooling it eventually forces some out. The dew point is the temperature where that happens, worked out here with the Magnus formula. Any surface colder than that will grow condensation — that is why a window streams while the middle of the room feels fine, and why a cellar floor sweats in June rather than January. Mold does not wait for standing water: it takes hold when the humidity right against a surface passes about eighty percent, which happens several degrees before you see a drop, and that is the temperature worth watching. The way out is nearly always the same three things — warm the cold surface, get the moisture out at the source, or move some air across it.