Mold risk calculator
Mold does not read your hygrometer. It responds to the humidity of the air touching the surface it is growing on — and a cold wall pushes that far above the room reading.
Enter your numbers
Moderate risk
Not yet germinating, but with no margin.
- Humidity at that surface
- 78%
- Dew point
- 53.0 °F
The 80% rule
Mold germinates at roughly 80% relative humidity at the surface, given time and something to eat — and ordinary dust on paint is enough to eat. It does not need liquid water, which is why growth appears on walls that never look wet.
Relative humidity is relative to temperature, so the same unchanged air reads differently against different surfaces. Air at 70 °F and 55% humidity is at about 78% against a surface at 60 °F, and above 90% against one at 55 °F. Your room is fine. The corner is not.
Finding the surface temperature
An infrared thermometer costs very little and gives you the missing number in seconds. Take readings on a cold morning, when the gap between room air and wall surface is at its widest: external corners, window reveals, the wall behind large furniture, the lintel above a window, and the floor-to-wall junction on the ground floor.
Without one, assume an uninsulated exterior wall sits 3–7 °F below room air, an external corner a few degrees below that again, and a window frame colder still.
What the result tells you to do
If the surface humidity is high because the room humidity is high, ventilate and dehumidify — the whole house needs drying.
If the surface humidity is high because the surface is cold while the room reads normally, no amount of dehumidification will fix it. That is an insulation and heating problem, and it is why so many people find that mold returns after they buy a dehumidifier.
Common questions
Can mold grow when my room humidity looks normal?
Routinely. Mold responds to humidity at the surface, and a cold wall or corner pushes that far above the room reading — a room at 55% can be at 80% against a surface just 10 °F cooler, which is enough for growth.
Why is there mold in the corner but not the rest of the wall?
External corners lose heat in two directions at once, so they are the coldest interior surface in the room. The air touching them is at a much higher relative humidity than the room average.
Can mold grow without visible condensation?
Yes, and it usually does. Liquid water is not required — 80% humidity at the surface for long enough is sufficient, and that happens on walls that always look dry.