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: exterior corners, window reveals, the wall behind large furniture, the lintel above a window, and the floor-to-wall junction on the first floor.
Without one, assume an uninsulated exterior wall sits 3–7 °F below room air, an exterior 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.
A worked example
A worked example for the most common real case: a room that reads fine and a corner that does not.
Take the room reading
70 °F and 50% relative humidity. This is squarely inside the recommended band and would pass any general advice on the internet.
Measure the actual cold surface
Point an infrared thermometer at the exterior corner behind the dresser on a cold morning. 60 °F is a realistic figure for an uninsulated solid wall in winter.
The surface humidity comes out near 70%
The same air, ten degrees colder, carries the same water against a much smaller capacity. Seventy percent is below the germination threshold, so this corner is not yet growing anything.
Now drop the surface to 55 °F
A harder freeze, or a corner with a thermal bridge, and the surface figure passes 80%. That is the line: sustained above it for days, common indoor molds germinate. Nothing about the room reading changed.
How to read the result
The point of this calculator is that mold does not respond to the number on your hygrometer. It responds to the humidity of the thin layer of air touching a surface, and that figure is set by how cold the surface is. Two rooms with identical readings can have completely different risks depending on their construction.
The threshold to remember is 80% at the surface, sustained. Below it, common indoor molds do not germinate however long you wait. Above it, the clock starts — a few days at 90%, a couple of weeks at 80%. Brief excursions during a shower do not count; it is duration that matters.
Where the calculator has to assume a surface temperature, it uses about 5 °F below room air, which is typical of an uninsulated exterior wall or a window reveal. If you can measure the real figure with an infrared thermometer, do — the answer improves considerably, and the thermometer costs about twenty dollars.
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?
Exterior 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.
How long does mold take to appear once conditions are right?
At around 80% surface humidity, expect a couple of weeks of sustained conditions. At 90% and above, a few days. Below 80% it does not start at all, which is why the practical goal is to keep the surface below the line rather than to keep it perfectly dry. Existing growth restarts much faster than new growth begins, so a cleaned patch that keeps getting wet will return quickly.
Does a warmer room always mean lower mold risk?
Usually, but not always, and the exception matters. Heating a room raises the surface temperature of its walls, which lowers surface humidity and reduces risk. But if you also add moisture — drying laundry, a humidifier, more people — the dew point rises with the temperature and a cold corner can end up no better off. Raise the temperature and hold the moisture steady, and the risk genuinely falls.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions