Enter your room's temperature and humidity to find the dew point — the temperature at which air can no longer hold its moisture. Then point an infrared thermometer at your coldest surface to see whether condensation, the earliest signal of hidden moisture and mould, is already in play.
Read from a thermostat, hygrometer, or smart sensor.
Enter air temperature and humidity to calculate.
A surface check is not an enclosure check
This tells you about surfaces you can see and point a thermometer at. The most damaging condensation usually happens where you can't — inside the wall assembly, where moisture-laden air reaches its dew point against cold sheathing or framing and feeds mould for years before it ever shows on the wall. No handheld reading catches that. It's why a vapour-open enclosure that can dry in both directions matters more than any single measurement — this tool is an early-warning aid, not a substitute for one.
Relative humidity is exactly that — relative. It tells you how full the air is at the current air temperature, not whether water will actually condense somewhere. A room can sit at a comfortable 50% RH and still grow condensation on a cold window or an under-insulated corner, because those surfaces are colder than the rest of the room.
Dew point is an absolute temperature. Any surface at or below it collects liquid water, full stop — regardless of what the RH number reads. So the practical move is to find your dew point above, then walk the room with an IR thermometer and check your coldest surfaces against it. If a surface is at or near the dew point, condensation — and the mould risk that follows — is already in play there.
As a starting point, aim to hold indoor RH between 40% and 60%. The classic Sterling Diagram shows this band simultaneously minimises fungi, bacteria, dust mites, viruses, and respiratory irritants — arguably the single most biologically effective indoor target. Below ~40% the air turns drying and irritating; above ~60% you start feeding mould and dust mites.
On a warm, humid day a glass of ice water “sweats” — beads appear on the outside. That water didn't leak through the glass; the surface is colder than the air’s dew point, so moisture condenses onto it. Your cold window or wall corner is that glass.
Warm air is a big sponge — it holds a lot of water vapour. Cool it and the sponge shrinks. A small sponge can't hold as much, so it wrings out the excess as liquid water. That wringing-out temperature is the dew point.
Overnight the ground radiates heat to the sky and cools below the air's dew point. The air touching it becomes the “small sponge” and releases its moisture as dew on the grass. Your walls and windows do the same on a cold night — same physics, indoors.
If you're seeing fogged windows, damp corners, or musty smells, answer a few quick questions and we'll point you to the next step that fits where your building actually is.
Find my next step →Disclaimer. This calculator is an educational tool. Dew point is computed from your entered air temperature and relative humidity using the Magnus formula; results are only as accurate as your inputs and instruments. Surface readings from a handheld IR thermometer are a spot check of visible surfaces only and cannot detect condensation inside a wall, roof, or floor assembly. This is not a substitute for a professional building-enclosure or moisture assessment. Persistent condensation, damp, or suspected mould should be evaluated by a qualified professional.