Condensation is about the temperature of a surface relative to the air’s dew point. Room temperature alone cannot diagnose it.
1. Record the pattern before changing anything
Note when moisture appears: after showers, during cooking, overnight or after rain. Record room temperature and relative humidity at the same time. Check a second instrument if a reading looks implausible. A damp patch driven by a roof leak will not be solved by a dew-point calculation.
Photograph affected surfaces and their surroundings. A problem confined to glass differs from dampness extending through plaster after rain. Avoid covering or painting a recurring damp area before identifying the source.
2. Compare the surface with the dew point
Dew point is the temperature at which the air reaches saturation when cooled without changing its moisture content. A sufficiently cold surface can therefore collect moisture even while the room feels warm. The calculator uses air temperature and relative humidity to estimate that threshold.
Its equation is an inversion of the August–Roche–Magnus relationship described by EPA. We limit inputs to ordinary indoor conditions. It is a screening calculation, not a model of moisture movement through a multi-layer wall.
EPA · Humidity calculation method ↗3. Address the moisture source and cold surface
EPA emphasises moisture control and recommends keeping indoor relative humidity below 60%, ideally 30–50% where possible. Those values do not guarantee freedom from condensation on very cold surfaces. Water leaks and persistent dampness still require attention.
Vent moisture-generating activities appropriately, use working extract systems and investigate unusually cold junctions. Air sealing, insulation and ventilation need to be considered together. Do not simply shut all ventilation to make a room feel warmer.
EPA · Mold prevention and moisture control ↗4. Recheck under comparable conditions
Compare temperature and humidity before and after a change, preferably under similar outdoor conditions. Surface-temperature readings depend on measurement technique; reflective glass can mislead an infrared thermometer. A small calculated margin should not be treated as precise evidence of safety.
A positive surface-to-dew-point margin only means this simplified condensation threshold has not been crossed. It does not rule out mold, hidden damp, rain entry or air leaks. Persistent problems deserve a building assessment.
Four observations, four different questions
| Observation | Question to ask | Useful evidence |
|---|---|---|
| Water on the inside of glazing | Is the glass below indoor dew point? | Temperature and humidity at the same time |
| Dampness following rain | Is bulk water entering? | Rain timing and exterior defects |
| Cold stripe at a junction | Is there a thermal bridge? | Construction detail and surface temperature |
| Persistent musty area | Is moisture trapped or recurring? | Inspection of the moisture source |
Worked example
A room at 20°C and 60% humidity
The Magnus approximation gives a dew point of about 12°C.
Surface temperature = 10°C Surface − dew point ≈ −2°C
The surface is below the estimated dew point, so condensation is possible. Raising the surface temperature or reducing moisture changes the result; neither identifies a hidden water leak.
What to have ready
- Room temperature and humidity readings
- Time and weather when moisture appears
- Surface-temperature measurement with its limitations
- Ventilation and water-leak inspection
Common mistakes
- Calling every damp patch condensation.
- Treating a positive calculated margin as proof against mold.
- Adding a vapour barrier without an assembly-specific plan.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.