Guides
Dew point and condensation limitations
Air dew point is not an interstitial-condensation or mould assessment.
Dew point is the temperature at which cooling air with a given moisture content reaches saturation. Air temperature and humidity help interpret a reading but do not establish wall temperature or resolve mould risk.
Key points
- Comparison needs a separate surface-temperature measurement.
- One moment does not describe a season.
- Air dew point is not interstitial condensation analysis.
In this guide
Prepare your inputs
- Air temperature in °C and relative humidity in % at the same time and place.
- Sensor accuracy and measurement conditions.
- Separate surface temperature when making a comparison.
1. Collect consistent readings
Do not combine bathroom humidity with another room's temperature. Record time, location, ventilation and events such as cooking. Allow a stable reading according to the sensor instructions.
Temperature in this form is always °C and humidity is %. Accepted inputs are −45 to 60°C and humidity greater than 0 through 100%. These are tool boundaries, not a promise of equal accuracy throughout.
Source: [1] US National Weather Service — dew point and humidity
2. Understand the approximation
The application uses a Magnus approximation with a = 17.62 and b = 243.12°C. ln is the natural logarithm. Treat the rounded result as an estimate that also depends on input accuracy.
The formula describes saturation over liquid water; frost point over ice uses a different relation. A calculated dew point below −45°C is outside NPL's stated approximation range even when air temperature is within the form's range. At 100% humidity this model's dew point equals air temperature. Lower humidity at the same temperature gives a lower dew point. The output does not predict when condensation will occur.
γ = ln(RH / 100) + 17.62 × T / (243.12 + T); Td = 243.12 × γ / (17.62 − γ)
Source: [3] NPL — dew point and relative humidity, equations 2–3
3. Compare the surface and assessment scope
A local surface cooler than estimated dew point may favour condensation when that air contacts it. Enter a separate reading in the optional surface-temperature field to obtain the difference in kelvins (a 1 K difference equals a 1°C difference). The tool does not measure the surface or boundary air conditions. Small differences may fall within measurement uncertainty.
No visible condensation does not establish absence of mould risk. Surface humidity and interstitial condensation assessment need other inputs and methods, as described by ISO 13788's scope. For recurring dampness, gather readings and investigate the cause rather than choosing insulation from this number alone.
Air and a cold surface
Our example uses 20°C air at 60% relative humidity and a separately measured surface of 11°C.
| Quantity | Value |
|---|---|
| Air temperature | 20°C |
| Relative humidity | 60% |
| Magnus dew point | ≈ 11.995°C |
| Surface temperature | 11°C |
| Surface minus dew point | ≈ −0.995 K |
The surface is below estimated dew point. This indicates local conditions to investigate, not a diagnosis or full moisture assessment.
Sensor accuracy and variation matter; the tool does not calculate their uncertainty.
Try this example in the calculatorCommon mistakes
| Mistake | How to avoid it |
|---|---|
| Treating air temperature as wall temperature. | Measure the surface separately. |
| Calling one dry afternoon a seasonal assessment. | Record a series under relevant conditions. |
| Designing layers from air dew point. | Use an assembly assessment method with appropriate conditions. |
Check before using the result
- T and RH share time and location.
- Units and sensor accuracy are known.
- Interpretation separates air, surface and assembly interior.
Method limits
- An air approximation without layer temperature or moisture balance.
- Not mould assessment, ISO 13788 implementation or dynamic simulation.
Questions and answers
Can I enter 0%?
Not here. The logarithm RH/100 requires positive humidity. Zero does not produce a finite result from this formula.
Can I enter °F?
Not in a °C field. Convert temperature using (°F − 32) / 1.8. The feet preference does not change this field's unit.
Sources and verification scope
- US National Weather Service — dew point and humidity
Definition of air dew point and its relationship to air temperature and relative humidity.
Access checked: - ISO 13788:2012 — official scopePublic standard abstract
Public abstract separates surface humidity and interstitial condensation assessment. Air dew point alone is not either complete assessment.
Access checked: - NPL — dew point and relative humidity, equations 2–3
Magnus coefficients 17.62 and 243.12 for saturation over liquid water, their stated temperature range, and the separate ice/frost-point formula. Vapour-pressure uncertainty is not a promised dew-point or sensor accuracy.
Access checked: