How to use the dew point calculator
Condensation and mould are among the most common complaints about homes, and they come down to two numbers: how much moisture the room air holds, and how cold the coldest surface gets. This calculator gives you both. It works out the dew point of the air, then estimates the inside surface temperature of a wall, roof or window from its U-value, and tells you whether that surface will get wet or stay damp enough for mould.
- Enter the inside air temperature and relative humidity. A hygrometer gives you real readings; 20 °C and 50–60% are typical for a lived-in home in winter.
- Enter the outside temperature you want to check, such as a cold winter night or a monthly mean.
- Pick an element or enter a U-value. Presets are typical values; use the R-value and U-value calculator for your actual build-up.
- Read the results: the dew point, the surface temperature, the temperature factor, the humidity at the surface, and the highest room humidity the surface can take before condensation or mould risk.
Dew point and surface temperature formulas
γ = ln(RH ÷ 100) + 17.62 × T ÷ (243.12 + T)
Dew point Td = 243.12 × γ ÷ (17.62 − γ) (T in °C)
Surface temperature θsi = θi − U × Rsi × (θi − θe)
fRsi = (θsi − θe) ÷ (θi − θe)
Surface RH = psat(Td) ÷ psat(θsi) · mould risk when ≥ 80%
psat is the saturation vapour pressure, 6.112 × exp(17.62 × T ÷ (243.12 + T)) hPa. The surface calculation is one-dimensional: it describes the middle of a wall, not corners, lintels or other thermal bridges, which are colder and where mould usually starts.
| Air at 20 °C (68 °F) | Dew point | Surface temperature for 80% surface RH |
|---|---|---|
| 40% RH | 6.0 °C (42.8 °F) | 9.3 °C |
| 50% RH | 9.3 °C (48.7 °F) | 12.6 °C |
| 60% RH | 12.0 °C (53.6 °F) | 15.4 °C |
| 70% RH | 14.4 °C (57.8 °F) | 17.9 °C |
Worked examples
A cold wall in a damp bedroom
A bedroom at 20 °C and 60% RH has a dew point of 12.0 °C. Its uninsulated cavity wall has a U-value of 1.5 W/m²K, so on a 0 °C night the surface is 20 − 1.5 × 0.25 × 20 = 12.5 °C. That is just above the dew point, so no water forms, but the surface RH is about 97% and the fRsi is 0.63, well under 0.75. Mould is likely, especially behind wardrobes.
The same wall after insulation
Insulating the wall to U 0.30 raises the surface to 20 − 0.30 × 0.25 × 20 = 18.5 °C, an fRsi of 0.93. Surface RH drops to about 66%, safely under 80%.
Single glazing
A single-glazed window at U 5.7 with Rsi 0.13 sits at 20 − 5.7 × 0.13 × 20 = 5.2 °C, far below the 12.0 °C dew point, so it streams with condensation. Double glazing at U 2.8 reaches 12.7 °C, just above the dew point, so it stays clear at 60% RH but will mist up if the humidity rises. ISO 13788 applies the 80% mould test to opaque surfaces, not to glass, so the calculator only flags condensation for windows.
Who this calculator helps
- Architects and building surveyors diagnosing damp and mould and making the case for insulation or ventilation upgrades.
- Energy assessors and retrofit coordinators checking surface temperature factors before and after a retrofit.
- HVAC designers checking humidity targets against cold surfaces and duct or pipe dew points.
- Landlords and homeowners who want to understand why a wall or window gets wet.
Practical tips
- Lowering indoor humidity from 70% to 50% drops the dew point by about 5 °C, which often fixes window condensation on its own. Size extract and whole-house ventilation with the ventilation calculator.
- Mould usually starts at junctions: wall to ceiling, around lintels and in external corners. Treat those as colder than the plain wall result.
- Upgrading the walls changes heating demand too; check it with the heat loss calculator and pick an insulation thickness with the insulation calculator.
- For internal insulation on solid walls, get a hygrothermal check of the build-up, since moisture can collect behind the insulation.
Planning the room's new look after the damp is fixed? Try finishes on a photo with AI Wall Paint.
