Surface Condensation and Mould Risk Calculator
Temperature of the inside face of a wall, window or corner on a cold day, and whether it will get condensation or mould at the room's humidity.
Results
The dashed line marks the value you entered.
What this tool does
A poorly insulated wall is cold on the inside, and the air touching it cools down too: its relative humidity rises, and above about 80 % for a few days mould can grow even without visible water, as the European standard EN ISO 13788 assumes. At the dew point water condenses outright. Corners, window reveals and walls behind wardrobes are colder still because less warm air reaches them, which is why the larger surface resistance of 0.25 is used there. Better insulation raises the surface temperature; ventilation lowers the humidity — the chart shows how much each U-value helps.
Formula
T_si = T_i − U × R_si × (T_i − T_e) · condensation if T_si < dew point · mould risk if surface RH ≥ 80 %
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ti | Indoor temperature | °C |
rh | Indoor relative humidity | % |
te | Outdoor temperature | °C |
u | U-value of the wall, window or corner | W/(m²·K) |
rsi | Inner surface resistance (0.13 open wall, 0.25 corners or behind furniture) | m²·K/W |
TS | Temperature of the inner surface | °C |
TD | Dew point of the room air | °C |
RS | Relative humidity right at the surface | % |
FR | Temperature factor fRsi | — |
VD | What to expect | — |
Worked example
- Indoor temperature20 °C
- Indoor relative humidity60 %
- Outdoor temperature0 °C
- U-value of the wall, window or corner1.6 W/(m²·K)
- Inner surface resistance (0.13 open wall, 0.25 corners or behind furniture)0.13 m²·K/W
- Temperature of the inner surface15.8 °C
- Dew point of the room air12.0 °C
- Relative humidity right at the surface78 %
- Temperature factor fRsi0.792
- What to expectUnlikely in these conditions
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.