Why mould almost always grows on exterior walls
Mould in homes rarely appears on interior walls. Typical spots are exterior walls, especially room corners, window reveals, the junction with the ceiling and the areas behind wardrobes. The reason is surface temperature: exterior walls are colder on the inside than the room air. On cold surfaces the relative humidity rises, even though the amount of water in the air does not change.
The 80% criterion
Mould does not need dripping water. It is enough for the surface to be very damp for several days. The German standard for minimum thermal insulation, DIN 4108-2, therefore uses a relative humidity of 80% at the component surface as its criterion. With indoor air at 20 °C and 50%, this limit is reached when the wall is colder than about 12.6 °C. Condensation only forms at the dew point of about 9.3 °C — so mould can appear well before visible wetness.
If indoor humidity rises to 60%, the critical surface temperature shifts to about 15.5 °C. A wall that is uncritical at 50% can already grow mould at 60%.
The temperature factor fRsi
So that building components can be assessed independently of the current weather, the standard uses the temperature factor fRsi = (θsi − θe) / (θi − θe). It describes how close the inner surface temperature θsi is to the room temperature θi — measured against the distance to the outdoor temperature θe. An fRsi of 1 means the wall is as warm as the room air. An fRsi of 0 means it is as cold as outside.
DIN 4108-2 requires a temperature factor of at least 0.70 at thermal bridges. Under the standard conditions of 20 °C and 50% inside and −5 °C outside, this corresponds to a surface temperature of about 12.5 °C, matching the mould-critical temperature of 12.6 °C.
How cold is an exterior wall?
For the undisturbed wall area, the surface temperature can be estimated from the U-value: fRsi = 1 − U · Rsi with Rsi = 0.25 m²·K/W. This gives some guideline values at 20 °C inside and −5 °C outside:
Uninsulated old wall, U ≈ 1.4 W/(m²·K): fRsi ≈ 0.65, surface about 11.3 °C — below the 80% limit at 50% humidity.
Older wall, U ≈ 1.2 W/(m²·K): fRsi = 0.70, surface about 12.5 °C — right at the limit.
Well-insulated wall, U ≈ 0.24 W/(m²·K): fRsi ≈ 0.94, surface about 18.5 °C — uncritical.
These values apply only to the open wall area. In external corners, little warm interior surface meets a lot of cold exterior surface; it is often several degrees colder there. Roller-shutter boxes, ceiling supports and radiator niches are also typical thermal bridges.
Furniture against the exterior wall
A wardrobe placed directly against the exterior wall acts like insulation on the wrong side: it keeps the warm room air away from the wall, the wall surface behind it cools down, while humid air still gets in. A gap of a few centimetres so that air can circulate reduces the risk considerably. Place large furniture against interior walls where possible.
What really helps?
1. Lower humidity: burst-ventilate regularly, especially after showering, cooking and in the bedroom in the morning. Tips in the article Ventilating correctly in winter.
2. Heat all rooms sufficiently: cool rooms have colder walls. According to Germany's Environment Agency (UBA), the temperature in bedrooms should ideally not drop below 16 °C.
3. Place furniture with a gap to the exterior wall and do not block radiators.
4. In the long term, raise the surface temperature: insulating the exterior wall or fixing thermal bridges works permanently. You can roughly estimate the cost with the renovation cost calculator.
Check your wall
In the dew point calculator you enter your indoor climate, the outdoor temperature and the U-value of your wall — or a surface temperature you measured yourself. The mould-risk traffic light shows whether surface humidity is below 70%, between 70 and 80% or above, and states the maximum indoor humidity at which your wall stays below the 80% limit.
Don't simply paint over visible mould. The UBA points out that mould can affect health when spores are inhaled in large numbers. For larger areas or recurring growth, professional help is advisable.