Why visible condensation is only the tip of the iceberg
Most people think condensation on the window is the first warning. In reality, mould growth can start months before you see anything at all. Here we explain the building physics behind it — and what it means for how you work with moisture.
Introduction
Mould growth arises from the interplay between three factors: temperature, relative humidity and time. The three are inseparably linked — and have to be understood together in order to assess the risk in a given home.
In our work with indoor-climate data we often see that the risk is present long before the visible signs appear. Condensation on the window is not the first warning — it is a late symptom of a problem that has developed over time.
1. Temperature — warm air holds, cold air squeezes
Warm air can hold more water vapour than cold air. At 20 °C the air can hold up to 17 grams of water per cubic metre. At 10 °C it can hold only around 9 grams — roughly half.
As the temperature falls, the air's ability to hold moisture is reduced, and relative humidity rises even though the absolute amount of water in the air is completely unchanged. Air at 20 °C and 50% relative humidity contains around 8.5 grams of water per cubic metre. If that air is cooled to 10 °C without losing any water, relative humidity rises to almost 100%.
That is why cold surfaces are a problem. They literally squeeze the moisture out of the air.
2. Relative humidity — the critical temperature
The dew point is the temperature at which the air's current moisture content causes water vapour to start condensing. At 20 °C and 60% relative humidity the dew point is around 12 °C.
That may not sound critical, but cold surfaces in corners, behind cupboards placed against external walls or around windows can easily reach that temperature in winter. Thermographic surveys often show surface temperatures of 11-15 °C in corners and at window reveals, where thermal bridges reduce the temperature significantly.
Mould starts before condensation
Myth: visible condensation is a precondition for mould growth. Many common mould species can grow at a sustained relative humidity of 75-80% at the surface. That is well below the dew point.
Mould growth can begin after just a few weeks of constantly high humidity, but it can take months before the growth becomes visible to the naked eye. By the time you see dark patches on the wall or notice the characteristic musty smell, the process has been under way for a long time.
That is why reactive damp control — waiting for visible signs — is a losing strategy.
3. Time — the underestimated variable
Short-lived rises in humidity are normally harmless. When you take a hot shower, humidity rises sharply — perhaps to over 90%. But if the ventilation works, the moisture falls back to a normal level within a couple of hours. That is not a problem.
It is persistent, high humidity that creates problems. If the moisture level stays above 75-80% for days or weeks at a time, mould growth begins.
IoT data makes it possible to distinguish between passing peaks and persistent load. A single high reading is not necessarily a cause for concern — what matters is how long the condition lasts.
What the data reveals
When we analyse time series, we see the risk long before it becomes visible:
- Moisture that does not fall after a shower suggests insufficient ventilation.
- Night-time rises in humidity that do not disappear during the day indicate a lack of air change.
- Moisture levels that rise when it gets cold outside point to thermal bridges.
These patterns are invisible to the eye, but stand out clearly in the data.
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