Moisture in buildings accumulates from simply occupancy and the everyday activities, such as cooking or bathing. However, it can also be caused by moisture rising from the soil beneath the building.
As a result, mould growth may occur when moisture condenses onto cold surfaces such as those in evidence at a thermal bridge. This can lead to damage to the structure through frost or corrosion. Furthermore thermal insulation performance is significantly reduced by moisture, thus resulting in a higher level of heat loss, with possible catastrophic results on the structure.
In order to prevent these negative effects, it is essential that the requirements for condensation control are met. For the building fabric, the main parameter that indicates surface condensation risk is the Temperature factor, which is governed by values contained within Part L.
When air cools, part of the resulting water vapour turns into condensation. This is a typical problem on cold surfaces in heated rooms. When the relative humidity is add comma cold surfaces are also prone to mould formation, even before condensation occurs.
Mould growth can occur with moisture levels as low as 80% humidity. This means that mould begins to grow on cold surfaces if the surface is at least cold enough for a moisture level of 80% to be reached in the layer of air directly adjacent to the surface. The temperature at which this occurs is the so-called mould temperature θS. Mould formation therefore already begins at temperatures above the dew point temperature.