Mould starts to grow if it is damp enough, i.e. if the relative humidity (RH*) exceeds approx. 75%. Mould in loft spaces is a relatively new phenomenon, which can be due to a number of reasons. These include:
Nowadays our homes have a lot of heat insulation between the living areas and the attic. This prevents heat from rising into the attic, which becomes colder and thereby relatively damper.
In the past homes were often heated by means of an oil boiler, for example, and the chimney stack heated the attic, at the same time also improving the ventilation in the living areas.
All in all, this meant that the loft was less vulnerable to damp. In other words, replacing the heating system can mean that there is an increased risk of damp problems in the attic.
Damp internal air comes up into the loft through gaps in the joist system. The lower the temperature in the attic, the higher the RH.
*RH= Relative Humidity. Relative humidity means, in simple terms, “the percentage volume of water in the air compared to the air’s maximum possible water volume at the current temperature”. At 100% RH condensation is formed.