Why does moisture damage occur?

Mould and dry-rot need moisture. When the air humidity is above 70-75% they can develop.
Outdoors the air humidity is often over 70%, and inside a cellar the humidity is higher than this for most of the year. This means that mould can grow and, in time, become a major problem.

Some houses with cellars suffer major problems with mould after only a few years, while others only experience minor growth even after many years. The only thing that you can be sure about is that all houses with crawl spaces ventilated by outdoor air run the risk of mould problems, even if the cellar has been built in compliance with all the regulations.

During the summer, when the air is hot and humid, the crawl space is still cool. When the air comes into the cool crawl space from outdoors it is cooled down, causing the relative humidity to increase. During an average year the relative air humidity inside the crawl space will be more than 70% relative humidity (RH) for a number of months every year (see graph), and during the summer condensation may even form in the crawl space.

The diagram shows how humidity in a typical crawl space varies over a year. The black line denotes 70% relative air humidity, which is the maximum you should have in your cellar. The air humidity curve is blue in the months when air humidity is below 70% and red in the months when it is above 70%. The measurements are made by AK-Konsult Indoor Air AB, on a villa in Bromma.