A normal person evaporates approximately 40-50 grams of water every hour while resting at room temperature. Therefore a family can easily produce a bucket of water a day in moisture, which accumulates in the indoor air. Most of this moisture is transported outside by the indoor ventilation system. However, humid air can flow through holes in the ceiling and condense in the attic space. While ceilings today are built to be air and vapour tight, it is very difficult to achieve a completely sealed barrier.

Moisture problems in attics and pitched roofs can be caused by a number of factors. One such source is the building damp in materials such as concrete and wood, which are exposed to moisture during the construction process. Moisture can also accumulate through the outside air ventilation and even be transported up from inside the dwelling.

If too much moisture accumulates in the attic space, there is a risk of increased levels of relative humidity. Possible consequences of this include rot and mould on the building materials and other such problems. The risk of mould growth is particularly high in cold spots where the relative humidity is highest, such as a north gable close to the ventilation inlet. Furthermore, if the temperature becomes low enough, the moisture laden air will condense, which can result in further problems. It can get particularly cold during nights with a clear sky, due to long-wave heat radiation from the roof to the sky.

An exhaust ventilation system, whether mechanical or natural, will reduce the vapour leakage to the attic due to the pressure reduction. Today it is common to replace traditional heating systems in a building, such as a furnace, with more modern alternatives such as a heat pump or district heating. The driving forces of natural ventilation are affected by these. Therefore it is important to control the ventilation of the dwelling in order to avoid the moisture problems that result from increased vapour transportation to the attic. A balanced ventilation system is therefore critical in avoiding costly moisture related damage.