Protection against mould and damage to structures
Avoiding dampness inside structures
Thermal insulation structures have to be protected against the moisture load contained in warm indoor air. This task is carried out by vapour retarders and airtight membranes.
If indoor air is able to flow through the thermal insulation in an unhindered manner, it is increasingly cooled as it penetrates further into the structure until eventually the water vapour present in the air liquefies in the form of condensation. This condensation can cause significant damage. Components that have important load-bearing functions can rot and lose their structural strength.
Moisture can also lead to the formation of mould that causes health problems.
A vapour retarder and airtight layer on the inside of the thermal insulation can help to prevent damage to structures of this kind.
The cause: Condensation – Dew point – Amount of condensation
Thermal insulation in the building envelope separates warm indoor air with its high moisture content from cold outdoor air, which has a low absolute moisture content.
If warm indoor air penetrates into a building element during the cold season, it will gradually cool down along its path through the structure. The water vapour contained in the air can then condense in the form of liquid water. The physical behaviour of the air is responsible for the formation of condensation:
warm air can hold more water than cold air. At higher relative humidities (e.g. around 65% in newly built buildings), the dew point temperature rises and, as a direct result, the amount of condensation increases too.
Fig.: With an indoor climate at 20 °C / 50% relative humidity, the dew point is reached at 8.7 °C. At -5 °C, the amount of condensate formed is 5.35 g/m³ of air.
An example: 800 g of condensation through a 1 mm gap
0.5 g of water per square metre will diffuse into the building structure each standard winter day through a gap-free insulation structure with a vapour retarder with an sd value of 30 m. In the same period, 800 g of moisture per metre of gap length will flow into the structure by convection through a gap with a width of 1 mm in the vapour retarder. The moisture amount in the latter case is 1,600 times larger!