Avoiding moisture damage
Protection against mould and damage to structures
Thermal insulation structures have to be protected against the moisture loading contained in warm indoor air. This task is carried out by vapour check (Oceania: vapour control; North America: vapor retarder) 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 insulation structure until eventually the water vapour present in the air liquefies in the form of condensation. This condensation can cause significant damage to building structures. Crucial load-bearing components can start to rot and may lose their structural strength.
Moisture can also lead to the formation of mould that causes health problems. Many mould fungi release poisons – such as MVOCs (microbial volatile organic compounds) – and spores as secondary metabolic products that are harmful to human health. These are a leading cause of allergies. Humans should avoid all contact with mould fungi. In this context, it does not make any difference whether the MVOCs or spores enter into the human body through the stomach from our food or else through the lungs if they are present in the air we breathe.
A vapour check and airtight layer on the inside of the thermal insulation can help to prevent this kind of moisture damage to structures.
Intelligent airtight membranes offer significantly more reliable protection than conventional sheeting
Condensation – Dew point – Amount of condensation
Thermal insulation in the building envelope separates warm indoor air, which has a high moisture content, from cold outdoor air with its 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 this air may 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.: Starting out from an indoor climate at 20 °C and 50% relative humidity, the dew point is reached at 8.7 °C. At a temperature of -5 °C, the amount of condensate formed will be 5.35 g/m³ of air.
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 check 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 check. The amount of moisture is 1,600 times larger in the case of convection through a gap!