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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.

Intelligent airtight membranes offer significantly more reliable protection than conventional sheeting

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.

Air flow (convection) is problematic

A flowing movement of air is referred to as convection. This can occur in thermal insulation structures if there are gaps in the vapour retarder layer. The temperature difference between the interior and exterior climates leads to an air pressure gradient, which the air flow attempts to balance out.

Several hundred grams of moisture can enter the insulation due to convection in a single day and accumulate there in the form of condensation water.

Diffusion is desirable – Convection is not

In contrast with convection, diffusion is a predictable and desirable process. Diffusion takes place due to a concentration or partial pressure difference between the inside and outside. In this case, mass transfer does not occur through leaks or gaps, but instead in the form of moisture passing through a monolithic, airtight material layer.

Diffusion is generally from the inside to the outside in winter and from the outside to the inside in summer. The entry of moisture into a structure depends on the diffusion resistance (sd value) of the material. In Central Europe, the period with warm exterior temperatures is longer than the period with winter temperatures, which means that more moisture can dry out of the structure.

A vapour retarder with an sd value of 2.3 m allows approx. 5 g of moisture per square metre to penetrate into the building structure each day in winter according to DIN 4108.

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!

Additional unplanned sources of moisture

Unplanned entry of moisture at the sides of building components

Flank diffusion: In this case, moisture enters into the thermal insulation at the sides of an adjacent component. This adjacent component is generally airtight, but has a lower sd value than the vapour retarder. Example: an enclosed masonry wall with a coating of airtight plaster. If structures that are closed to diffusion on the outside have vapour retarders on the inside that allow little or no drying to the inside, there is a danger of an accumulation of moisture and of resulting damage to structures in the case of airtight design.

Unplanned moisture from building materials

Moist construction materials: By their very nature, newly built structures often contain a lot of moisture in addition to the building materials themselves. This example illustrates the amounts that can be involved: a roof with 6/22 rafters, e=70 cm and a wood density of 500 kg per cubic metre will have approx. 10 kg of wood per square metre; When this wood dries, the following amounts of water will be released per square metre

  • 1% drying: 100 g of water/m²
  • 10% drying: 1000 g of water/m²
  • 20% drying: 2000 g of water/m²

These quantities of moisture can subsequently enter into other parts of the building structure.

The key takeaways

  • Moisture can enter into a building structure in many different ways. It is impossible to prevent a certain level of moisture loading.
  • However, if moisture levels are too high, moisture damage to structures can result.
  • Vapour retarders provide more reliable protection than vapour barriers. Vapour barriers with high diffusion resistances allow for barely any drying from the component to the inside and thus quickly become moisture traps.
  • The decisive factor in preventing damage to structures is the presence of significant drying reserves.

Systems for building sealing to reliably prevent moisture damage to structures