Paths taken by moisture

Thermal insulation structures have to be protected against the humidity loading from warm indoor air. This task is fulfilled by vapour retarder and airtight membranes.


Diffusion occurs in a planned manner

Diffusion takes place due to the pressure difference between the inside and outside. The exchange does not occur through 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 the structure depends on the diffusion permeability (perm rate) or resistance in case of sd/g-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, g-value of 11.5 MNs/g respectively a water vapor permeance of 1.43 US perms allows approx. 5 g of moisture per square metre to penetrate into the building structure each day in winter according to DIN 4108-3.


Unforeseen: Entry of moisture through adjacent components

Flank diffusion:
In this case, moisture enters into the thermal insulation through an adjacent component. This adjacent component is generally airtight, but has a lower sd/g value (higher permeance) than the vapour retarder.
A connecting masonry wall with a coating of airtight plaster would be an example here. 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 structural damage in the case of airtight design.

Feuchte Baustoffe

Unforeseen: Moisture from building materials

Moist construction materials:
Newly built structures often include a lot of water together with the building materials themselves.
This example illustrates the amounts that can be involved: a roof with 6/22 rafters, e= 70 cm (2'4") and a wood density of 500 kg per cubic metre will have approx. 10 kg of wood per square metre. If this wood dries the following quantities of water will be released

  • drying by 1%: 100 g water/m²
  • drying by 10%: 1 000 g water/m²
  • drying by 20%: 2 000 g water/m²

Those amounts of moisture can enter into other parts of the building structure.


Unforeseen: Air flow (Convection)

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 also leads to a pressure difference, which the air flow attempts to balance out.

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

Formation of condensation

Condensation formation at 50% relative humidity

The physical behaviour of the air is responsible for the formation of condensation: warm air can hold more water than cold air.
The thermal insulation in building structures separates warm indoor air with its high moisture content from cold outdoor air with its low absolute moisture content.
If warm indoor air enters into a building component during the cold season, it will cool down on its path through the structure. Liquid water can then condense out of the water vapour contained in the air.

Under standard climate conditions (20 °C; 68 °F / 50% relative humidity), the dew point is reached at 9.2 °C (48.6 °F). At -10 °C (14 °F), the amount of condensate formed is 6.55 g/m³ of air.

800 g (28.2 oz) of condensation through a 1 mm (0.04") gap

An example:
0.5 g (0.17 oz) of water per square metre will diffuse into the building structure per standard winter day through a gap-free insulation structure with a vapour retarder with an sd-value of 30 m (g-value: 150 MNs/g).
In the same period, 800 g (28.2 oz) of moisture per metre of gap length will flow into the structure by convection through a gap with a width of 1 mm (0.04") in the vapour retarder.
This corresponds to an increase by a factor of 1600.

Note: The humidity of air increases when it is cooled. When the temperature falls below the dew point, condensation will form. The dew point temperature increases at higher indoor air humidity. The result is that condensation forms earlier.


  • 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, structural damage can result.
  • Vapour retarders are more reliable 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 keeping a building structure free of damage is the presence of significant drying reserves.