Flank diffusion – what it is and how to prevent it

pro clima clients ask – pro clima engineers answer

Airtightness is good, but it is not everything: there is also the matter of diffusion! When we speak of diffusion in civil engineering, we are referring to the gradual transport of moisture through a building component. This transport is driven by the pressure difference between the interior and exterior. As water molecules always want to move from warm environments into the cold, diffusion normally occurs from the inside to the outside in wintertime. A vapour retarder hinders the transport of these molecules.

Unforeseen: Entry of moisture through adjacent components

For example, if an interior wall penetrates through an insulation layer (the vapour retarder is cleanly attached to the wall on the left and right, i.e. everything is airtight), so-called flank diffusion can occur. The water vapour looks for the path of least resistance – which in this case is the interior wall. In the case of structures that are open to diffusion on the outside, this may represent a manageable risk. However, if the structure is impermeable to diffusion on the outside, moisture problems can soon occur. After all, there is a fundamental problem here. The interior wall has a large surface area, so it absorbs a lot of moisture, but there is only a smaller area available for drying to the inside – i.e. the wall cap.

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Moisture-variability – what it is and how it works

pro clima clients ask – pro clima engineers answer

Moisture-variable vapour retarder membranes are able to adapt their diffusion resistance to the time of year in an optimal manner. As a result, they are more closed to diffusion in winter and can thus protect building components against moisture penetration. In contrast, they are open to diffusion in summer and thus ensure the best possible conditions for drying to the inside.

Moisture-variable membranes react to the humidity of the surrounding air
The moisture of the surrounding air is around 40% on average in wintertime for a vapour retarder installed on a building component in the case of typical usage of the building as a home. This is relatively dry.
Nonetheless, the diffusion flow is from the warm indoor space, through the building component and towards the outdoor air, which is cold and effectively even drier. A high vapour resistance on the inside of the building component inhibits or stops the movement of moisture that would condense along its route towards the colder outer layers, and in this way the structure is protected against the condensation that would otherwise form. Continue reading