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.

In the summertime, the diffusion flow is in the opposite direction: from the outside to the inside. In addition, the average moisture of the surrounding air at the vapour retarder is over 80%. At these ambient moisture values, moisture transport is activated in moisture-variable vapour retarder membranes. This means that the membranes now become highly permeable to diffusion and thus allow for the drying of outer components towards the inside.

In the case of building components that are already permeable on the outside by design, this moisture-variable behaviour offers additional protection against the unforeseen entry of moisture. For building components that are impermeable to diffusion on the outside, this potential for drying is essential as the only path that remains for drying of the building component is towards the inside.

Additional information:

pro clima technical team

Marcus Willink of pro clima Germany answered this question. The engineers in Timber Construction can help you quickly and simply in an expert manner and can offer solutions on cost-effective, reliable constructions that lead to healthy living environments.
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