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Variable vapor retarder

A variable vapor retarder is an intelligent film that adjusts its vapor resistance according to the amount of moisture it is exposed to. If the structure on the cold side or in the insulation package cannot breathe sufficiently, the risk of condensation arises. An intelligent self-regulating vapor retarder will have a vapor retarding function in the winter. But should there still be moisture between the vapor retarder and the roof boarding in the summer, the membrane will release this vapor into the room.
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Always sure of the right indoor climate in your home!

A variable vapor retarder is a unique film of its kind. Because this film self-adjusts its functionality based on the temperature and humidity of a room, these films are also called intelligent or climate films. Variable vapor retardant films have a dual function; vapor-open in the summer and vapor retardant in the winter.

When do you use a variable vapor retarder?

A variable vapor retarder is most appropriate when a vapor retarder or vapor-tight film has been used on the exterior of the structure. This is often the case, for example, in a flat roof construction where moisture does not escape through the cold side. Variable vapor retarder films are only suitable for spaces with climate class 2 and 3 such as living rooms, bedrooms or offices. This film is not suitable for humid areas such as a bathroom or sauna.

All our variable vapor retarder films at a glance

Where is variable vapor retarder film used?

Technical drawing Tyvek AirGuard Smart
Technical drawing Miofol Active - Flat roof
Technical drawing Miofol Active

Vapor-open in the summer

In summer, the air contains a lot of moisture, both inside and outside. Because it is often warmer outside than inside in the summer, the vapor pressure will cause moist air to enter the structure where it can condense. In this case, a variable vapor retarder film has a vapor-open function and thus ensures that the moisture has a chance to leave the structure back into the interior space. Especially in the case where structures contain a thick layer of insulation, it is essential that moisture be allowed to dry to two sides. Most vapor retardant layers have a constant vapor retardant effect throughout the year. As a result, the drying process of the structure will only be able to go to one side in the summer, so it will take longer for a structure to dry again. The dual effect of a variable vapor retarder such as the Miofol® Active gives the material a breathable effect in the summer. This way the construction dries faster!

Vapor retardant in winter

In the winter, it is cold outside and warm inside. The warm air inside contains more water vapor, causing vapor pressure to build up from the inside to the outside. Water vapor is therefore contacted by the wall and roof, pushed out from the inside. Depending on the performance of the vapor barrier layer applied, some of this moisture will enter the structure. If you don’t make sure that this moisture can leave again, condensation will form, causing wood rot and mold. A variable vapor retarder has a vapor retarding function in the winter and thus ensures that moisture does not affect the structure. This way the construction is always optimally protected!

How do you process a variable vapor barrier film?

A variable vapor retarder film such as the Miofol® Active or the Tyvek® AirGuard Smart is always used on the warm side (inside) of the structure in combination with mineral wool such as rock wool or glass wool. The film can be attached directly to the mineral wool by stapling. We recommend using an air cavity between the film and the wall finish for optimal moisture control. Apply the film preferably horizontally, taking into account overlaps of at least 10 cm. For optimal performance of the film, it is essential to seal the overlaps airtight with VAST-R® Spider Fleece tape or Tyvek® tape.

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