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Vapor retarders

Vapor retarders are mostly films made of polyethylene (PE). The unit of measurement used to indicate the degree of permeability of vapor retarders is the sd-value.

The sd-value of a vapor retarder/vapor barrier is obtained by multiplying the water vapor diffusion resistance factor (?) with the layer thickness of the insulating material, in metres.

An sd-value of 1 m means that the film has a water vapor diffusion resistance to a water molecule equal to that of 1 m of a stagnant air layer. The higher the sd-value of a vapor retarder film, the higher its resistance to vapor diffusion.
  • Applications in screed flooring
To avoid vapor diffusion from the ground, sufficiently vapor-proof films must be installed. These films must be equal in function to a vapor-diffusion-equivalent air-layer thickness that is higher than that of the overlying flooring material.

Precautions against vapor diffusion have to be taken in the following cases:
  • For surfaces in contact with soil
  • Insufficiently dried floor ceilings and subfloors
  • Above doorways, entrances, moisture-prone rooms, heating rooms,
  • floors above garages, heat sources underneath raw ceilings
  • Vapor-proof floorings and coatings
  • Wooden floors
Folded films must be installed with the folding facing downward.


  • Screed flooring
  • Attics
  • Applications in attic areas

Vapor retarders are intended to prevent warm and humid air from penetrating into the roof structure and forming condensation water therein. They are only effective in combination with thermal insulation and they always need to be installed on the side of the insulation facing towards the living space. To ensure that no condensation water forms on the underside (living-space facing side) of the vapor retarder, this underside has to be kept warm.

Critical conditions are reached whenever – under normal surrounding conditions (inside + 20°Celsius, 55 % relative air humidity; outside – 10°Celsius, 80 % relative air humidity) - surface temperatures drop below 12.3°Celsius.

If, moreover, vapor retarders are planned and installed inexpertly, mold formation will result. Unfavourable room climates said to be caused by the use of impermeable vapor barriers are not to be expected though if installations are carefully planned and executed. Sufficient ventilation is another important factor. Regular ventilation by opening the windows completely for short periods of time reduces air humidity in rooms to a much greater extent than would ever be accomplished by diffusion through the structures of inclined roofs.
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