A sheet or membrane that is steam-tight. Vapour barriers prevent steam from crossing or stagnating in the walls. They are therefore placed facing inwards, on the warm side, in front of the insulation.

When is a vapour barrier necessary?

Whatever ventilation and wall processing system is used, it is necessary to make sure that the steam does not enter the walls. 
In over 90% of cases, good ventilation, combined with thermal continuity and air tightness, is enough to avoid condensation but, in all other cases as in professional kitchens, swimming pools, ice rinks, mountain constructions, etc. where the interior temperature and humidity vary greatly from the outside and for wood frame or mobile houses, when is necessary to stop humidity from damaging the wood, completely vapour-proofed walls are needed. For that, vapour barriers are placed on the warm side of the wall (in front of the insulation). 

How to choose a vapour barrier?

Vapour barriers are characterised by their ability to resist steam diffusion. The Sd factor, expressed in metres, represents the resistance of a vapour barrier in comparison with the resistance produced by the equivalent thickness of an air layer.

  • The higher the Sd value, the less steam is let through by the product. It is more or less resistant to the diffusion of steam.
  • The lower the Sd factor, the more steam is let through by the product. It is permeable to its diffusion. 

NB: When it is necessary to set up a vapour barrier, it should be independent, placed on the wall surface and perfectly joined. There should be no holes at all. If the vapour barrier is perforated, it is necessary to do a join around the perforation. Any perforations could cause a concentrated steam leak which could develop pathologies. 

Back to FAQ list