Vapour barrier

As warm, moist internal air permeates through the external envelope of a building towards the outside, it will tend to cool. When it reaches its ‘dew point’ temperature, the moisture it holds will begin to condense as water. If this happens within the structure of the building itself, this is known as interstitial condensation.

If interstitial condensation occurs within the inner skin of the building envelope, it can cause problems such as:

• Mould growth, migration of salts, mildew and staining.
• Corrosion and decay of the building fabric.
• Frost damage.
• Poor performance of insulation.

To prevent this, vapour control layers (VCL) or vapour barriers are positioned on the warm side of the structure, preventing the warm moist air from penetrating to a point where it might reach its dew point temperature. In a traditional cavity wall construction for example, a vapour barrier might be introduced between the cavity insulation and the inner masonry skin.

Vapour barriers might also be used in timber frame constructions, roof and floor constructions as well as in high humidity buildings such as swimming pools and factories.

Care must be taken to ensure that vapour barriers are installed properly, so that they are continuous, and that joints, edges, junctions, cuts and penetrations are sealed. Particular care must be taken around openings such as ceiling hatches, and service penetrations such as cables, ducts, sockets, light fittings, and so on.

As well as preventing interstitial condensation, vapour barriers can improve the airtightness of the buildings. However, typically vapour barriers have a very low permeability, but are not completely airtight. Where they are airtight, they may be described as air and vapour control layers (AVCLs). Preventing the passage of air through the structure, can improve its thermal performance. In addition, some vapour barriers may include a low-e (low emissivity) metalised foil in their construction to improve thermal efficiency.

Vapour barriers may include an adhesive face, or adhesive laps and may have some ‘elastomric’ properties, giving a limited self-sealing capability if penetrated.

Some types of rigid foam insulation have a foil bonded to their surface which will act as a vapour barrier, however, care must still be taken to ensure that joints and edges are properly sealed.

NB Approved document C: Site preparation and resistance to contaminants and moisture, 2004 edition incorporating 2010 and 2013 amendments defines a vapour control layer as: ‘Material of construction, usually a membrane, that substantially reduces the water vapour transfer through any building in which it is incorporated.’

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