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.
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 improving 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.
 Related articles on Designing Buildings Wiki
- Breather membrane.
- Cavity tray.
- Cavity wall insulation.
- Cavity wall.
- Cold bridge.
- Damp proof course.
- Defects in brickwork.
- Defects in stonework.
- Dew point.
- Dew point.
- Floor insulation.
- Interstitial condensation.
- Penetrating damp.
- Rising damp.
- Roof insulation.
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