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.
- 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
- Barrier wall system.
- Breathable membranes market.
- Breather membrane.
- Cavity tray.
- Cavity wall insulation.
- Cavity wall.
- Chemical injected DPC.
- Cold bridge.
- Damp proof course.
- Defects in brickwork.
- Defects in stonework.
- Dew point.
- Floor insulation.
- Interstitial condensation.
- Penetrating damp.
- Rising damp.
- Roof insulation.
Featured articles and news
Urban Heritage, Development and Sustainability: international frameworks, national and local guidance.
What will the General Data Protection Regulations (GDPR) mean for you when they come into force in May?
Business Secretary chairs a new taskforce to monitor and advise on mitigating the impacts of Carillion’s liquidation.
Sir John Armitt is appointed the new chair of the National Infrastructure Commission.
High quality and high density homes - is it what we need or is it storing up trouble?
Government announces its intention to strengthen planning rules to protect music venues and neighbours.
National Audit Office reports that there is little evidence that PFI offers better value than other forms of contracting.
What is liquidation and how does it apply to contractors in the construction industry?
Scrutiny is placed on Carillion's controversial 2013 decision to extend subcontractor payment terms to 120 days.
RSHP unveil their involvement in a boundary crossing which will provide a new entry point into Hong Kong.
With PFI currently under the spotlight due to Carillion, this introductory article explains what they are.
Estimates suggest that up to 30,000 small firms could be at risk of non-payment as a result of Carillion's collapse.