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.’
[edit] Related articles on Designing Buildings
Featured articles and news
A threat to the creativity that makes London special.
How can digital twins boost profitability within construction?
The smart construction dashboard, as-built data and site changes forming an accurate digital twin.
Unlocking surplus public defence land and more to speed up the delivery of housing.
The Planning and Infrastructure bill oulined
With reactions from IHBC and others on its potential impacts.
Farnborough College Unveils its Half-house for Sustainable Construction Training.
Spring Statement 2025 with reactions from industry
Confirming previously announced funding, and welfare changes amid adjusted growth forecast.
Scottish Government responds to Grenfell report
As fund for unsafe cladding assessments is launched.
CLC and BSR process map for HRB approvals
One of the initial outputs of their weekly BSR meetings.
Architects Academy at an insulation manufacturing facility
Programme of technical engagement for aspiring designers.
Building Safety Levy technical consultation response
Details of the planned levy now due in 2026.
Great British Energy install solar on school and NHS sites
200 schools and 200 NHS sites to get solar systems, as first project of the newly formed government initiative.
600 million for 60,000 more skilled construction workers
Announced by Treasury ahead of the Spring Statement.
The restoration of the novelist’s birthplace in Eastwood.
Life Critical Fire Safety External Wall System LCFS EWS
Breaking down what is meant by this now often used term.
PAC report on the Remediation of Dangerous Cladding
Recommendations on workforce, transparency, support, insurance, funding, fraud and mismanagement.
New towns, expanded settlements and housing delivery
Modular inquiry asks if new towns and expanded settlements are an effective means of delivering housing.