This technical document provides additional guidance relating to the use of vapour control layers (VCLs) in construction.
VCLs are often specified in construction projects in a variety of scenarios, however, their use is often misunderstood and their importance is often understated. This technical document will provide background information on what a VCL is for and where they should be used.
A VCL is a membrane used throughout construction projects and it is designed to protect building elements from degradation by restricting the movement of water vapour from inside a building to within the building structure (interstitial condensation). A VCL therefore helps prevent interstitial condensation.
Condensation will occur where the water vapour in warm air inside the property condenses into liquid form – this occurs when there is a temperature difference between the air and the surface it lands on. Condensation which occurs within elements of the building fabric is called interstitial condensation and it can occur within walls, pitched and flat roofs.
The most effective way to prevent interstitial condensation is to use a VCL. A VCL essentially prevents warm moist air getting into the structure or the cold side of the insulation/structure.
For framed structures such as timber frame and light gauge steel frame systems, a VCL should be used on or near the warm side of the insulation. The VCL should have a minimum vapour resistance of 250 MNs/g.
The VCL may take the form of:
For pitched roofs, a VCL should be used on the warm side of the insulation for both warm and cold roofs.
For flat roofs, a VCL should be used on the warm side of the insulation for warm deck flat roofs. Cold deck flat roofs are not recommended and an alternative form of flat roof should be adopted.
The above is not an exhaustive list of every situation where a VCL is required, however it should provide some guidance for the most common locations for a VCL to be located.
We’ve recently seen a surge in proprietary vapour control membrane products used on site. The developer should ensure they hold third party accreditation from a UKAS accredited body and this should be forwarded to the warranty surveyor as early as possible to confirm acceptance. They often have differing and confusing names, properties and certifications, so ensure they are reviewed very closely.
The above article should provide some background information on what a VCL is for and where they should be used. If a proprietary vapour control membrane product is proposed, the developer should carefully review the third party accreditation to ensure it is suitable for use and forward it to the warranty surveyor as early as possible.
Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.