VCLs are often specified in construction projects in a variety of scenarios, however, their use is often misunderstood and their importance is often understated.

Warranty surveyors are being presented with information for vapour control membrane products that do not have appropriate information to allow a determination of whether they meet our requirements or not.

Often information, when reviewed, reveals that products are not suited to their intended purpose or have performance characteristics that do not meet the values expressed within supporting calculations and the Technical Manual guidance. 

Our stance is that VCLs must protect building elements from degradation by effectively restricting the movement of water vapour from inside a building to within the building structure.

Failure to provide an appropriate VCL is considered to compromise the ability of the elements construction and associated materials in meeting with the functional requirements and performance standards stipulated by the Technical Manual, notably those relating to the control of moisture and the reduction of risk to building fabric from the effects of interstitial condensation.

When selecting a product that is to be used as a vapour control layer, the developer should ensure the VCL performance aligns with their condensation risk analysis requirements and clearly proves and demonstrates attainment of any required characteristic performance using published data from a recognised source.

The below is not an exhaustive list of every situation where a VCL is required. However, it should provide some guidance for the most common position for a VCL to be located:

For flat roofs, a VCL should be used on the warm side of the insulation - for warm deck flat roofs its location is typically a layer placed immediately above the structural deck.

Cold deck flat roofs are not acceptable for warranty unless they are 3m² or less in area. In these situations the VCL is fixed to the underside of the flat roof joists.

In respect of the above, depending upon the situation, the VCL may take the form of:

• A vapour control plasterboard comprising a metallised polyester film bonded to the back face of the plasterboard or;
• A minimum 125 micron thick (500 gauge) polythene sheet or;
• A proprietary vapour control membrane product supported by third party accreditation from a UKAS accredited body

Developers should ensure that any information demonstrating product or material performance is forwarded to the warranty surveyor as early as possible alongside all relevant designs and condensation risk analysis.