11.2.13 Air and vapour control layers
Air and vapour control layers shall be designed and installed correctly to restrict the passage of water from within the home to the structural frame.
A high resistance air and vapour control layer should be provided, unless a condensation risk analysis shows that it is not necessary, and the air and vapour control function is being provided by another solution compliant with NHBC Technical Requirements. Modelling where appropriate should be undertaken in accordance with BS EN ISO 13788 or BS EN 15026. The following boundary conditions should be used for the purposes of calculation to BS EN ISO 13788:
- at 21ºC internally
- at -2ºC externally
- >60% internal RH.
Where they are provided, AVCLs should be:
- sealed around service penetrations
- 500g polyethylene sheet, vapour control plasterboard, or material assessed in accordance with Technical Requirement R3
- in accordance with the design
- overlapping the base rail
- fixed on the warm side of the wall insulation and frame
- placed to cover the external wall, including base rails, head rails, studs, lintels, and window reveals
- taped or sealed in accordance with the manufacturer’s recommendations and punctures made good.
Air and vapour control layers should be:
- stapled at 250mm centres to the top and bottom of the frame, at laps and around openings to closed panel timber frame
- fixed with double-sided tape or adhesive as a temporary fixing before the wall board is fixed to closed panel light steel frames.
Joints in air and vapour control membranes should:
- have 100mm minimum laps
- be located on studs or noggings.
Where vapour control plasterboard is used:
- joints between sheets should be positioned on studs or noggings
- joints should be filled, taped, and finished
- care should be taken not to displace the vapour control material when cutting vapour control plasterboard.
Where internal linings or sheathing boards are factory installed to panelised units, provision should be made to allow adequate access to achieve correct lapping and sealing of vapour control layers at junctions when installed in the final position on site.
Multiple layers of AVCL-type material through the thickness of the element should be avoided, except where condensation risk analysis shows it to be acceptable.
Closed panel timber frame and structural insulated panels (SIPs)
For closed panel timber frame systems and SIPs, where they incorporate structural timber, all structural timber elements (including the OSB of the SIP, if applicable) should have a moisture content of less than 20% before any internal AVCLs, insulation or plasterboard linings are installed.
Last updated: 2nd January 2025