6.2 External timber framed walls

6.2.12 Air and vapour control layers

Air and vapour control layers shall be installed correctly and restrict the passage of air and water vapour from within the home into the timber 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. An analysis in accordance with BS EN ISO 13788 (Glaser method), using the following boundary conditions will generally be acceptable:

  • >60% internal relative humidity
  • 21°C internal air temperature
  • -2°C external air temperature.

Air and vapour control layers should be:

  • installed once framing timbers have a moisture content of less than 20%
  • installed once the building is weathertight
  • minimum 500 gauge (125 micron) polyethylene sheet, vapour control plasterboard or a product assessed in accordance with Technical Requirement R3
  • adequately fixed to the warm side of the insulation and frame
  • fixed at 250mm centres to the top and bottom of the frame and at laps and around openings, vapour control plasterboard should be fixed in accordance with Clause 9.2.4
  • placed to completely cover the external framed wall area, including rails, studs, lintels, and sills
  • lapped with the DPC/DPM and at junctions to improve air tightness
  • lapped into reveals and sealed to window and door frames
  • sealed around service penetration
  • made good where damage has occurred.

Joints in the air and vapour control layer:

  • should have 100mm minimum laps
  • should be located on studs, rails, or noggings and be mechanically fixed
  • may be sealed with adhesive tape for enhanced air tightness (but joints should still occur over studs, rails, or noggings and be mechanically fixed).

Where vapour control plasterboard is used, joints should be:

  • positioned on studs, rails or noggings
  • filled, taped and finished
  • cut with care to avoid displacing the vapour control material.

Where floor zone air tightness membranes are specified for enhanced air tightness, they should be of a breathable material with a vapour resistance less than 0.6MNs/g (0.12 Sd) when tested in accordance with BS EN ISO 12572 using the set of conditions C and using five test specimens.

Last updated: 2nd January 2024

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