Light steel framing

6.10.18Vapour control layers

Vapour control layers (VCLs) shall restrict the passage of vapour from within the home to the steel frame and be correctly installed.

A VCL should be provided, unless a condensation risk analysis shows it is not necessary. An analysis in accordance with BS EN ISO 13788 (Glaser method), using the following boundary conditions, will generally be acceptable:

  • „>60% internal RH
  • „at 21ºC internally
  • „at -2ºC externally.

Split layers of VCL-type material should be avoided, except where condensation risk analysis shows it to be acceptable.

Where they are provided, VCLs should be:

  • 500g polyethylene sheet, vapour control plasterboard, or material assessed in accordance with Technical Requirement R3
  • fixed on the warm side of the wall insulation and frame
  • „in accordance with the design
  • placed to cover the external wall, including base rails, head rails, studs, lintels and window reveals
  • overlapping the base rail
  • fully sealed and punctures made good.

Where polyethylene sheet is used:

  • each joint in the VCL should be located on studs or noggings and lapped by a minimum of 100mm
  • double-sided tape or adhesive should be used as a temporary fixing before the wall board is fixed.

Where vapour control plasterboard is used:

  • joints between sheets should be positioned on studs or noggings
  • care should be taken not to displace the vapour control material when cutting vapour control plasterboard.