External timber framed walls

6.2.13Breather membranes

Breather membranes shall be correctly installed to protect the sheathing and frame from moisture, and allow water vapour from within the frame to pass into the cavity.

Breather membranes should be:

  • vapour resistant to 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
  • Type 1 to BS 4016 in areas of very severe exposure to wind driven rain, unless impervious or rendered masonry cladding is used
  • capable of resisting water penetration
  • durable and adequately strong when wet, to resist site damage
  • self extinguishing
  • fixed so that vertical joints are staggered where possible, and at regular intervals, to prevent damage by wind
  • lapped so that each joint is protected and moisture drains outwards and downwards
  • lapped so that upper layers are over lower layers to ensure rain runs away from the sheathing
  • lapped so that water is shed away from the lowest timber
  • lapped with a minimum 100mm overlap on horizontal joints and 150mm on vertical joints
  • fixed at a maximum spacing of 600mm horizontally and 300mm vertically
  • fixed at a maximum spacing of 150mm around openings
  • marked with stud positions for wall tie fixing
  • applied using fixings that are in accordance with this chapter
  • repaired or replaced before proceeding with the cladding, if damaged.

When bitumen impregnated fibre building board is used and a breather membrane is not specified, the joints of the boards should be closely butted and horizontal joints sealed to prevent water ingress.

When a breather membrane is not required, the bottom frame members should be protected from water in the cavity.