6.2 External timber framed walls

6.2.13 Breather 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
  • at least Class W2 to BS EN 13859-2 with no water leakage during testing. In areas of very severe exposure (see Clause 6.1.6 for classification of exposure zones) or where liquid water penetration of the cladding is anticipated, for example open-jointed cladding, Class W1 should be used. When open-jointed claddings are used or the membrane is likely to be left exposed during construction for a duration longer than normally to be expected (also see the membrane manufacturers recommendations on exposure times), performance should be based on artificial aged behaviour in accordance with BS EN 13859-2. Where a vented and ventilated cavity with full rainscreen and no gaps, for example masonry or rendered board claddings are used, performance should be based on artificial aged behaviour in accordance with BS EN 13111
  • 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 or cladding fixing
  • applied using fixings that are in accordance with this chapter
  • repaired or replaced before proceeding with the cladding, if damaged.

Last updated: 2nd January 2024

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