Premier Guarantee’s Research and Development Surveyor, Victoria Barwood, advises on breather membranes, their function, when they are required, where should they be positioned and other recommendations
What are Breather Membranes?
Breather membranes are sheet or liquid applied materials that contain many very small holes or pores. These are large enough to allow water vapour to pass but resist the passage of liquid water.
What is their function?
The function of breather membranes is to provide a barrier against water penetration from the outside while allowing migration of water vapour through the membrane from inside the building.
While being typically associated with rainscreens, their use is not always well understood and they are not always required.
When are they required?
A breather membrane should be provided unless it can be established that:
- Any insulation in the cavity is resistant to wetting. For example, some closed cell foams and mineral fibre insulation with water repellent additives.
- No water reaches the backing wall, either directly or by migration through the insulation.
When a non-absorbent insulation is used (such as rigid foam), a breather membrane may not be required to protect the insulation. However, one may still be used as an additional line of defence against water penetration.
Where should they be positioned?
Typically, a breather membrane would be used on the outer face of insulation within the rainscreen cavity to protect it from any water passing through the outer panels. In order for the breather membrane to effectively protect the insulation from wetting it is important that it is continuous across the surface.
It is important that any penetrations due to rainscreen brackets should be fully sealed.
An alternative position is on the outside face of the sheathing board, if the dew point is in-board of the sheathing board. As this is against a flat surface, it’s easier to install and penetrations due to fixings are essentially sealed by the bracket which is attached. Depending on the type of sheathing boards used and the sealing between adjacent boards, a breather membrane in this position may not be necessary but may still be specified.
Recommendations
If a breather membrane is required, there is a need to consider factors such as:
- Sealing around penetrations.
- Can the material used act as an air barrier, or is a separate air barrier required?
- How the slab edge deflections are accommodated.
- How the membrane will be secured to resist cyclic wind loading.
- How the membrane will be secured around steel edge beams.
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