When designing or installing façade through-wall systems, the question sometimes arises: is a breather membrane always necessary, given the different specifications and performance levels of the various sheathing boards available? The answer is that a breather membrane should always be used on the face of a sheathing board, regardless of which sheathing board is used.
The breather membrane provides an extra layer of water resistance and is included within the through-wall structure to increase the resistance to water ingress. Its inclusion or otherwise should not depend on the type of sheathing board being used.
Although most sheathing boards offer a measure of durability and water resistance, they are rigid panels with joints and fixings. This means their ability to accommodate building movement, and to seal interfaces and penetrations while maintaining water tightness, is limited.
On the other hand, breather membranes can be easily dressed, wrapped, and sealed (typically with tapes) around these interfaces and penetrations, providing a much greater level of resistance to water ingress.
The breather membrane should satisfy several key criteria:
- It should carry suitable third-party accreditation.
- Its vapour resistance should be less than 0.6MNs/g, when tested in accordance with EN ISO 12572 set C and five specimens.
- The membrane should possess a minimum B-s1,d0 fire-classification according to EN 13501-1
- It should be rated at least Class W2 to BS EN 13859-2. In areas of extreme exposure or anticipated liquid water penetration of the cladding, Class W1 should be used.
Even though the sheathing board may possess BBA certification, it does not eliminate the need for a breather membrane. It uniquely acts as a barrier, preventing water from seeping inward through the wall, while still allowing water vapour to diffuse outward, making it an essential element in façade through-wall systems.