Rigid air barriers (or external sheathing boards) are sometimes incorporated within open facade and rainscreen systems behind the rain screen/cladding and ventilation cavity. Here, they act as a protective skin over the internal lining and insulation and thus are a proven alternative to flexible building membranes. They are typically adopted in mid to high-rise building structures that are subject to challenging site condition but can be used on low-rise buildings.
In many ways, rigid air barriers are similar to pliable membranes in terms of condensation management potential as they are vapour permeable akin to a Class 4 vapour permeable membrane. When tapped at the joints and continuously sealed, they become a secondary water control layer and can be left exposed for extended periods throughout construction. An additional membrane may also be adhered to the rigid air barrier if further protection is required. This also allows them to becomes part of an air control layer and part of a more comprehensive air-tightness strategy.
Where they differ is their materiality, being rigid in nature and often made from a fibre cement sheet or similar rigid substrate. This allows them to have higher performance than traditional pliable membranes against wind forces or wind washing. This also allows them, in some cases, to be deemed as non-combustible in the case of fibre cement or similar variants of rigid air barriers.
Rigid air barriers have no thermal performance as a direct result of the material itself as it is typically between 13 – 16 mm thick. However, as a part of a wider thermal and condensation management strategy, they can allow dryer wall systems and thus improved R-values in practice.