Vapour barriers installed to provide resistance to vapour diffusion through building assemblies, and through interfaces between assemblies, that are subject to differentials in temperature and water vapour pressure and where a lack of control of vapour diffusion will not adversely affect health, safety, the intended use of the building or the operation of building services.

This applies to building materials, components, assemblies and their interfaces described in Sentence 5.1.2.1.(1) that are installed in buildings to which Part 5 applies [see Sentence 1.3.3.2.(1) of Division A for application of Part 5].

This also applies to the position of low air and vapour permeance materials within the building assembly relative to the position of materials providing thermal resistance in the assemblies of buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9], where the building assembly:

• incorporates low permeance materials that act as other than cladding or that are installed on the warm side of the assembly, and
• is more susceptible to condensation [see Sentences 9.25.1.2.(4) and (5) for combinations of interior and exterior environmental conditions that create a higher risk for condensation].

This also applies to the design of vapour barriers in buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9], where the building assembly is more susceptible to condensation [see Sentences 9.25.4.2.(2) and (3) for combinations of interior and exterior environmental conditions that create a higher risk for condensation].