5.5.1.1.Required Resistance to Vapour Diffusion (See Note A-5.5.1.1.)

(1) Where a building component or assembly is subjected to differentials in temperature and water vapour pressure, the properties and position of the materials and components in those components or assemblies shall be such that they control vapour diffusion or permit venting to the exterior so as to minimize accumulation of condensation in the building component or assembly. (2) Except as provided in Sentence (3), a vapour barrier shall be installed to provide the principal resistance to water vapour diffusion. (3) A vapour barrier is not required where it can be shown that uncontrolled vapour diffusion will not adversely affect any of (a) the health or safety of building users, (b) the intended use of the building, or (c) the operation of building services.