Non-Residential Buildings

There is no requirement for underlay under the roof of an unlined building, underlay is normally used in internal situations.

The main reason why aluminium foil underlay is installed in commercial and industrial buildings under roof cladding is to maintain surface reflectance, limit heat emission, and enhance lighting conditions. These may be non-permeable, or permeable.

Permeable options normally use the same underlays as those for residential applications. Non-permeable is often reflective foil, either double-sided or white-faced.

Foil can be double-sided aluminium or one side may be white polythene faced.  Some types may have an anti-glare coating on one side. White polythene faced foils are often used in public buildings with an exposed roof structure to enhance the appearance from below.

They may also be lightweight, standard weight or heavyweight (light, medium or heavy duty under AS/NZS 4200.1) and may or may not be fire-retardant (having a Flammability Index of 5 or less when tested to AS/NZS 1530.2).

Although foil itself is not permeable, a foil underlay will not function as a vapour barrier unless all joints, penetrations, and intersections are fully sealed.
Non-permeable underlays should comply with AS/NZS 4200.1:2017.

• An underlay is regarded as Fire Retardant if it has a FI (Flammability Index) of 5 or less when tested to AS/NZS 1530 Part 2.
• Self-supporting (S/S) is defined as strong enough to support its own weight up to a 1200mm span.
• pH between 5.5 and 8.0