Description

The use of rigid wall underlays in residential construction has increased over recent years as more designers, builders and building owners recognise the advantages associated with their inclusion in exterior wall assemblies.

The requirement for rigid wall underlays (RWUs) to be installed behind exterior wall claddings in conjunction with drained and vented cavities on buildings in extra high wind zones when using Acceptable Solution E2/AS1 as a means of compliance with Building Code clause E2 has led to greater understanding of their benefits. It has also led to an increase in the development of proprietary systems and ultimately greater use.

E2/AS1 calls for generic RWUs to act as barriers to airflow. The pressure differential associated with higher-pressure air acting on the building exterior drives air through any gaps in the exterior wall assembly to the lower-pressure building interior. These air leakage paths can become water leakage paths when moisture is present on the exterior of the building.

The interior linings of the building act as a suitable air barrier at lower wind pressures – air enters the wall assembly and is restricted from entering the building by the interior linings. Pressure moderation occurs within the assembly, negating the higher-pressure exterior driving force.

Under higher wind pressures, rigid wall underlays provide an effective air barrier on the outside face of the wall framing.

Scope

This article covers:

• Generic rigid wall underlays
• Proprietary rigid wall underlays
  • Pre-cladding/early close-in
  • Containment of bulk insulation
  • Structural bracing
  • Fire resistance and acoustic performance
  • Building envelope airtightness
• RWU system considerations