BU573 Flexible wall underlays (2014)

Wall underlays reduce air movement into insulated frame cavities, restrict the occasional transfer of liquid water to the framing, allow water vapour to escape from the inside and, where absorbent, temporarily absorb condensation.

This bulletin focuses on the performance, specification and installation of flexible wall underlays applied to wall framing. It replaces Bulletin 470 Wall underlays.

The bulletin covers:

• function of wall underlays
• codes and standards
• performance parameters
• specifying a wall underlay
• installation, including important installation requirements, fixing, limiting exposure to weather, intermediate restraints and flashing tape
• factors affecting performance.