The aim of this project is to provide guidance to industry about the use of rigid sheathing - often referred to as a rigid air barrier or rigid underlay in New Zealand. We are focusing on two factors - airtightness and weathertightness. In New Zealand, we currently do not have a Code requirement for the airtightness of a building.
Walls of newly built timber framed buildings in New Zealand generally have a drainage cavity but do not have sheathing. Overseas, particularly in North America, the typical construction is slightly different - sheathing is generally nailed to the framing before attaching the building wrap, and often a separate air barrier system is used as well to meet building code requirements for airtightness.
In New Zealand, sheathing is now mandatory for buildings in the extra high wind zone as it is perceived to offer a lower risk of weathertightness failure - sheathing provides a more robust cavity and prevents the insulation bulging, which would increase the chance of water bridging from the cladding to the framing line.
A contributing factor to the leaky home disaster was the use of products and techniques that hadn't been assessed for use in New Zealand. The use of sheathing is not perceived to represent a huge risk - quite the opposite - but we want to ensure we do not walk straight into another leaky home situation. We also want to be able to quantify the benefits of using sheathing.
Three study reports related to the work on rigid air barriers are availble.