Pliable building membranes and underlays
Specification and installation of sarking, wall wrap and underlays.
Overview
AS 4200 has two parts. Part 1 is the materials specification — vapour permeance, water control, fire indices. Part 2 covers installation — laps, fixing, supporting. A key 2022-era concept is vapour permeability classification, which interacts with condensation management in Part 10.8 of the Housing Provisions.
Key Requirements
- Membrane classified by water control (low, medium, high) and vapour permeance (class 1–4).
- Installation laps (typically 150 mm horizontal, 100 mm vertical).
- Continuous support behind sarking — should sag slightly between supports to drain.
- Fire indices for cavity applications.
How it's used in the NCC
Called up by Part 7.3 of the Housing Provisions and by Part 10.8 condensation management requirements.
Practical Notes
- Climate Zone 6, 7 and 8 require vapour-permeable wraps to manage condensation — don't substitute a non-permeable wrap.
- Sarking pulled drum-tight defeats the drainage path — it needs slight sag.
Where to obtain
The full text of AS 4200 is published by Standards Australia and must be purchased — typical price A$150–400 per part. Always work from the current edition referenced in the NCC, not an older one.