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NZ Building Code · WeathertightnessBuilding wraps & underlays: your second line of defence
The wall underlay sits behind the cladding to catch and drain any water that gets past, while still letting the wall dry outward.
A building wrap (wall underlay) is the second line of defence behind your cladding. If water gets past the cladding, the wrap catches it and drains it down and out — while still letting water vapour escape from inside so the wall can dry. Get the wrong product or a sloppy lap and you turn your framing into a moisture trap.
Why ‘breather’ matters
Walls get damp from both sides: weather on the outside, and living moisture — cooking, showers, breathing — on the inside. That’s why the wrap must be vapour-permeable (a “breather”).
- A vapour-permeable wrap blocks liquid water but lets vapour pass, so the wall dries outward.
- A non-breathing wrap traps internal moisture and rots the frame from the inside.
- Self-supporting wraps are mandatory in all wind zones under the current E2/AS1.
Compliant products (AS/NZS 4200.1)
These products meet AS/NZS 4200.1 and are commonly specified in New Zealand:
- Tyvek HomeWrap (DuPont) — vapour-permeable, self-supporting.
- Hydra Hydroguard (Marshall) — locally made, vapour-permeable.
- Tekton (Thermakraft) — vapour-permeable, premium.
- Watergate (Pro-Clima) — German-made, intelligent vapour control.
- Covertek 403/407 — older bituminous wrap, still acceptable.
Installation rules
Follow these steps to keep the wrap watertight and compliant:
- Use a self-supporting wrap — mandatory in all wind zones.
- Lap horizontal joints 150mm and tape them (Stick Stuck or Tyvek tape).
- Lap vertical joints 75mm.
- Keep UV exposure to a maximum of 3 months (most products).
- Cover the wrap with cladding within 90 days.
- Seal penetrations — windows and services — with flashing tape, keeping wrap continuity.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
Why does a building wrap have to be vapour-permeable?
Walls get damp from both sides — weather outside and living moisture (cooking, showers, breathing) inside. A vapour-permeable “breather” wrap blocks liquid water but lets vapour pass, so the wall dries outward. A non-breathing wrap traps internal moisture and rots the frame from the inside.
Do I need a self-supporting wrap in every wind zone?
Yes. Self-supporting wraps are mandatory in all wind zones under the current E2/AS1.
How much lap and taping do the joints need?
Lap horizontal joints 150mm and tape them (Stick Stuck or Tyvek tape). Lap vertical joints 75mm. Seal penetrations such as windows and services with flashing tape while keeping wrap continuity.
How long can the wrap stay exposed before cladding goes on?
Most products allow a maximum of 3 months UV exposure, and you should cover the wrap with cladding within 90 days.
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