The 20 mm cavity behind your cladding, explained

A plain-English walk through E2/AS1 cavity construction — why NZ builds a drained-and-vented gap behind the cladding, and how to detail it.

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The 20 mm drained-and-vented cavity behind the cladding is the single biggest lesson of the leaky-homes era, and the heart of E2/AS1 §9. If you build exterior walls in New Zealand, this is the detail that keeps water off your framing.

The idea is simple: accept that some water always gets past a cladding, and give it a clear air gap to drain back out and dry before it can reach the framing — instead of trapping it against the building paper like the failed direct-fixed monolithic walls of the late 1990s.

Cavity vs direct-fix

A cavity separates the cladding from the frame so water drains and air dries the gap. Direct-fixed cladding — screwed straight to the frame over the wrap — has no drainage path.

Only specific low-risk claddings and situations still allow direct-fix (see the Risk Matrix).
For most modern NZ builds, cavity is the default and often mandatory.

The 4Ds principle (Morris + Hazelden 1999)

The whole approach comes back to four jobs the wall has to do:

Deflection — get water off the cladding (flashings, drips).
Drainage — get water out of the wall (cavity drains to outside).
Drying — dry any moisture in the cavity (top vent for stack effect).
Durability — cladding, batten and framing survive the expected wetting.

Cavity batten specs

The batten is what holds the cladding off the frame and forms the gap. The key numbers to detail:

18–20 mm H3.2 treated timber batten (or a proprietary metal/plastic equivalent).
Vertical orientation preferred — it drains better than horizontal.
Cavity drains at the bottom, with vermin mesh at the base.
Top vent: a 10 mm gap to the soffit, or omit the mortar in the top course (brick), or drilled weeps.
Fixings 75 mm minimum through the batten into the stud.

Sheathing

Rigid wall underlay / sheathing (RAB, Triboard, EcoPly, JH HardiSheath) is mandatory in the Extra High wind zone (E2/AS1 §9.4).

It backs up the wrap if the cladding fails and adds bracing.
Common in coastal Wellington and Christchurch.

Reference: NZBC E2 — External Moisture (building.govt.nz).

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 NZ use a 20 mm cavity behind the cladding?

Because some water always gets past a cladding. The drained-and-vented cavity gives that water a clear air gap to drain back out and dry before it reaches the framing, instead of trapping it against the building paper the way failed direct-fixed monolithic walls did in the late 1990s. It is the heart of E2/AS1 §9.

What is the difference between a cavity and direct-fixed cladding?

A cavity separates the cladding from the frame so water drains and air dries the gap. Direct-fixed cladding is screwed straight to the frame over the wrap and has no drainage path. Only specific low-risk claddings and situations still allow direct-fix; for most modern NZ builds, cavity is the default and often mandatory.

What are the cavity batten specs?

An 18–20 mm H3.2 treated timber batten (or a proprietary metal/plastic equivalent), preferably run vertically because it drains better than horizontal. The cavity drains at the bottom with vermin mesh at the base, vents at the top (a 10 mm gap to the soffit, omitted mortar in the top brick course, or drilled weeps), and fixings are 75 mm minimum through the batten into the stud.

When is rigid wall sheathing mandatory?

Rigid wall underlay / sheathing (such as RAB, Triboard, EcoPly or JH HardiSheath) is mandatory in the Extra High wind zone under E2/AS1 §9.4. It backs up the wrap if the cladding fails and adds bracing, and is common in coastal Wellington and Christchurch.

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