Drained cavity (external wall)
Drained cavity: ventilated gap between cladding and wall wrap that drains incidental water and vents vapour. NCC 2025 makes it mandatory in climate zones 6, 7, 8.
Ask Chalkline about this →A drained cavity is a continuous air gap between the external cladding and the wall wrap that lets incidental water drain to the base of the wall and vapour ventilate upward, rather than soaking into the wall assembly. The cavity is formed by fixing cladding to cavity battens (commonly 12 mm or 20 mm timber or proprietary plastic battens) installed over the wall wrap, rather than directly through the wrap and into the studs. Openings at the base of the wall (weep holes) and at the top of the cavity (vent strip or fascia gap) allow air to move through.
Under NCC 2025 Part 10.8 (Condensation management), direct-fix cladding will generally no longer be permitted in climate zones 6, 7 and 8 (cool temperate, cool, alpine). Lightweight external cladding in these zones must sit over a continuous drained and ventilated cavity (verified 2026-05-16 against ABCB NCC 2025 and HIA confirmation).
Commencement timeline (verified 2026-05-16):
| Jurisdiction | NCC 2025 commencement |
|---|---|
| Victoria | 1 May 2026 (in force) |
| NSW | Delayed beyond original 1 May 2026 |
| Qld | Delayed beyond original 1 May 2026 |
| Other states | Phased; check ABCB current adoption table |
Typical construction:
- Cavity batten: 12 mm minimum (lightweight cladding), 20 mm common (brick veneer), proprietary battens for specific systems.
- Battens: vertical (most common), at 600 mm centres aligned to studs.
- Wrap behind the cavity: Class 4 vapour permeable membrane per AS 4200.
- Base opening: weep holes or open base course at slab edge or DPC.
- Top opening: vent strip at eaves or fascia, often combined with insect mesh.
- Penetration sealing: each pipe, fixing or service penetration sealed to maintain the drainage and vent path.
Why it matters. Drained cavity construction is the building-physics fix to the condensation-and-leak problem that hit Australian residential housing in the late 2010s. Direct-fix cladding without a cavity puts the wall wrap immediately behind cladding, where any liquid water that gets past the cladding has nowhere to drain and any condensing vapour cannot escape. A drained cavity gives both a place to go.
Also known as: drained and ventilated cavity; rain-screen cavity (when paired with a non-airtight outer cladding); battened cavity; cavity-batten install.
Category: Building science.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.