From 1 July 2026, updated National Construction Code provisions bring in stricter condensation management requirements for wall and roof cavities — part of a broader push for more energy-efficient, airtight homes. As homes get tighter, moisture generated inside (from showing, cooking, even breathing) needs somewhere to go, or it migrates into cavities, condenses on cooler surfaces, and can lead to mould, timber rot and reduced insulation performance. This page answers the questions builders and designers ask most about drained and ventilated cavities, vapour-permeable membranes and roof space ventilation under the new provisions.
This guidance applies Australia-wide, but the specific ventilation and membrane figures on this page relate to climate zones 6, 7 and 8 — which cover Melbourne and much of Victoria, but not the whole country. If you're building outside these zones, check the requirements that apply to your climate zone before relying on the numbers below.
Drained and ventilated cavities are mandatory in climate zones 6, 7 and 8 behind many external wall claddings, where required by the applicable building code and climate/water-management provisions — particularly in high-exposure or moisture-risk situations. They may also be required as part of a cladding or external wall lining installation.
Not required in these climate zones for:
While not mandatory in other climate zones, membrane class requirements still apply with and without cavities — see NCC Part 10.8 — Condensation Management [confirm edition name/year before publishing — see note].
In colder climates, moisture drive tends to run from inside the home to the outside. A ventilation cavity lets moisture vapour moving through the wall assembly dry out once it reaches the outer cavity — there's more detail on this in the Performance Construction section of the SBA Roadmap.
Cavities sit on the outside of the vapour-permeable membrane, letting vapour travel out into the cooler cladding zone to dissipate and dry — helping maintain a drier internal condition and reducing condensation risk. Risk is highest at the bottom plate, where water gathers, and especially on the colder south-facing side of the house, where things don't dry well at the best of times.
OPTION A
Include a ventilation/drainage cavity with a permeable Class 4 membrane. This lets water vapour pass through and, if it condenses on the cladding, drain away. Add a ventilation opening at the top (subject to BAL rating and cladding requirements).
OPTION B
On a cold night, water may still condense before reaching the Class 4 membrane — minimise how long it sits in the studwork. Ventilation at the top of the cavity lets the stack effect get going once the sun warms up, accelerating evaporation and drying potential in the wall.
For more detail and strategies, head to the Performance Construction section of our Roadmap.
This depends on where the insulation layer sits in relation to the roof plane.
In climate zones 6–8, a ventilation cavity immediately above the insulation — or above a Class 4 membrane directly above the insulation — must be provided, with a minimum depth of 18mm and openings at the top and bottom of the roof. Opening sizes vary with roof slope, but ventilation openings top and bottom must always be maintained.
These requirements don't apply to:
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