Under NCC2025 drained and ventilated cavities are mandatory in climate zones 6, 7 and 8 behind 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:

• a single skin masonry wall
• a single skin concrete wall
• a wall constructed from insulated sandwich panels for the full extent of the external wall
• a wall that does not form part of the building envelope
• any portion of the external wall below natural ground level

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].

• Cavity battens should be a minimum of 12mm.
• The cavity should be unobstructed by any control layer.
• It should have ventilation openings to the exterior at the top and bottom.
• The cavity should be drained to the exterior, with a free area of not less than 1,000mm²/m at the base and top of each cavity — in plain terms, a continuous 10mm gap top and bottom of the cavity.

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.

Some suggested approaches
‍

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.

• Insulation parallel to the roof plane (roof area greater than 10m²): the roof space must be ventilated with a minimum 18mm height for unrestricted airflow, a low-level/eaves opening of not less than 20,000mm²/m, and a high-level/ridge opening of 5,000mm²/m.
• Insulation not parallel to the roof plane: the same requirements apply for roofs under 10°. For roofs above 10°, the low-level/eaves opening must be not less than 7,000mm²/m, with a high-level/ridge opening of 5,000mm²/m.

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:

• a concrete roof
• a roof made of insulated sandwich panels
• a roof subject to Bushfire Attack Level FZ requirements under AS 3959
• a tiled roof without a control layer located above the primary insulation layer

• Having a cavity but not ventilating it.
• Cavity closers that don't open enough to allow adequate ventilation.
• BAL rating compliance — FZ has different requirements, and lower-level roof ventilation needs to be larger than the top ventilation.
• Not adjusting for different climate zone requirements — this page covers climate zones 6, 7 and 8 only.
• Blocked or missing ventilation pathways.
• Incorrectly installed water-resistive barrier (WRB) systems.
• Not following manufacturer installation procedures.

• Use a vapour-permeable membrane with a permeance of around 1.14 µg/N·s as the control layer [verify unit/figure] — this lets the cavity sit outside the primary insulation area, avoiding disturbance to its performance and limiting wind-washing of open-cell insulation.
• Keep insulation within 18mm of the roof plane.
• Above-membrane ventilation is an option, but insulation (and potentially beam thickness) will need to be upsized under the membrane to offset the loss of a foil-backed blanket.
• Where the ventilation pathway passes through a roof space with flat-ceiling insulation, use raised-heel trusses so insulation has room to sit without blocking the required 18mm ventilation path at the wall edge.
• Match the product to the home and system.