I'm building brick veneer in Melbourne and my energy assessor says the walls need a drained and ventilated cavity. Is that right, and what exactly does it involve?

Yes, the assessor is right, and it is worth understanding why, because two different parts of the code are in play and they are often conflated: [1]

• Part 10.8 (condensation management) drives the cavity requirement.
• Part 13.2 (energy efficiency) drives the wall insulation R-value.

The cavity mandate is climate-zone-driven, not cladding-driven. Table 10.8.1 lists "no cavity" as not permitted in climate zones 6, 7 and 8, and the provision expressly names masonry veneer as in scope. Melbourne is climate zone 6, so standard brick veneer must incorporate a drained and ventilated cavity. [1] [2]

A compliant cavity must: [1]

• sit between the cladding and the external side of the primary insulation layer;
• be at least 12 mm deep and unobstructed by any control layer;
• drain to the exterior;
• have openings of not less than 1,000 mm2 per metre of wall, located at the base and top of the cavity (or at each storey where the cavity is closed).

Drawings showing brick veneer with no cavity vents fail the last point. Weep holes and vents must deliver that free area, and vermin devices are only acceptable if they still allow the required drainage and ventilation. [2] If a sarking or pliable membrane sits between the cladding and the insulation, in climate zone 6 it must be vapour permeable to AS 4200.1 Class 4. [1]

Wall type does matter at the margins. The cavity requirement does not apply to a single skin masonry or concrete wall, a wall of full-height insulated sandwich panels, a wall outside the building envelope, or any portion below natural ground level. Standard brick veneer is none of these. [1]

One honest caveat: the commonly quoted R2.5 wall insulation figure for this scenario comes from ABCB guidance written against NCC 2022. Victoria is on NCC 2025, so confirm the operative R-value against the NCC 2025 Part 13.2 tables directly rather than relying on that figure. [3]