The National Construction Code (NCC) 2025 introduces changes to the way cavities in external walls are detailed. Moving forward, masonry cavities will need to be ventilated, with openings provided at both the top and bottom of the cavity where required.
Key Requirements
- An external wall that does not include a pliable building membrane or water barrier between the cladding and the exterior side of the primary insulation layer must incorporate a drained and ventilated cavity.
- A drained and ventilated cavity must:
- Be located between the cladding and the external side of the primary insulation layer, or the outermost control layer;
- Have a depth of at least 12 mm;
- Be unobstructed by insulation and/or control layers;
- Be drained to the exterior; and
- Have openings with a free area of not less than 1,000 mm² per metre of wall at either:
- The base and top of the cavity; or
- Each storey level where the cavity is closed.
- These requirements do not apply to single skin masonry walls.
Vented Versus Ventilated Cavities
Traditionally, masonry cavities have been vented using weepholes at the base of the wall, as well as above window head and sill flashings. Under the NCC 2025 changes, a ventilated cavity requires continuous airflow between the base and the top of the cavity. This helps reduce humidity and moisture build-up within the wall system.
Ventilation at the Base of the Cavity
A free open area of 1,000 mm² per metre of wall is required at the base of the cavity. Weepholes in masonry perpend joints can be used to achieve this requirement. A weephole is created by omitting mortar from the perpend joint, including the bed mortar immediately above and below the joint.
For a standard brick, the weephole area can be calculated as follows: 96 mm x 10 mm = 960 mm², where 96 mm is made up of a 76 mm brick height plus 10 mm bed joints above and below.
To achieve the required ventilation area, weepholes should be placed at a maximum spacing of every fourth perpend joint for a standard brick. This provides 960 mm² of open area over a length of 960 mm, which is equivalent to approximately 1,000 mm² per metre.
Ventilation at the Top of the Cavity
The top of the masonry cavity must also provide openings with a free area of 1,000 mm² per metre of wall. The following options may be considered to achieve compliance.
Option 1: Weepholes
Weepholes may be placed at every third or fourth perpend joint, provided the total open area meets the NCC 2025 requirement. The weephole size must be checked to determine the appropriate spacing. Where weepholes are 96 mm x 10 mm, as with the base weepholes described above, spacing at every fourth perpend joint may be sufficient. Smaller weepholes will need to be installed more frequently.
Option 2: Continuous Opening
A continuous opening may be provided above the topmost course of masonry to achieve the required open area. The opening should be at least 5 mm to reduce the risk of blockage. An optional bead may be installed to conceal the opening, provided the required gap is maintained.
Option 3: Ventilation into the Roof Space
In climate zones 6 to 8, where roof spaces must be ventilated, the cavity may remain open at the top to allow air to ventilate into the roof space. This is only permissible where the roof space is ventilated in accordance with Section 10.8.3 or 10.8.4 of the NCC 2025 ABCB Housing Provisions Standard.
Option 4: Vented Cavity with Membrane
Where a membrane is installed as the moisture control layer, top and bottom ventilation openings are not required. In this arrangement, the wall may be designed with a vented cavity rather than a ventilated cavity. This may be advantageous in Climate Zone 2, where there are no minimum membrane requirements. The membrane must meet the required class for the relevant climate zone.
| Climate Zone | Membrane Class |
| 1 | 2 or 3 |
| 2 | No minimum requirement |
| 3 | 3 or 4 |
| 4–5 | 4 |
| 6–8 | Not permissible |
Download the Think Brick Australia NCC 2025 Ventilation of Cavities Factsheet.
