Scale model of a house cut away to show a ventilated wall cavity, wall wrap, insulation and a vented roof space with a bathroom exhaust duct running outside.

How to manage condensation in residential buildings: NCC 2022 and NCC 2025 compared

In short

  • Condensation rules for houses sit in Part 10.8 of the ABCB Housing Provisions. For sole-occupancy units in Class 2 buildings and Class 4 parts of buildings, they sit in Part F8 of NCC Volume One.
  • NCC 2025 brings climate zones 1 and 3 into the wall vapour permeance rules for the first time, makes drained and ventilated cavities mandatory in zones 6, 7 and 8, and splits roof ventilation into two clauses.
  • Climate zone 2 is still not covered by the wall permeance table, and roof ventilation still applies only in zones 6, 7 and 8.
  • NSW and Tasmania have varied NCC 2025 so their NCC 2022 condensation rules continue, even after they adopt NCC 2025.
  • The house exhaust rules (25 L/s, 40 L/s, 10 minute run-on, make-up air) read the same in both editions.

Correct as at September 2026.

As houses get tighter and better insulated, moisture has fewer ways out. When warm, moist air reaches a surface below its dew point inside a wall or roof, it turns to water. Do that often enough and you get mould, rotting timber, corroded fixings and insulation that no longer performs.

The NCC deals with this through a short set of Deemed-to-Satisfy provisions. NCC 2025 reworks them substantially, and the change most likely to catch designers out is not in the cold south. It is in the tropics.

Where the rules sit

For Class 1 buildings, the provisions are in Part 10.8 of the ABCB Housing Provisions, which grew out of Part 3.8.7 of NCC 2019 Volume Two. For sole-occupancy units of Class 2 buildings and Class 4 parts of buildings, the matching clauses are in Part F8 of NCC Volume One: F8D3 for walls, F8D4 for exhaust, and F8D5 and F8D6 for roofs. The technical content of the two lines up closely.

Which edition applies to you

NCC 2025 applies in Victoria and, alongside NCC 2022 until 1 May 2027, in the ACT and WA, while the other states and the NT are on NCC 2022 for now. Our NCC 2025 vs NCC 2022 guide has the state by state detail.

NSW and Tasmania keep the 2022 rules. NSW has varied NCC 2025 so that Housing Provisions Part 10.8 and Volume One Part F8 do not apply, and the NCC 2022 Amendment 2 versions apply instead. Tasmania replaces Part 10.8 with the 2022 version. So the NCC 2025 condensation changes below will not reach NSW or Tasmanian houses when those states adopt NCC 2025.

External walls: what changed

NCC 2022 set a minimum vapour permeance for membranes, sarking-type materials and insulation on the outside of the primary insulation layer in climate zones 4 to 8, and required a drained cavity where no pliable building membrane was installed. Zones 1, 2 and 3 had no permeance requirement.

NCC 2025 introduces the defined term control layer and ties the permeance you need to both the climate zone and whether the wall has a drained and ventilated cavity (clause 10.8.1 and Table 10.8.1, or F8D3):

Climate zoneNCC 2022 (µg/N.s)NCC 2025, no cavity (µg/N.s)NCC 2025, drained and ventilated cavity (µg/N.s)
1No requirement≥ 0.0022 to < 1.1403≥ 0 to < 1.1403
2No requirementNo requirementNo requirement
3No requirement≥ 0.1429≥ 0.0022
4, 5≥ 0.143≥ 1.1403≥ 0.1429
6, 7, 8≥ 1.14Not permitted≥ 1.1403

Two things stand out. First, zone 1 sets an upper limit, because in the tropics a low permeance layer helps stop outdoor moisture being driven inwards. A highly vapour-open wrap copied from a Melbourne job will not comply in Cairns. Our article on condensation in climate zones 1 and 3 covers this in detail. Second, in zones 4 and 5, a wall without a cavity now needs a Class 4 level of permeance, well above the NCC 2022 minimum, while a wall with a cavity keeps roughly the NCC 2022 figure.

Drained and ventilated cavities

In zones 6, 7 and 8 a cavity is now mandatory. In every zone, a wall with no pliable building membrane or water barrier behind the cladding needs one. Where a cavity is used, it must:

  • sit between the cladding and the outside of the primary insulation layer or the outermost control layer;
  • be at least 12 mm deep, formed with battens, spacers or the like;
  • be unobstructed by any control layer and drained to the exterior; and
  • have openings of at least 1,000 mm² per metre of wall at the base and top, or at each storey where the cavity is closed.

Single skin masonry and concrete walls, full-height insulated sandwich panel walls, walls outside the building envelope and any part of a wall below natural ground level are exempt.

Exhaust systems: unchanged

Clause 10.8.2 of the Housing Provisions reads the same in both editions. Exhaust systems need at least 25 L/s for a bathroom or sanitary compartment and 40 L/s for a kitchen or laundry, and must discharge to outdoor air, directly or through a shaft or duct. That includes range hoods and venting clothes dryers, but not condensing dryers. Exhausting into the roof space does not comply.

A fan that does not run continuously, serving a bathroom or toilet without natural ventilation, must be interlocked with the light switch and run on for 10 minutes after the light goes off. Rooms without natural ventilation also need make-up air, through openings to an adjacent room with a free area of 14,000 mm² or in line with AS 1668.2. The ABCB notes a 20 mm undercut on a 700 mm door achieves 14,000 mm². For apartments, F8D4 in Volume One sets the same flow rates, discharge and run-on rules, with its own wording for clothes dryers and make-up air.

Roof ventilation: split into two clauses

Roof ventilation still applies only in climate zones 6, 7 and 8. NCC 2022 had one clause for all roof spaces, with a 20 mm minimum space. NCC 2025 splits it in two:

  • 10.8.3 (F8D5), insulation not parallel to the roof plane. The usual ceiling-insulated roof. The roof space must be at least 18 mm high, sit directly above the primary insulation layer and be vented to Table 10.8.3. For pitches from 10 to under 75 degrees, that is 7,000 mm² per metre at the eaves or low level and 5,000 mm² per metre at high level or the ridge.
  • 10.8.4 (F8D6), insulation parallel to the roof plane. New in NCC 2025, for cathedral and skillion roofs. It needs an unobstructed space of at least 18 mm above the insulation or control layer, vented at low and high level, and any control layer directly above the insulation must have a vapour permeance of at least 1.14 µg/N.s.

Concrete roofs, insulated sandwich panel roofs, roofs subject to BAL-FZ requirements under AS 3959 and tiled roofs without a control layer above the primary insulation layer are exempt from both.

Where condensation claims start

The designer chooses the wall system, membrane and ventilation strategy. The builder installs them. The certifier checks against the documents. Common errors:

  • one wall wrap specified for every climate zone, without checking the permeance table;
  • no cavity in a zone 6, 7 or 8 wall on a job built under NCC 2025;
  • foil or rigid board insulation outside the frame that acts as a low permeance control layer;
  • bathroom and laundry fans ducted into the roof space;
  • cathedral and skillion roofs detailed without the 10.8.4 roof space.

Photos of the control layer, cavity battens, junctions and exhaust terminations are the best evidence you can hold if a claim comes later.

Your next steps

  1. Confirm the climate zone and the NCC edition for every live project.
  2. Check the permeance of every wrap and sarking in your standard details against the table above.
  3. Decide cavity or no cavity at concept stage, because it changes the membrane you can use.
  4. Show exhaust duct terminations to outside on the drawings.

CPD on condensation

Each of these live courses runs for one hour:

For the heat loss side of the same walls, read thermal bridging in buildings, and see the CPD requirements guide and the CPD deadlines calendar.

Frequently asked questions

Where are the NCC condensation requirements?

For Class 1 buildings, in Part 10.8 of the ABCB Housing Provisions. For sole-occupancy units of Class 2 buildings and Class 4 parts of buildings, in Part F8 of NCC Volume One. The technical requirements of the two are closely aligned.

What changed for condensation in NCC 2025?

NCC 2025 brings climate zones 1 and 3 into the wall vapour permeance rules, sets permeance by climate zone and by whether the wall has a drained and ventilated cavity, makes cavities mandatory in zones 6, 7 and 8, and splits roof ventilation into separate clauses for insulation parallel and not parallel to the roof plane.

Do the NCC 2025 condensation rules apply in NSW and Tasmania?

No. NSW has varied NCC 2025 so Housing Provisions Part 10.8 and Volume One Part F8 do not apply, and the NCC 2022 Amendment 2 versions apply instead. Tasmania replaces Part 10.8 with the 2022 version.

Is a wall cavity mandatory under NCC 2025?

In climate zones 6, 7 and 8, yes, unless the wall is exempt, such as single skin masonry or concrete or full-height insulated sandwich panels. In other zones a drained and ventilated cavity is required where there is no pliable building membrane or water barrier behind the cladding. Otherwise it is a choice, but it changes the vapour permeance the membrane must meet.

What exhaust flow rates does the NCC require?

At least 25 L/s for a bathroom or sanitary compartment and 40 L/s for a kitchen or laundry, discharged to outdoor air. The figures are the same in NCC 2022 and NCC 2025.

Does climate zone 2 have condensation requirements?

The exhaust rules apply in every climate zone, but climate zone 2 is not covered by the wall vapour permeance table, and roof ventilation rules apply only in zones 6, 7 and 8.

Sources