Design Considerations / Comfort & Energy / Construction & materials
Condensation and building breathability
A well-insulated, airtight house keeps the heat in, which also makes the outer layers of its walls and roof colder. Any moisture that reaches them can condense where no one sees it. The current National Construction Code now dictates how walls and roofs in Canberra and the surrounding region must be built to let that moisture dry out.
Design ConsiderationsComfort & EnergyAt a glance
Tight, well-insulated houses have colder outer layers, where moisture can condense out of sight. In Canberra and the surrounding cold zones, the code now requires walls and roofs that let moisture drain and dry out: a ventilated cavity behind the cladding, a ventilated space above the ceiling insulation, and vapour-open membranes.
- Canberra, the Southern Tablelands and the alpine areas are in climate zones 6 to 8, where the strictest rules apply.
- Direct-fixed cladding is no longer allowed there. Lightweight cladding must sit on battens over a drained and ventilated cavity at least 12 mm deep.
- The wrap behind the cladding must be the most vapour-open class, Class 4.
- Vertical cladding needs cross-battening, or ventilated battens, so the cavity can still drain and breathe.
- Roofs need a clear ventilated space of at least 18 mm above the insulation, with vents at the eaves and near the ridge.
- Raked and cathedral ceilings are the hardest to detail. Decide the roof build-up at concept stage.
- The first defence is still keeping moisture out: exhaust fans ducted outside, and a well-sealed inner lining.
The problemWhy condensation is now a design issue
Warm indoor air carries moisture. A family of four adds 10 to 14 litres of water to it every day by breathing, cooking, showering and drying clothes. When that air reaches a surface below its dew point, the moisture turns to water. On a window you can see it. Inside a wall or roof you can't. There it wets timber, corrodes fixings, ruins insulation and grows mould.
Canberra is a hard climate for this. The region has about 60 frosty nights a year, and 9 am humidity in July averages 85%. Older houses got away with it because they leaked: draughts dried them out, at the cost of comfort and energy. The better insulated and sealed a house becomes, the colder its outer layers get and the less accidental drying there is. Moisture has to be managed by design.
The NCC has tightened its rules in stages, in 2019, 2022 and again in NCC 2025. The strictest requirements apply to climate zones 6, 7 and 8, which covers Canberra (zone 7), the Southern Tablelands (zone 7), western Sydney (zone 6) and the alpine areas (zone 8).
The principleAirtight, but able to breathe
Breathability is not the same as leakiness. A good wall or roof is:
- Airtight, so warm, moist indoor air can't flow into it through gaps. Air leaks carry far more moisture into a wall than vapour soaking through the lining.
- Vapour-open towards the outside, so any moisture that does get in can dry outwards. Membranes on the outside of the insulation must let water vapour through while keeping liquid water out.
- Drained and ventilated behind the cladding, so rain that gets past the cladding runs out, and moving air carries away moisture coming through the membrane.
The rule of thumb is tighter on the inside and more open on the outside. A wall that traps moisture between two vapour-tight layers can't dry in either direction.
WallsCavities are now required
In climate zones 6, 7 and 8, NCC 2025 requires external walls to have:
- A Class 4 vapour-permeable membrane (the most vapour-open class under the Australian Standard) for any wrap or barrier between the insulation and the cladding
- A drained and ventilated cavity between the cladding and the outside of the insulation. Direct-fixed cladding, nailed straight through the wrap into the studs, is no longer permitted in these zones.
The cavity must:
| Requirement | Minimum |
|---|---|
| Depth | At least 12 mm, formed by cavity battens, spacers or similar |
| Obstruction | Clear of any membrane or other layer |
| Drainage | Drains to the outside, at each level where the cavity is closed |
| Ventilation openings | At least 1,000 mm² per metre of wall, at both the base and the top of the cavity, or at each storey |
Some walls are exempt: single-skin masonry and concrete, insulated sandwich panels over the full wall, walls outside the building envelope, and walls below ground.
What it means in practice
- Brick veneer already has a cavity. Keep the weepholes open at the base and the cavity clear of mortar droppings, and make sure it's ventilated as well as drained.
- Lightweight cladding (weatherboards, fibre cement, metal or timber panels) now sits on battens over the wrap. With horizontal cladding, vertical battens fixed to the studs form channels that drain downwards and ventilate upwards.
- Vertical cladding needs fixing points across the wall. Fixing horizontal battens directly to the studs would block the cavity. The answer is cross-battening: vertical battens against the wall first, then horizontal battens over them for the cladding. Proprietary ventilated battens with notches or grooves do the same job in one layer.
- The wall gets thicker. A cavity adds to the depth of every wall, which affects window reveals, flashings, sill and head details, the base of the wall, and on tight blocks the setbacks. It has to be drawn from the start, not added on site.
- Openings need protection. Vents at the top and base of the cavity need insect mesh, and in bushfire-prone areas, ember protection.
RoofsA ventilated space above the insulation
In climate zones 6, 7 and 8, the roof must have a ventilated space directly above the ceiling insulation, at least 18 mm high and not blocked by insulation. Moist air that reaches the roof space can then escape before it condenses on the underside of cold roofing or sarking.
The ventilation openings required depend on the roof:
| Roof type | Low-level openings (eaves) | High-level openings (at or near the ridge) |
|---|---|---|
| Pitched roof, 10° or steeper, ceiling below the roof space | 7,000 mm² per metre | 5,000 mm² per metre |
| Low-pitched roof, under 10° | 20,000 mm² per metre | Not specified |
| Skillion and raked roofs, with insulation following the roof slope | 20,000 mm² per metre | 5,000 mm² per metre |
Low-level openings are calculated on twice the longest plan dimension of the roof, or once for a skillion. High-level openings must be at the ridge or within 900 mm of it. Concrete roofs, insulated panel roofs, roofs at the highest bushfire level (BAL-FZ), and tiled roofs with no membrane above the insulation are exempt.
What it means in practice
- Pitched roofs with a roof space usually have room. The risk is at the eaves, where ceiling batts get pushed hard against the roof and close the gap. Lower-profile perimeter batts, or eave spacers, keep the airway open.
- Metal roofs with an insulation blanket need ventilation between the ceiling batts and the blanket, or a vapour-permeable blanket with a ventilated space above it.
- Raked ceilings and skillion roofs are the hardest. The insulation runs up the slope, so a clear ventilated gap has to be built above it. Usually that means cross-battening over the rafters, or deeper rafters, plus eave and ridge vents. That adds depth to the roof, which changes fascia heights and ceiling levels.
- Ridge and eave vents have to be designed into the roofing and gutter details, and in bushfire areas, ember-screened.
Cathedral ceilings need deciding early
A raked ceiling looks simple on a sketch. Under the current code it needs a ventilated cavity above the insulation, enough insulation below that cavity to reach seven stars, and vents at both ends. All of that has to fit within the roof depth. Decide the roof build-up at concept stage, and the ceiling heights, fascia line and roof pitch will look the way you expect when it's built.
InsideStop the moisture at its source
Walls and roofs that can dry out are the second line of defence. The first is keeping moisture out of them:
- Exhaust fans and rangehoods ducted outside, never into the roof space. A bathroom fan without enough natural ventilation must run with the light and for ten minutes after.
- A good air barrier on the inside. Sealed plasterboard, sealed penetrations and sealed downlights stop moist air being carried into wall and roof cavities.
- Warm inner surfaces. Good glazing and continuous insulation keep the inside faces of walls and windows above the dew point.
The Ventilation and air leakage guide covers this side in more detail.
DeliveryGetting it built right
- Choose the wall and roof systems at design stage. A cavity behind the cladding and a ventilated space above the insulation change dimensions, details and costs.
- Draw every junction. Base of wall, top of cavity, window head and sill, eaves, ridge, and every change of cladding.
- Specify the products by class. Wraps and sarking must meet the required vapour permeance class and be installed to the Australian Standard, with laps, tapes and flashings as the manufacturer specifies.
- Check before lining. Once the plasterboard and cladding are on, cavities, vents and membranes can't be inspected.
- Check substitutions. A different wrap, batten or vent can change the vapour class or the ventilation area.
Applying it locallyCanberra and New South Wales
Canberra, the Southern Tablelands and western Sydney (zones 6 and 7). All of the wall and roof requirements above apply. Plan on a cavity behind every lightweight cladding and a ventilated space above every ceiling.
Alpine areas (zone 8). The same rules apply, and the risk is highest. Cold outer layers, heavy condensation on cold mornings and snow cover on roofs make careful detailing essential.
The NSW coast (zone 5). The wall membrane still has to meet a vapour permeance class, which is Class 4 for direct-fixed cladding and Class 3 or 4 with a cavity. Direct fixing is still permitted. Check the current roof ventilation requirements for the site, as the code has been extending them beyond the cold zones.