Ventilated cavities: how walls and roofs are built to dry

One of the most important parts of a high-performance wall is not a part at all. It is a gap.

Behind the cladding of a well-built wall — and above the membrane of a well-built roof — there is a deliberate air space, usually around twenty millimetres, held open by battens. It looks like nothing. It is one of the things that decides whether the wall lasts fifty years or quietly rots from the inside.

That gap is the ventilated cavity — and on a lot of Adelaide homes, it simply is not there.

What is a ventilated cavity?

A ventilated cavity is the air gap between the cladding and the weather-resistant barrier: open at the bottom so water can drain out, and able to move air so the wall can dry.

It is held open with battens fixed vertically over the frame, on top of the weather-resistant barrier. The cladding — weatherboard, fibre cement, panel, timber — hangs off those battens, so it never sits hard against the membrane. Together, the cladding, the gap and the membrane behind it form what building science calls a rainscreen.

Two words get used around this, and they are not quite the same. A drained cavity has an opening at the base only: water that gets in runs down the back of the cladding and out the bottom. A drained and ventilated cavity adds a continuous opening at the top of the assembly as well — wall or roof — so air can move right through the gap and dry it, not just drain it. A good wall, and a good roof, does both.

How big those openings are is not guesswork. They are sized to the situation — the type of assembly, the membrane manufacturer’s specification, the climate zone, and, on a roof, the pitch — so the cavity moves enough air to dry without opening a path for weather or, where it matters, embers.

Brick veneer has always had a gap of its own, the cavity behind the brickwork. The catch is the lightweight claddings more and more homes are built with today: they go straight onto the frame unless someone deliberately builds the cavity in.

Why a wall needs one

Because no cladding is waterproof, and a wall that gets wet has to be able to dry.

That is the part most people have backwards. Cladding is not a seal; it is a screen. Wind-driven rain finds its way past brick perpends, weatherboard laps and panel joints — it always has. The cavity assumes water will get in, and gives it somewhere to go: down the back of the cladding and out the bottom, before it reaches the frame.

Then it lets the wall dry. Building-science testing — including decades of it by the Building Science Corporation — shows a wall with a ventilated cavity can dry several times faster than one where the cladding sits tight against the membrane. Water will drain through a gap of just two or three millimetres; but to actually dry, a wall wants something closer to twenty. That is why the cavity is sized the way it is.

There is a quieter benefit too. A vented gap moderates the air pressure behind the cladding, which takes some of the force out of wind-driven rain trying to push through the joints. It is a real effect, though a modest one — the dependable work is the draining and the drying, not the pressure.

And in a sunny climate like ours, the cavity does one more thing. Brick, render, fibre cement and timber all soak up rain like a sponge. When the sun hits a wet wall, it drives that moisture inward — toward the cool, air-conditioned inside of the house, where it can condense. A ventilated cavity carries most of that moisture away to the outside before it ever crosses the membrane. (It is the same uncontrolled-moisture story we tell in why air leaks cause condensation in walls — the cure is always a way for the moisture to leave.)

The cavity is what makes the membrane work

A weather-resistant barrier can only drain and dry a wall if there is somewhere for the water to go.

This is the part that ties the wall together. The weather-resistant barrier on the outside is built to shed water and let vapour out — but pressed flat under hard-fixed cladding, it cannot drain and it cannot dry. The cavity is what lets it do its job. Take the cavity away and even the best membrane is half-wasted.

Read the whole wall and a pattern appears. On the warm inside face, an intelligent vapour control membrane keeps indoor moisture out of the wall. On the cold outside face, the weather-resistant barrier sheds the weather. And between the membrane and the cladding, the ventilated cavity gives the whole assembly room to dry. The principle under all three is the same: a wall stays sound when its ability to dry is greater than the moisture it will ever have to shed.

Roofs need it too

The same logic applies overhead — and the roof is where it is hardest to get right.

A traditional roof has a ventilated space above the ceiling that does this drying job. But the moment you insulate at the rafter line — a cathedral ceiling, a skillion roof, a room in the roof — that space is gone, and the roof loses its way to dry. These are exactly the roofs that fail quietly: there is no attic to inspect, and nowhere for trapped moisture to go.

The fix is to build the cavity into the roof. Counter-battens over the roof membrane create an air space that runs from the eave up to the ridge, so the roof can breathe the way the wall does. In our climate, with a hot sun on a dark roof for half the year, that drying path matters as much up top as it does in the walls. A wet roof is the hardest, most expensive thing in a house to put right.

Getting it right is the hard part

A cavity only works if it is continuous and open in the right places — and that takes detailing most builds skip.

It is easy to draw and fiddly to build. The cavity has to be opened at the base to drain and vented so it can dry, and closed off where it should not feed moist air into the roof. It has to stay continuous — not blocked at corners, not bridged by a stray batten, not packed with offcuts. Every window and door has to be flashed so water in the cavity is shown back out, not led into the frame. The base openings have to keep insects and vermin out while staying clear — and the most common failure of all is simply a cavity whose vents have blocked with dirt, render or insect nests, so it stops draining and drying with no one the wiser.

In bushfire country there is a real tension to resolve. The cavity wants open vents to breathe; the bushfire standard, AS 3959, wants every opening screened down to a two-millimetre mesh to keep embers out. Both have to be satisfied at once, with the right rated vents and closers. It is detail, and it is the kind that does not forgive being rushed. It is also far harder to add later — which is why a renovation or addition is the moment to get the wall build-up right, not the year after. This is the as-built-versus-as-designed gap in miniature: the cavity that performs is the one that was actually detailed and actually built, not the one assumed on the drawing. (We write about that gap in why a home underperforms its rating.)

Adelaide doesn’t require it yet — the colder states already do

In Adelaide, the National Construction Code does not yet require a ventilated cavity. In the colder climate zones, it effectively does.

Adelaide sits in climate zone 5, where the Code still lets a wall comply with a vapour-permeable membrane alone, no cavity needed. Move to the cooler zones 6 to 8 and the latest Code, NCC 2025, has effectively ended direct-fixed cladding: a wall there must now have a drained and ventilated cavity, at least twelve millimetres deep, behind the membrane. South Australia has deferred NCC 2025, so we are still building to the 2022 Code for now — part of why the cavity remains optional here.

It is worth knowing this is not new thinking. New Zealand, after a national leaky-building crisis, has required a drained and ventilated cavity of around twenty millimetres behind most claddings for two decades. Australia’s colder-climate rules are now arriving at the same place. Adelaide will get there; we would rather build it now than be caught up to.

Because the Code, here, is a floor — and a low one on this point. We treat a ventilated cavity as standard on the homes we build in Adelaide, the same way we treat airtightness and a tested envelope as standard: not because a rule compels it, but because it is what keeps a healthy, durable wall sound.

What we build

On a high-performance home, the cavity is not an upgrade — it is part of the wall:

• A ventilated cavity behind the cladding, drained at the base and able to dry, over a vapour-open weather-resistant barrier.
• A counter-batten cavity above the roof membrane on cathedral and skillion roofs, running eave to ridge, so the roof can dry as well as the walls.
• Continuous and unobstructed — opened and closed in the right places, flashed at every window and door, and kept clear at the base.
• Detailed for bushfire where the site calls for it, so the wall can breathe and still keep embers out.

It adds a little cost and a lot of detailing, and almost no one will ever see it. But over the fifty-plus years someone lives in the home, that hidden gap is the difference between a wall and roof that shrug off the water they inevitably take on, and ones that hold it until something rots. It is the cheapest insurance in the building, and we do not leave it out.