What is an intelligent vapour control membrane?

Most new Adelaide walls have no air-control layer inside them at all.

Plasterboard isn’t one. It leaks air at every join, gap, power point and pipe penetration — and once air can move through a wall, so can the moisture it carries. Moving air drives far more water into a wall than slow diffusion ever does, so a wall with nothing sealing it has no real defence against the main way water gets in.

That is the gap worth closing. The real question isn’t which barrier to reach for — it’s whether your wall has an airtight layer doing this job at all, and, in a mild two-way climate like Adelaide’s, what the right kind of layer is.

Does an Adelaide wall need an internal membrane?

It needs an air-control layer — and because the threat is air, that is what the layer has to stop.

That sounds obvious, but it’s where most walls fall short: there is nothing in them sealing the air, so moisture rides in unchallenged. (We follow that mechanism through in our piece on why air leaks cause condensation in walls.)

An internal membrane is the usual way to add that missing layer. But in a mild, two-way climate like Adelaide’s, the right kind isn’t a fixed barrier — it’s an intelligent one.

What is an intelligent vapour control membrane?

An intelligent vapour control membrane is an interior airtight layer — it goes on the warm, inside face of the wall or ceiling, behind the plasterboard — whose resistance to water vapour rises and falls with the humidity around it. It’s also called a humidity-variable membrane. In dry conditions it tightens up to keep household moisture out of the wall; in humid conditions it opens up to let the wall dry inward.

That is exactly what our climate asks for. Adelaide’s cool, drier winters push moisture from inside the home outward into the wall; our hot summers reverse the flow. A wall has to resist moisture in one season and shed it in the other. A membrane that adapts can do both. A fixed sheet of plastic holds one resistance all year — it’s either helping or hurting depending on the season, and you don’t get to choose which. That’s why a fixed barrier is the wrong way to add an internal layer here, and an adaptive one is the right way.

What an sd value actually means

The number behind all this is the sd value: the vapour-diffusion-equivalent air-layer thickness, in metres.

In plain language, it tells you how hard a material is for water vapour to push through, expressed as the depth of still air that would resist diffusion just as much. A high sd value (many metres) is a strong vapour brake. A low sd value (a fraction of a metre) lets vapour pass freely.

For reference, a sheet of builder’s polythene sits fixed around an sd value of 50 to 100 metres — a permanent barrier in both directions. That’s the thing Adelaide rightly leaves out.

An intelligent membrane moves instead. Pro Clima’s INTELLO, for example, sits above 25 metres in dry winter conditions — a strong brake that keeps household moisture out of the wall — and drops below 0.25 metres in humid summer conditions, opening the wall to dry inward (the maker’s own figures). That’s a swing of more than a hundredfold across the year: less than 7 grams of moisture per square metre per week held back in winter, against more than 500 grams let out in summer. We verify the installed airtightness on site with a blower-door test rather than trusting a spec sheet, which is part of the real cost of airtightness.

Airtight is not the same as vapour-tight

The most common confusion in this whole subject is that an airtight wall must be a sealed, non-breathing box. It is not.

Airtight stops bulk air moving through the wall. Vapour-tight stops water vapour diffusing through the material itself. They are different things — and the best interior membrane is airtight while staying vapour-open. That’s the point most builders miss: the job of the internal layer is to stop the air (which carries nearly all the water), not to stop the vapour (which the wall needs to be able to pass so it can dry). We unpack that mechanism, and why air leaks cause condensation in walls, in its own article.

How does it differ from the weather barrier on the outside?

It sits on the opposite face of the wall and does the opposite job. A well-built wall has two breathable membranes, and they are not interchangeable.

On the outside, behind the cladding, sits the weather-resistive barrier — the weather-resistant wrap. It faces the elements: it sheds wind-driven rain and stops wind washing through the insulation, while staying vapour-open so any moisture inside the wall can dry outward. These are the German weather-resistant membranes we use on the outer face of a healthy wall — Pro Clima’s SOLITEX MENTO and EXTASANA are examples on that side, the outward-facing counterpart to an INTELLO layer inside.

On the inside, behind the plasterboard and on the warm side, sits the intelligent vapour control membrane — the layer this article is about. It works the other way around: it stops warm, moisture-laden indoor air being pushed into the wall, and it adapts with the seasons. The weather barrier keeps water and wind out from the cold side; the internal membrane manages air and vapour from the warm side.

The air-sealing job doesn’t strictly have to live on the inside — on some build-ups it’s handled by a vapour-open air barrier on the outside instead, combined with that weather layer. What never changes is the principle: one continuous airtight layer, vapour-open enough to dry, that never becomes a moisture trap. An intelligent internal membrane is the most common way we deliver it; an external air barrier is the alternative when the wall calls for it.

How a wall dries

A wall built this way has a drying path in both directions across the year, which is its insurance policy.

In winter, the interior layer is tight and the exterior layer is open: moisture that gets into the wall dries to the outside, and very little gets in from the inside. In summer, an intelligent interior membrane opens up: moisture is pushed inward by the heat and dries into the home, where heat-recovery ventilation carries it away.

That last step is why the membrane and the ventilation are one system, not two. A wall that dries inward needs somewhere for the moisture to go once it reaches the room — and in an airtight home that job belongs to mechanical ventilation, which is where the choice between heat recovery and energy recovery for Adelaide’s climate comes in.

What we specify, and why we don’t name one brand

We specify the layer to suit the wall, not the other way around.

The right approach depends on the build-up: what the exterior layer is and how vapour-open it is, the climate zone, and whether it’s a new build or a renovation worked into existing fabric. Pro Clima’s INTELLO is useful to quote because its figures are public and well documented — it’s an example, not a rule. The principles are what hold across every project:

• An airtight layer that’s vapour-open — sealed to air, but able to pass vapour so the wall can dry.
• That layer kept continuous right around the envelope — walls and roof or ceiling — because it only works where it has no gaps.
• A weather-resistant, vapour-open layer on the outside, so water stays out but the wall still breathes.
• A drying reserve built in deliberately, so the wall forgives the moisture that inevitably gets in.
• The airtight layer verified with a blower-door test, not just trusted to a spec sheet.

In Adelaide the comparison was never intelligent membrane versus plastic — almost no one here builds with plastic. It’s a considered internal layer versus none at all. Over the fifty-plus years someone lives in the home, that’s the difference between a wall that stays sound and one that quietly takes on water it can’t shed.