Why air leaks cause condensation inside your walls.

Airtightness is usually sold as an energy story. It’s really a water story.

Keeping warm air inside a home in winter saves money, and that’s the version most people hear. But the more important job of an airtight layer is to keep liquid water out of the wall — water that never fell as rain, never came through the cladding, and that you will never see until the damage is done.

That hidden water is the difference between a home that is merely compliant and a home that is durable. They are not the same thing.

Why do air leaks cause condensation in walls?

Air leaks cause condensation because moving air carries water vapour with it, and when that air cools inside the wall, the vapour turns to liquid.

Warm air holds more moisture than cold air. Indoor air in an Adelaide winter is warm and reasonably humid — from breathing, cooking, showers, drying clothes. When that air finds a gap in the wall lining and pushes out through the structure, it cools as it goes. At the point where it drops below its dew point — the temperature at which it can no longer hold its moisture — the vapour condenses into liquid water.

In a leaky wall, that point sits somewhere in the cold outer half of the insulation. Water collects there, soaks into the timber framing and the back of the sheathing, and stays. That is the mechanism. It’s ordinary physics, and it’s happening inside a great many new Australian homes right now.

Air leakage carries far more water than vapour diffusion

There are two ways moisture gets into a wall: it diffuses slowly through solid materials, or it rides through gaps on moving air. They are not remotely equal.

Diffusion is the gentle one. Water vapour seeps through plasterboard, paint and timber by pressure difference — a slow, predictable, manageable trickle. A wall designed to handle it usually can.

Air leakage is the violent one. According to the building-science figures published by the membrane manufacturer Pro Clima — calculated to the German standard DIN 4108 — about 800 grams of moisture will flow through a single one-millimetre gap, per metre of gap length, on a standard winter day by air convection. Through a gap-free vapour-control membrane, diffusion moves around 0.5 grams per square metre over the same day. That is roughly 1,600 times more moisture carried by a draught than by diffusion.

Eight hundred grams is most of a litre of water, through a gap the width of a stack of business cards, every cold day. It doesn’t evaporate harmlessly. It accumulates as condensation inside the insulation.

This is why a sealed air barrier matters more than a heavy vapour barrier. Stop the air movement and you remove the mechanism that delivers nearly all the water. We covered the on-site discipline of building that barrier in the real cost of airtightness — the point here is what it’s protecting against.

Where the water actually forms: interstitial condensation

The dangerous condensation in a home isn’t the kind you wipe off a window. It’s the kind you never see.

Condensation on a cold window is annoying but honest — it’s on the surface, it dries, you know it’s there. Interstitial condensation forms inside the wall assembly, in the gap between the materials, where no one is looking. By the time it shows itself as a stain, a soft skirting or a smell, it has usually been wetting the structure for several winters.

In building-science terms, condensation forms wherever the air inside the assembly reaches its dew point. You can’t simply move that point by adding insulation — more insulation makes the outer layers colder, which can push the dew point further out, not away. What you can do is stop humid indoor air reaching the cold zone in the first place. That is the entire logic of putting a continuous airtight layer on the warm, inside face of the wall.

Get the air-control layer right and the wall stays dry because the wetting mechanism never starts. Get it wrong and no amount of insulation or vapour barrier reliably saves it.

Compliant is not the same as durable

A new home in South Australia can meet every line of the code and still be built to fail slowly.

The National Construction Code sets minimum R-values and, in its 2022 form, allows prescriptive cladding paths where a wall wrap is effectively optional. Meeting those minimums says the home is legal. It says very little about whether warm indoor air is being kept out of the cold parts of the wall — which is the thing that actually determines whether the framing stays dry for fifty years and more.

That gap between legal and lasting is the whole argument for building above code. It’s the same reasoning behind our Passive House approach: treat measured airtightness and controlled ventilation as the baseline, not an upgrade. A blower-door-tested envelope isn’t about a certificate. It’s proof that the air — and the water it carries — is going where the design intended.

The fix is two layers, not one

A dry wall in Adelaide’s climate comes from a system, not a product. Two things have to be true at once.

First, a continuous airtight layer on the warm inside face, with every junction and penetration sealed — because air carries the water, and a gap anywhere undoes the layer everywhere. Tighter is the goal: the Passive House Institute’s benchmark is no more than 0.6 air changes an hour at 50 pascals of pressure, against the 15-plus typical of an untested new Australian home.

Second, controlled ventilation, so the moisture that occupants generate is removed on purpose rather than pushed into the structure by accident. A tight house without ventilation is a problem; a tight house with heat-recovery ventilation is the answer. Which type of unit suits the local climate is its own decision — we work through it in HRV vs ERV for Adelaide.

There’s a third refinement worth naming. The membrane on the inside doesn’t have to be a fixed plastic sheet. Intelligent, humidity-variable membranes hold moisture out in winter and open up to let the wall dry in summer — a safety margin a static vapour barrier can’t offer. We unpack that in intelligent vapour control membranes.

What this means for an Adelaide build

Adelaide’s climate is gentler than Tasmania’s or Melbourne’s, and that lulls people. The cold, humid mornings of an Adelaide winter are still cold enough, and humid enough, to drive condensation into a leaky wall — and our long, hot summers mean a wall that gets wet has plenty of opportunity to grow what wet timber grows.

This is a building-durability problem before it is anything else. We’re talking about whether the structure of the home — the timber you’re paying to build and never see again — stays sound for the life of the house. On the custom homes we build across West Lakes, Henley Beach, Grange, Lockleys and greater Adelaide, the airtight layer is detailed and tested for exactly this reason: not to chase a number, but to keep the water that air carries out of the part of the wall that can’t dry on its own. Where the wall can dry — outwards, behind the cladding — we make sure it has the means to, with a ventilated cavity.

The short version: insulation keeps the home warm, but airtightness keeps the wall dry. Build the first without the second and you’ve built a wall that condenses its own demise, quietly, behind the plasterboard, for years before it tells you.