Condensation in external walls: the layers nobody sees until the lining stains

When a wall stains, bubbles or grows mould, the first assumption is a leak. Sometimes it is. But a growing number of wall problems in well-built, recently finished homes come from something quieter: moisture that formed inside the wall, from the warm, humid air that every household produces, and that had nowhere to go.

This is condensation within the wall assembly, and it is one of the most commonly overlooked parts of building. The wall looks perfect on the day of handover. The problem takes a season or two to show, the cause is hidden behind linings and cladding, and by the time it is diagnosed, the fix often means opening up the wall.

This article explains how it happens, what the National Construction Code (NCC) requires, how it gets overlooked, and what to ask for. It is the first of three on condensation. The others cover exhaust systems and roof space ventilation.

How condensation forms inside a wall

Air always carries some water vapour, and warm air carries more. When that air meets a surface colder than its dew point, the vapour turns to liquid. In a wall, the sequence is:

1. Indoors, showers, cooking, breathing and drying clothes put moisture into the air.
2. Warm, moist air moves toward the cooler outside, either by diffusing through materials or by being carried through gaps in the lining.
3. Somewhere in the wall, a layer is cold enough, and the vapour condenses on it.
4. If the moisture can leave again, to the outside or back inside, the wall dries. If it cannot, it accumulates.

Winter nights in a heated house are the classic case, but not the only one. In humid weather, an air-conditioned house can drive condensation the other way, with warm outdoor air meeting cool inner surfaces.

Modern homes make this more important, not less. Better insulation means the outer layers of the wall run colder, and better air sealing means less accidental drying. The Australian Building Codes Board (ABCB) explains that sealed buildings with poor ventilation face a higher condensation risk.

What the NCC requires for walls

For houses, Part 10.8 of the NCC 2022 Housing Provisions sets out the deemed-to-satisfy rules in clause 10.8.1:

• Where a pliable building membrane (wall wrap) is installed, it must comply with AS 4200.1, be installed in accordance with AS 4200.2, and be on the exterior side of the primary insulation layer.
• Vapour permeance. A membrane, sarking-type material or insulation layer on the exterior side of the primary insulation layer must have a vapour permeance of not less than 0.143 µg/N.s in climate zones 4 and 5, and 1.14 µg/N.s in climate zones 6, 7 and 8.
• Where no membrane is installed, other than single-skin masonry or single-skin concrete, the primary water control layer must be separated from water-sensitive materials by a drained cavity.

In plain terms: the layers outside the insulation must let vapour through, so that moisture that gets into the wall can dry to the outside, and the wall must either have a membrane in the right place or a cavity that drains.

Apartments (Class 2) are covered by Part F8 of NCC Volume One. Climate zones vary across NSW, and greater Sydney spans more than one, so check your site against the ABCB climate zone map. These provisions are from NCC 2022. Which edition applies depends on when the work was approved, so confirm with your certifier.

How it gets overlooked

"Foil sarking is just insulation." Reflective foil wraps are used for their thermal benefits, and many have very low vapour permeance. Put one on the outside of a wall in a zone where it does not meet the permeance figure, and it can trap moisture in the insulation and frame, exactly where you do not want it. The product data sheet will state the vapour permeance. Ask for it and compare it with the figures above.

A membrane in the wrong place. Wall wrap that sits inside the insulation instead of outside it, or a second layer added later, can create a cold, damp plane behind the lining. The clause requires the membrane outside the primary insulation.

Mixed layers from different trades. The framer, the wrap installer, the insulation installer and the cladder each do their part. Nobody owns the question: are the layers together vapour-open to the outside? A good product in the wrong combination is a risk.

Gaps and penetrations in the wrap. Unsealed laps, torn membrane, uncapped fixings and unflashed penetrations let rain and moist air into the wall. Our article on building envelope leak points lists where this happens. Water that gets behind the wrap and cannot drain turns a drying wall into a wet one.

No drained cavity, or a blocked one. A cavity behind cladding lets water that gets past the cladding run down and out. If it is filled with mortar, construction debris or insulation, or is closed at the bottom with no weep path, it cannot do that job.

Cold bridges. Steel framing, concrete slab edges and lintels conduct heat and can create cold spots where condensation concentrates, often showing up as mould lines on the inside at corners and edges.

Retrofit insulation and lining over old walls. Adding insulation or a new internal lining to an existing wall changes where the dew point falls. Doing it without checking the existing layers is a common cause of problems in renovations.

Treating the symptom. Repainting a stained wall, or sealing a bathroom wall with a mould-resistant paint, does not change where the moisture is coming from. The stain returns.

Why it is hard to fix later

• It is concealed. The cause is behind the lining and under the cladding, and often cannot be confirmed without opening the wall.
• It looks like something else. Condensation, a rain leak and a plumbing leak produce similar stains. Our guide to what a damp patch is telling you helps tell them apart, but a definite diagnosis can need moisture readings or a cavity inspection.
• The fix is structural. Changing the order or type of layers means removing cladding or linings, replacing wrap and insulation, and rebuilding. That is many times the cost of getting it right during construction.
• Damage accumulates. Damp insulation loses its performance, framing can decay and mould spreads. Our article on timber frame myths explains that keeping the frame dry is what lets it last.
• Responsibility is hard to establish. The longer between handover and discovery, the more difficult it is to show what was built and who did it. The statutory warranty periods in our article on statutory warranties apply, but evidence matters.

What to ask for

1. Which climate zone is the site in, and what permeance does the wall wrap need to meet?
2. Show me the data sheets for the wall wrap, any sarking-type material and the insulation, with their vapour permeance.
3. Where does the membrane sit relative to the insulation? It should be outside the primary insulation layer.
4. If there is no membrane, where is the drained cavity?
5. Who checks the wall layers before they are covered? This should be a hold point, with photographs of the wrap, laps, flashings and insulation before cladding or linings go on.
6. What is in the handover record? The product data and photographs are what let someone diagnose a problem years later. See anatomy of a handover record.

For renovations and strata buildings

In a renovation, ask what is already in the wall and what the change will do to it. In older buildings, walls were often designed to dry in both directions. Sealing one side can change that.

For committees, facade and wall repairs are an opportunity to check the layers when they are exposed, and to record them. Include the wall assembly in the scope, as described in our article on tendering a strata defects rectification.

Working with us

We treat the wall as a system, not a set of separate trades, and we inspect and record the layers before they are covered. See How We Build. If you are planning a build, renovation or repair and want to discuss it, get in touch.

Sources

• NCC 2022 Housing Provisions, Part 10.8 Condensation management, Australian Building Codes Board
• NCC 2022 Volume One, Part F8 Condensation management, Australian Building Codes Board
• Improved condensation management in NCC 2022, Australian Building Codes Board
• Climate zone map, Australian Building Codes Board

This article is general information, not technical advice for your project. Wall design depends on the climate zone, construction type, products and NCC edition, so confirm with your designer, certifier and a building consultant. Full website disclaimer