Ever wondered why some new homes end up with mould on the ceilings or damp in the wardrobes a few years after handover? More often than not it's condensation, and much of it could have been avoided at the design stage.

Modern homes are better insulated and more airtight than ever, which is good for energy bills. But it also means moisture has fewer ways to escape. If the design doesn't manage that moisture, it ends up inside walls, roof spaces and on cold surfaces, where it feeds mould and rots timber.

What condensation is and why it happens

Condensation forms when warm, moist air meets a surface that's cold enough to drop it below its dew point. You see it on the outside of a cold drink or on a window on a winter morning. The problem is when it happens where you can't see it: inside a wall, under the roof or behind a wardrobe on a cold external wall.

New homes are especially prone in the first year or two, as the slab, timber and plaster dry out and release construction moisture.

What a condensation risk analysis looks at

A condensation risk analysis looks at how heat and moisture move through the walls, roof and floor of your home, and where condensation is likely to form. It considers:

  • Climate. Temperatures, humidity and how they swing between day and night and across seasons.
  • Orientation and exposure. In Australia, north walls get the most sun. South-facing walls and rooms stay coldest and are the most prone to condensation and mould. East and west walls get harsh low-angle sun.
  • Wall and roof construction. The type and position of insulation, wall wraps, sarking, linings and cladding, and whether each layer lets moisture escape.
  • Ventilation. Whether moisture from bathrooms, the laundry and the kitchen is removed at the source, and whether roof spaces can breathe.
  • How the home will be used. Cooking, showers, drying clothes indoors and the number of people all add moisture.

For most standard homes, this is done by the designer and energy assessor checking the design against the NCC and good practice. For more complex designs, or homes with unusual wall or roof systems, a qualified consultant can run a hygrothermal model (WUFI is a common tool) that simulates moisture movement through the actual wall build-up over a year or more.

Why the old "vapour barrier" advice doesn't suit Queensland

A lot of condensation advice comes from cold climates, where heated indoor air is the warm, moist side and a vapour barrier goes on the inside of the wall. In humid Queensland, the warm, moist air is often outside, especially in an air-conditioned home in summer. A plastic vapour barrier on the inside of the wall can trap that moisture in the frame.

There's no blanket rule that works everywhere. The right wall build-up depends on your climate, cladding and how the home is cooled. That's exactly why the analysis matters.

What NCC 2022 requires

NCC 2022 strengthened the condensation rules in the Housing Provisions (Part 10.8). The key points for homeowners:

  • Exhaust fans from bathrooms, toilets, laundries and kitchen rangehoods must discharge to outside air, not into the roof space.
  • Wall wraps (pliable membranes) on the outside of the insulation must be vapour permeable in climate zones 4 to 8.
  • Roof space ventilation is required in the cooler climate zones 6 to 8.

Most Queensland homes are in zones 1 to 3, so the wall wrap and roof space rules don't always apply. The exhaust rules do. The ABCB's free Condensation in Buildings Handbook is a good reference if you want to go deeper.

How builders and designers reduce the risk

  • Duct exhaust fans to outside and make sure they're sized properly. A bathroom fan that just pushes steam into the roof space is a mould factory.
  • Install insulation without gaps. Gaps create cold spots on ceilings and walls where condensation forms.
  • Choose the right wall wrap and sarking for your climate and cladding, as specified by the designer under AS 4200.1.
  • Pick suitable windows. Better-performing glazing (double-glazed or low-e) keeps the inside surface warmer and reduces condensation on the glass, particularly on south-facing rooms and in cooler climates.
  • Keep wet areas properly waterproofed. Behind showers and baths, that's a waterproofing membrane to AS 3740, not a vapour barrier. Our C13A Waterproofing Design and C13B Waterproofing Installation checklists cover what to check.
  • Vent the clothes dryer outside and design for good cross-ventilation.

Questions to ask your builder

Before you sign, ask:

  1. Has anyone assessed condensation risk for this design, and who did it?
  2. What wall wrap, sarking and insulation are specified, and why were they chosen for this climate?
  3. Where do the bathroom, laundry and rangehood exhausts discharge?
  4. Are any walls or rooms at higher risk, such as south-facing bedrooms or walk-in robes on external walls?
  5. Will the home be airtightness tested (a blower door test), and if so, how is ventilation designed to match?
  6. What does the contract and the home warranty cover if condensation damage turns up later?

A builder who's thought about this will answer quickly. One who hasn't will tell you it's "never been a problem".

What to check on site

  • At frame stage: wall wrap is the specified product, continuous and lapped correctly.
  • At rough-in: exhaust ducts run all the way to an outside vent, and wall insulation fills each cavity.
  • At enclosed and fixing stages: ceiling insulation has no gaps and the exhaust fans actually vent outside.

Our C10 Rough In and C11 Enclosed checklists include these checks.

After you move in

You play a part too. Run the exhaust fan during and after showers, use the rangehood when cooking, avoid drying clothes indoors without ventilation, and open up the house when the weather allows. If mould appears in the first 12 months, raise it with your builder in writing within the defects liability period so the cause can be investigated.

For more, see tackling home condensation and condensation and mould on builds.