Damp patches on walls and ceilings. Water streaming down the windows on a cold morning. A musty smell you can't track down. Condensation and mould are showing up in new Australian homes, and they're bad for your health and your house.

The good news: since NCC 2019, and more so since NCC 2022, the National Construction Code includes condensation management rules for houses. They don't cover everything, so it pays to understand the problem and ask the right questions.

Section through a house with a metal roof: warm moist air from a bathroom leaks through a downlight into the roof space and condenses under the cold roof sheet, dripping on the insulation and ceiling; fixes shown are eave vents in, a ridge vent out and an exhaust fan ducted outside.
Condensation, evaporation, precipitation (unless ventilation can modify the environmental conditions to mitigate the process)

Why condensation happens

Air holds water vapour. Warm air holds more than cool air. When warm, moist air touches a surface colder than its dew point, the excess water turns to liquid. That's condensation.

In homes, four things drive it:

  1. Moisture. Cooking, showers, drying clothes and even breathing add water to the air.
  2. Ventilation. If moist air isn't removed, humidity builds up. Newer homes are more airtight, so this matters more than it used to.
  3. Cold surfaces. Single glazing, aluminium window frames, uninsulated walls and cold spots behind cupboards or around pipes.
  4. Temperature differences. The bigger the gap between the air and a surface, the more likely water forms on it.
Chart of water vapour content against air temperature with relative humidity curves, showing 20 °C room air at 60% RH reaching its dew point at about 12 °C.
As air cools the same moisture means higher humidity, until it reaches dew point and condensation forms.

Why it matters in Queensland

Most people think condensation is a cold-climate problem. In Queensland, it often runs the other way.

On a hot, humid day, the outside air is loaded with moisture. Inside, the air conditioning keeps the walls and ceilings cool. When that humid outside air finds its way into the wall or roof cavity, it can hit a cool surface and condense inside the wall, where you can't see it.

A vapour barrier on the wrong side of the insulation can trap that moisture rather than let it dry out. Wall wraps and sarking need to suit the climate, and the details around them need to be right.

Section through a timber-framed wall with a vapour-open wrap outside the insulation and a smart vapour retarder inside, showing the wall drying outward in winter and inward in summer.
A vapour-permeable wrap lets moisture escape the wall while keeping outside water out.

What condensation does to your health and home

  • Health. Damp, mouldy homes can worsen asthma and allergies. Mould spores irritate airways, and damp air attracts dust mites. The World Health Organization's 2009 guidelines on dampness and mould recommend preventing damp and mould problems in all buildings.
  • Building damage. Timber rots, steel corrodes, plasterboard and paint break down, and in bad cases the structure is affected.
  • Warning signs. Water on windows, peeling paint, mould spots on walls and ceilings, stains and musty smells.

What the NCC now requires

NCC 2019 introduced condensation management requirements for houses, and NCC 2022 strengthened them. In the Housing Provisions (Part 10.8) they include:

  • Exhaust fans ducted outside. Exhaust from bathrooms, toilets, kitchens and laundries must discharge to outdoors, not into the roof space. Minimum flow rates are 25 litres per second for bathrooms and toilets and 40 litres per second for kitchens and laundries.
  • Vapour-permeable wall wraps. Pliable membranes on the outside of the insulation must allow water vapour to pass through, with the required permeance depending on climate zone.
  • Roof space ventilation in the cooler climate zones (6, 7 and 8).

Most of Queensland sits in the warmer zones, so the roof ventilation rule usually won't apply here. That's one of the gaps.

Where the gaps remain

Research led by the University of Tasmania for the Australian Building Codes Board in 2016, the Scoping Study of Condensation in Residential Buildings, compared Australia with New Zealand, the United States, Canada, the United Kingdom and Europe. Most of those places had introduced condensation and mould rules over the previous 10 to 20 years. Australia was behind, and while the NCC has since caught up in part, gaps remain:

  • Humid climates. The rules are strongest for cold climates. Air-conditioned homes in hot, humid areas get less attention.
  • Energy efficiency without moisture management. As homes get more airtight and better insulated, they need ventilation and good detailing to match. These should be designed together, not separately.
  • Education. Many designers, builders and occupants still don't understand vapour, dew point and ventilation. Guidance like the ABCB's Condensation in Buildings handbook is helpful but not mandatory.
  • Checking the work. A rule only works if someone checks the wrap is the right type, the laps and penetrations are sealed and the fans actually go outside.
Vertical section through a clad timber-framed wall: lapped cladding, a drained and vented cavity on battens open at the top and bottom, a taped wrap acting as the air barrier, insulation and plasterboard, with equal air pressure outside and in the cavity and a base flashing with a drip.
A vented cavity behind the cladding drains water and lets the wall dry, reducing condensation and mould.

What you can do

Before you build

  • Ask your builder which wall wrap and roof sarking they use, and why they suit your climate.
  • Ask how the roof space is ventilated, even if the code doesn't require it in your zone.
  • Confirm every exhaust fan (bathrooms, toilet, laundry and rangehood) is ducted to outside.
  • Look for gaps around pipes, cables and other services through the wall wrap. They should be sealed.
  • Consider better glazing, such as double glazing or thermally broken frames, on cold or air-conditioned rooms.

Living in the home

  • Run exhaust fans while showering and cooking, and for a while afterwards.
  • Dry clothes outside or in a vented dryer.
  • Open windows when the weather allows.
  • If your house is raised, make sure there's no standing water underneath and the subfloor is ventilated.
  • In your first winter, and first humid summer with the air con running, look for water on windows, damp patches or musty smells. Report them early, in writing, while you're within your builder's defects period.

Read the research

The study this post draws on is the 2016 Scoping Study of Condensation in Residential Buildings by Dr Mark Dewsbury and colleagues at the University of Tasmania, funded by the ABCB.

READ AND DOWNLOAD IT NOW