Despite its name, fungus in our homes is not fun. Over the past three years, Australia has experienced record-breaking rainfall and floods, generating the perfect storm for the spread of these unfriendly organisms in homes under construction and in water-damaged properties. 

Concerns have been raised about the impact on house frames from extended construction times and pauses in projects. During this time, an increasing number of HIA members have also raised the issue of mould build up, or more specifically black spores, on frames due to exposure to prolonged wet weather. 

Mould in relation to buildings is a multi-faceted issue and, in general, not well understood. It can occur due to a range of factors – some of which are within the builder’s control, such as ensuring the right products are used in the right circumstances and installed in the right way. However, other environmental and weather-related factors are much harder to plan for and control. Regardless, rectification works as a result of damage from mould build up can have costly consequences for both builders and homeowners. 

With changes to energy efficiency and condensation management under the National Construction Code (NCC) coming into effect (and more proposed under NCC 2025), how we deal with moisture in buildings is a big topic for industry. 

What causes mould?

Too much damp and moisture can lead to mould. Its growth can occur during construction or present in buildings after completion. It’s important to distinguish the multiple factors at play. 

If mould starts to appear on walls, ceilings or floors outside wet rooms, it is likely a sign of larger problems on the other side of the plaster. Its appearance might stem from a water leak, rising damp, or from inadequate sealing/weatherproofing or flashing. 

Mould growth can occur on the internal barriers of party wall systems of adjoining townhouses before the structure is roofed over or on frames exposed to weather for extended periods. Excessive mould growth is always a risk after flooding in a building with a high moisture content level. Frames should not be covered too quickly without giving them enough time to dry out. 

A home is at risk of potential mould growth if condensation becomes a persistent problem. Condensation typically forms from a lack of ventilation. Its build up can come from increased building sealing/energy efficiency and moisture in a frame cavity or from general household activities in kitchens, bathrooms or laundries where moist air lingers without a chance to escape. 

Prevention is better than cure 

As with most things, it’s better to prevent an issue than try to resolve one. 

To avoid moisture build-up, wet rooms should be vented to outside air. This means ensuring exhaust fans are ducted (connected to) outside the roof space. Avoid dumping the moisture into the ceiling cavity.

Adding insulation to the walls and ceiling of a bathroom is another way to help prevent mould because their surfaces may not get cold enough for water to condense. 

Double-glazed windows and thermally broken frames are also less likely to have water vapour condense due to an insulating material that breaks the pathway of heat energy transfer from inside and outside a home. 

If you discover only a few local patches, wipe them away with bleach, vinegar or mould killer and thoroughly dry the area. This may prevent further problems. Keep the room dry afterwards to prevent recurrence.

If frames are experiencing mould growth, or black spores, there are a number of proprietary products that can be used to remove and protect the frame. It is recommended though that you do your research on the various products and ensure follow the directions on its application and WHS procedures for its use. 

Solutions for industry

To prevent mould growth on interstitial structures, Siniat recommends using a combination of mould-resistant Intershield and Multishield in its popular Interhome separating wall system.

‘Interhome is recommended for adjoining residential dwellings such as duplexes or townhouses,’ says Erik Money, Senior Technical Services Engineer at Siniat. ‘The system is well-known for its ease of installation and the simple solution it provides when it comes to installing penetrations for electrical and plumbing services.’

An important design element that sets Interhome apart from conventional separating wall systems is the central fire barrier that typically incorporates Siniat Shaftliner – a 25mm fire-resistant plasterboard made with a gypsum core with recycled blue liner paper.

Erik says Siniat’s Intershield board offers the same fire and acoustic performance as Shaftliner but with the added benefit of superior mould and water resistance. ‘This significantly minimises the risk of interstitial mould in the built structure during construction and in case of any future water leaks,’ he says.

To enhance mould resistance in the Interhome system, Siniat also recommends using its mould resistant Multishield board laminated to the central fire barrier. Multishield is a fire- and water-resistant board that now comes standard with mould inhibitor.

‘When using the combination of Intershield and Multishield, builders can be assured of the best protection against mould forming on the Interhome separating wall system during all stages of construction,’ Erik adds.

Energy-efficient homes and condensation

Energy-efficient houses can be very well sealed and nice and warm inside. But if insulation isn’t installed correctly or the right products aren’t used, the air movement might be low and moisture levels high. This is the perfect recipe for mould. The Passive House standard, for example, requires the use of heat recovery ventilation. 

It goes without saying, but it is particularly important that houses are built well, and all insulation, wraps and construction layers are designed and installed with condensation management principles in mind. The National Construction Code sets out specific condensation (moisture) management measures as well as ventilation and ducting requirements to ensure fresh air enters a home and unwanted moist air leaves it. 

Builders and building designers play a key role in determining the selection and placement of materials – including insulation – so the conditions that enable condensation are avoided.

Condensation changes you need to know now

Over the past editions of the NCC, the ABCB has been progressively introducing more robust condensation management provisions. For NCC 2022, the introduced measures cover changes to wall sarking requirements to more vapour permeable types, equivalent to Class 3 or Class 4 membrane, depending on the climate zone. Other changes to ventilating roof spaces and exhaust systems, include:

• minimum flow rates for exhaust fans
• mandatory exhaust and ducting to the outside for rangehoods and exhaust fans
• banning recirculating rangehoods
• mandatory exhaust fans with 10-minute run-off timer for bathrooms without natural ventilation 
• additional provision for ventilation to laundries and bathrooms containing a dryer.

Many of these changes will need to be designed into the dwelling from the outset. For example, opting for a floor framing type to allow ducting or designing a bathroom/laundry with natural ventilation and exhaust fans to meet required airflow rates.

As a word of warning, there are lots of hidden details in applying and meeting these new provisions and they also apply differently based on climate zones. You can learn more about these at face-to-face HIA seminars in August and September or log on to our virtual on-demand sessions. See the HIA website for more details.

More to come in NCC 2025 

The ABCB has also commenced work on further condensation reforms for NCC 2025 which will consider additional measures. These include requirements for:

• ‘background’ ventilation, such as a bathroom exhaust fan, to run continuously at a low level
• enhanced ventilation measures at extreme sources of moist air in kitchens, bathrooms and laundries
• cavities for clad walls
• a ventilated cavity to be present between cladding and wall wraps
• ventilation openings in roof cavities
• window thermal performance levels and thermal breaks in windows
• thermal bridging mitigation
• party wall construction detailing and insulation
• considering the installation of vapour control layers, with some building materials to be impermeable to water vapour
• considering installation of external insulation to walls and roofs.

The new reforms may require builders to also investigate measures to mitigate condensation risks for buildings in bushfire-prone areas or take specific measures for buildings in tropical climates through the use of ventilation systems that continuously dehumidify indoor air. Airtight buildings will need to be provided with additional ventilation to manage condensation risks, while there is the potential for expanding condensation provisions to other buildings, not just houses and apartments. 

What is mould?

Mould is a fungal growth that thrives on moisture. In a home, mould generally grows where moist warm air meets a cold surface. The air moisture condenses, wetting the surface and creating the ideal environment for mould to thrive. 

Mould loves damp, dark, poorly ventilated environments, such as bathrooms and laundries. It can grow on many surfaces, including walls, ceiling tiles, insulation material or even glass and stainless steel. Apart from adding an unpleasant smell and look to a home, mould can damage building materials – costing you money in maintenance.

What is condensation?

The occurrence of condensation in buildings is the result of complex interactions between the environment, construction methods and occupant behaviour. Air contains invisible water vapour. 

The higher the temperature the more water vapour it can hold. The lower the air temperature the less water vapour. 

Condensation occurs in buildings where warm air encounters cold surfaces below dew point. The air cools and produces visible water droplets on the cold surface. The water formed is known as ‘condensate’ and the process is called ‘condensation’. 

If more water vapour (warm air) is present, further condensation will occur. Where material surfaces are warmer or ventilated, condensate may evaporate.

Want to be involved?

A number of members have reached out to us on this topic and put forward practical measures to safeguard against condensation and mould build up in homes. We have also heard from members who have conducted extensive research on mitigation measures suited to residential construction (and are climate specific), which could be considered for the next suite of NCC changes.

If you are interested in staying in touch or contributing to this body of work.

Email us

For members looking for further resources, HIA has information on securing frames if work needs to be paused for a period of time due to delays; frames exposed to weather; inspection of frames; and protecting frames.

First published on 11 August 2023