Does Passive House design increase the risk of indoor mould?

Passive House design is gaining popularity in Australia, but is building an airtight dwelling in some of our humid coastal areas not looking for a mould problem down the line?

The Passive House (Passivhaus) design concept originated in Germany about 30 years ago, and more Australian architects and designers are now starting to embrace these design specifications.

It’s easy to understand why Australians find the concept appealing. Passive House design is a voluntary standard for constructing buildings that deliver a healthy indoor environment that is comfortable in temperature and humidity, while using very little energy.

Read more about the 5 principles of Passive House design here.

In early January 2020 the Sapphire Passive House built by Blue Eco Homes in the Blue Mountains had an indoor temperature of 27°C while it was a sweltering 46°C degrees outside. This was achieved with no active heating and cooling in operation.

Sapphire Passive HousePassive House design provides improved resilience to heatwaves, ensures a bug and dust free indoor environment and many of the strategies employed in the design can help to make a property more fire resilient. The design includes the use of triple-glazed windows and an airtight building envelope, which can help prevent toxic gases and smoke from entering the building with the assistance of HRV (Heat Recovery ventilation system).

Airtightness

One of the key principles of Passive House design is an extremely airtight building envelope. A building needs to pass a blower door test to verify the airtightness before Passive House certification can be attained. In a certified Passive House, the minimum compliance is 0.6 air changes per hour at 50 Pascals (air pressure).

According to the Australian Passive House Association this is up to 25 times tighter than a conventional building in Australia.

Considering the high levels of humidity in many of Australia’s coastal regions, it naturally leads to the question of whether such airtightness does not create favourable conditions for mould and mildew to flourish.

And the short answer would be yes, without adequate ventilation it would. That is why mechanical ventilation and heat recovery is one of the five principles of Passive House design. When installed correctly and professionally, such a system will ensure a high indoor air quality and prevent mould from forming on indoor and interstitial built structures. 

Mechanical ventilation heat recovery

Mechanical Ventilation with Heat Recovery (MVHR) is the process of improving the indoor air quality without opening windows or doors, and part of the answer to mould prevention in the airtight space. It does not mean that windows and doors cannot be opened, only that they don’t need to be opened to achieve fresh air quality.

MVHR is for improving indoor air quality control and not for the heating or cooling of buildings, although MVHR systems do recover warm and cool air that would otherwise have gone to waste. They also clean the air from pollution and help to regulate humidity.

Condensation

Condensation is a major risk factor for mould growth in a building. Condensation occurs when moist air comes into contact with a colder surface like a wall, window, mirror etc.

Condensation can occur on various surfaces in houses simply because of a low external temperature (such as during winter) or due to the activities of the residents, such as showering, cooking - or simply breathing. It is estimated that a family of four can generate up to 20 kilograms of water vapour per day inside a home.

The Passive House design standard requires the installation of high-performance windows and doors to help maintain the indoor temperature. These products also have the benefit of maintaining a consistent surface temperature so that the frames and the glass never feel cold to touch, thereby preventing condensation from forming.

Another important Passive House principle is the absence of “thermal bridges” – there is a need for a continuous layer of insulation separating the inside to the outside. This also helps to protect the structure of the house, since it stops condensation from occurring in those spots where you would have thermal bridges in traditional construction.

Also read our blog Condensation and the NCC: What’s changed?

Don’t go half-way

Passive House materials supplier Andreas Lucci reportedly told a gathering of building designers to “either go full Certified Passive House, or else keep making leaky tents – don’t go halfway, you will just get condensation problems!”

In other words, if some Passive House standards are adopted and others sacrificed, mould problems could be lurking ahead. Passive House is a design standard that requires a set of key principles to work together to create high indoor air quality.

Passive House professionals are aware of how to avoid moisture and mould build-up and will design according to the unique circumstances of the house and the climate – whether in Germany or Queensland.

“Air leakages and uncontrolled moisture entering the building equal potential mould issues and a building that does not perform efficiently,” emphasises Joe Mercieca from Blue Eco Homes – a company that specialises in the construction of sustainable buildings and Passive House design.

“This leads to an unhealthy indoor environment as well as a waste in energy consumption.  The indoor environment is therefore not only uncomfortable but it results in high energy and health costs.”

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