Avoiding the Risk of Indoor Mold in Passive House Design

A building design concept that originated in Germany around 30 years ago, Passive House (Passivhaus) is becoming increasingly popular in Australia as architects and designers begin to adopt these design principles in their projects.

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

But isn’t building an airtight dwelling in some of our humid coastal areas looking for a mold problem eventually?

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 46°C outside. This was achieved without active heating or cooling in operation.

Passive house design provides better resistance to heat waves, ensures an indoor environment free of insects and dust, and many strategies used in design can help make a property more fire resistant. The design includes the use of triple-glazed windows and an airtight building envelope, which can help keep toxic gases and smoke out of the building with the help of HRV. heat recovery).

Air tightness

One of the key principles of Passive House design is an extremely airtight building envelope. A building must pass a blower door test to verify airtightness before Passive House certification can be achieved. 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, it’s up to 25 times more airtight than a conventional building in Australia.

Given the high levels of humidity in many coastal areas of Australia, this naturally leads to the question whether such airtightness is not creating favorable conditions for the growth of mold and mildew.

And the short answer would be “yes” – without proper ventilation it would. This is why mechanical ventilation and heat recovery is one of the five passive house design principles. When properly and professionally installed, such a system will ensure high indoor air quality and prevent mold growth on interior and interstitial built structures.

Heat recovery by mechanical ventilation

Mechanical ventilation with heat recovery (MVHR) is the process of improving indoor air quality without opening windows or doors, and part of the answer to preventing mold in the airtight space. This does not mean that windows and doors cannot be opened, only that they do not need to be opened to obtain fresh air quality.

MVHR is intended to improve indoor air quality control and not for heating or cooling buildings, although MVHR systems recover hot and cold air that would otherwise have been wasted. They also clean the air of pollution and help regulate humidity.


Condensation is a major risk factor for the development of mold in a building. Condensation occurs when moist air comes into contact with a cooler surface like a wall, window or mirror.

Condensation can occur on various surfaces in homes simply due to low outside temperature (such as in winter) or due to resident activities such as showering, cooking or simply breathing. It is estimated that a family of four can generate up to 20 kilograms of water vapor per day inside a home.

The passive house design standard calls for the installation of high performance windows and doors to help maintain indoor temperature. These products also have the advantage of maintaining a constant surface temperature so that frames and glass are never cold to the touch, thus preventing the formation of condensation.

Another important principle of the passive house is the absence of “thermal bridges” – there needs to be a continuous layer of insulation separating the interior from the exterior. It also helps protect the structure of the house, as it prevents condensation from occurring where you would have thermal bridges in traditional construction.

Don’t go halfway

Passive house material supplier Andreas Lucci reportedly told a group of building designers to “either go for a complete certified passive house or keep making leaky tents – don’t go halfway.” way, you’ll just have problems with condensation”.

In other words, if certain Passive House standards are adopted and others sacrificed, mold problems could loom. 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 know how to avoid damp and mold buildup and will design for the unique home circumstances and climate, whether in Germany or Queensland.

“Air leaks and uncontrolled moisture entering the building are synonymous with potential mold problems and a building that does not operate efficiently,” points out Joe Mercieca of Blue Eco Homes – a company specializing in building construction. sustainability and passive house design.

“This leads to an unhealthy indoor environment as well as wasted energy consumption. The indoor environment is therefore not only uncomfortable, but leads to high energy and health costs.