Assessment of mould growth risk in regulatory compliant 6 and 7 star new homes in Victoria

What we did and why

The BPC supported research by the University of Tasmania (UTAS) through a research grant to assess the risk of mould growth in external wall systems used in the construction of new housing in Victoria.

This research grant was awarded in 2021 and led by UTAS’ Dr Mark Dewsbury. Dr Dewsbury is an expert in building science, thermal performance, energy use, indoor environmental quality and condensation risk analysis in Australian buildings. In 2016, Dr Dewsbury co-authored a.Scoping Study of Condensation in Residential Buildings for the Australian Building Codes Board.

The research is an international and national collaboration between UTAS, the BPC, Commonwealth Scientific and Industrial Research Organisation (CSIRO) (Australia), Fraunhofer Institute of Building Physics (Germany) [the equivalent of the CSIRO in Australia], Master Builders Victoria and Forest and Wood Products Australia.

All new homes and some renovations, alterations and additions must comply with the minimum energy efficiency requirements of the National Construction Code (NCC). These requirements aim to reduce the environmental impacts of energy consumption and water usage within the built environment.

The energy efficiency of buildings can be demonstrated through a star rating. Under the Nationwide House Energy Rating Scheme (NatHERS), houses are given a star rating (out of 10 stars) based on the energy efficiency of their design. The higher the star rating, the less energy needed to heat and cool the home to keep it comfortable.

Increased energy efficiency requirements have led to the construction of ‘airtight’ buildings. This, combined with a lack of appropriate ventilation, can trap water vapour in building envelopes. Condensation in homes can cause mould growth, structural failure and serious health issues for occupants.

What we found

The research analysed Victorian housing information and climate types, the requirements of the NCC, and the risk of surface and interstitial mould growth within typical external wall systems for new housing in Victoria to achieve a 6-star energy rating (under NCC 2019), and a 7-star energy rating (under NCC 2022).

Using WUFI (WUFI Pro and WUFI VTT) software^[1], the research conducted 4,310 hygrothermal^[2] and 4,310 bio-hygrothermal^[3] simulations to assess moisture accumulation and mould growth in nine external wall systems for a 10-year period post-construction.

The research identified:

• no concerning moisture accumulation in the simulated external walls from the hygrothermal analysis
• many southern orientated external walls indicated higher mould growth risk in the bio-hygrothermal analysis because of the climatic conditions and properties of the wall systems
• wall systems performed better under NCC 2022 compared to NCC 2019 requirements due to expanded requirements for vapour permeable membranes in external walls (i.e. between the cladding system and timber frame).

The research tested and identified enhancements that could be made to external wall systems commonly constructed in Victoria. The research found that mould growth risk can be reduced by:

• further increasing the vapour permeance properties of the weather resistive pliable membrane in external wall systems
• adding a ventilated and drained cavity between the cladding system and the weather resistive pliable membrane layer
• adding a water vapour control pliable membrane to the interior side of the insulation layer (between plasterboard lining and timber frame), or
• a combination of these to provide the most robust external wall system with lowest mould growth risk.

The research results for the first two enhancements (increased vapour permeance of the external membrane and a ventilated and drained cavity) support proposed changes to NCC 2025. The inclusion of an interior vapour control layer is an added enhancement not yet considered for inclusion in the NCC.

The research also recommended further research including:

• simulation-based research on enhancements to the clay masonry external wall system to reduce mould growth risk, as this type of external wall system showed poorer than expected results,
• more complex simulations that assess the effects of intermittent conditioning on hygrothermal simulation results for climates in southern Australia (including Victoria).

What difference this made

The research adds to the body of knowledge and evidence base to help inform policy and regulatory decisions, including future changes to the NCC and guidance for Victorian practitioners. This will help better manage the risk of mould growth in future new homes.

We shared the research with the Australian Building Codes Board (ABCB) and other government policy makers. The research findings support the next phases of mitigation measures being proposed for NCC 2025.

The research has informed the BPC’s response and input into the Victorian position on the proposed condensation mitigation measures for NCC 2025.

Since 2016, the Australian Building Codes Board (ABCB) has been developing mitigation measures to reduce condensation and mould growth risk in buildings. Implementation of these measures has been phased to ease the burden on industry and the community.

NCC 2019 introduced the first stage of these changes – minimum vapour permeance membranes (Class 3) for external walls in climate zones 6, 7 and 8. NCC 2022 introduced the second stage of changes with higher value vapour permeance membranes (Class 4) for external walls in climate zones 6, 7 and 8, and minimum vapour permeance membranes (Class 3) in climate zones 4 and 5.

The third stage of changes proposed for NCC 2025 – drained and ventilated cavity for external walls in climate zones 6, 7, and 8 (in addition to the higher value vapour permeance membranes), and a combination of drained and ventilated cavity for external walls and/or vapour permeance membranes in the other climate zones – are currently under consideration after public consultation.

The third stage of proposed changes are consistent with the findings of the University of Tasmania research. ABCB impact analysis (PDF, 2 KB) shows significant overall benefits of the proposed changes for tropical (climate zone 1) and cooler climates (climate zones 5-8).

The BPC has developed a consumer safety guide to help consumers understand the causes of condensation, mould growth risk and dampness in their homes. The safety guide sets out simple steps that consumers can take to reduce mould risk in their homes and to protect their property and health.

A Practitioner Education webinar on Research Insights: Management of Condensation and Mould Growth Risks was held on 19 September 2024 to provide practitioners with a better understanding of how and why condensation and mould occurs, and ways to mitigate these risks through the research insights and measures in the NCC. The webinar was presented by Dr Mark Dewsbury and attended by over 870 practitioners.

The research is also an input into and supports research by the University of Sydney (also supported through a BPC research grant) to increase the resilience of external building envelopes to moisture damage. The latter research will further add to the body of knowledge to improve the design, construction and maintenance of building envelopes through details such as building corners, thermal bridges, balconies and wall/roof intersections.

^[1]Wärme Und Feuchte Instationär (WUFI) software is developed and owned by the German Fraunhofer Institute of Building Physics (a contributor to this research). The WUFI software has undergone empirical validation in Europe, United Kingdom, Canada, and the United States of America. The WUFI VTT software add-on allows the graphing of data about mould and corrosion risks.

^[2]Hygrothermal analysis evaluates the risk of condensation formation by studying heat and moisture movement within built spaces, and the diffusion of moisture and water vapour through the built fabric.

^[3]Bio-hygrothermal analysis evaluates the risk of mould growth within the built fabric by incorporating assessment of transient hygrothermal boundary conditions (e.g., temperature, humidity, and substrate material) against the growth conditions for typical moulds found on building materials.