Hygrothermal and mould growth analysis of typical Australian residential wall systems

The Australian National Construction Code’s (NCC) energy efficiency requirements have been continuously enhanced since 2002, to decrease greenhouse gas emissions. To achieve these energy efficiency requirements, new houses have included increased minimum levels of insulation and airtightness in their building envelopes (i.e., walls, floors, and roofs). In Australia's temperate and cool-temperate climates the focus has been to make houses warmer in winter and cooler in summer. With these new interior conditions, a problem has emerged. Design and construction professionals, and inhabitants of the newly built houses, are reporting increased incidents of condensation and mould, which is a significant concern as this substantively impacts human health. The World Health Organisation found that residential dampness is associated with a 50% increase in asthma rates. Asthma is a major health problem in Australia and is known to be aggravated by the presence of mould. Asthma is a prominent illness in children aged 0–14 years in Australia, accounting for 12.3% of the total asthma burden. There is also an international acknowledgement that mould spores are associated with several immunology and allergy conditions that may cause lifelong illnesses.
A national building profession survey and a Parliamentary enquiry have estimated that one in three Australian homes may be affected by condensation and mould. Compounding this, many elderly people, people with health problems, and children are known to spend substantial time indoors, and much of this in their homes. Moreover, the COVID-19 pandemic, where people spent most time indoors, has created an urgent situation where housing and health need consideration together in a more intentional way. In Australia, it is known that the built fabric of homes can support mould growth as there is no adequate guidance for the design and construction professions regarding the configuration of building materials in the external envelope. Some developed nations, such as the UK, Canada, the USA, Germany, and New Zealand have, in recognition of the importance of ensuring healthy indoor environments, incorporated guidelines on condensation and mould in their building regulations over the last two decades. Critically, Australia’s regulatory mitigation against condensation and mould substantially lags these nations. Considering Australia’s need, and to support the development of relevant regulation, this research assesses unintentional condensation and mould growth that has occurred through gradually increasing the NCC energy efficiency requirements.
Due to the emerging awareness that the Australian building regulations about moisture and mould growth in new housing may be potentially inadequate, this research utilises simulations of interior conditions in Australian homes to provide hygrothermal (condensation assessment) conditions and bio-hygrothermal (mould growth scenarios). Hygrothermal and mould growth simulations in this research were guided by international standards and best practice guidelines, including, an internationally accepted index to quantify mould growth. Simulations explore commonly constructed, code-compliant, Australian residential external wall types (clay masonry veneer, compressed fibre cement sheet, and weatherboard), in three different NCC defined climate zones (CZ 7 – cool temperate, CZ 6 – temperate, and CZ 2- warm humid). These wall types were analysed for the envelope energy efficiency enhancements made in the NCC from 2003 to 2021. The simulated external wall systems adopted standard, regulation-compliant, building material configurations that were constructed for pre-energy-efficiency regulation designs in 2002 and walls systems expected by the different step changes in the national building regulations in 2003, 2005, and 2010, and likely wall systems for 2021. The key findings of this research are that the simulated contemporary code-compliant external wall systems showed increased levels of moisture, moisture accumulation, and mould growth in all three of the selected Australian climate zones, in all three wall types. These findings could provide technical guidance for National Construction Code improvements in 2022 and 2025 related to energy efficiency, moisture, and mould, and be included in associated NCC condensation and water vapour management guidelines