Moisture accummulation and mould growth for 6-Star housing in Victoria

In 2008, the first meeting to discuss the looming ‘wet-buildings’ syndrome for Australia occurred
between University of Tasmania researchers, the Tasmanian government, the Australian Building
Codes Board and Building Research Association of New Zealand. Based on the experiences of many
developed nations and a lack of the application of their learned experiences within Australian
regulatory development, there was an acceptance that as Australia increased envelope thermal
performance requirements, our own ‘wet buildings’ experience would evolve quite quickly. By
2016, there was enough government and industry concern to prompt the Australian Building
Codes Board to complete a nationwide condensation survey. The survey had 2,662 usable
responses, from all Australian jurisdictions. The survey respondents advised that more than 40%
of new buildings constructed in the last decade had a concerning presence of condensation and/or
mould [2]. This led to the Australian Building Codes Board commencing actions to consider what
may need to be considered from a regulatory perspective. As Tasmania has some of Australia’s
coolest climates, condensation and mould problems started appearing from 2010. By 2014, the
number of regular enquiries from new homeowners, builders, building surveyors and design
professionals to the State’s building regulator, identified the need for ‘better-than-code’ design
and construction guidance for the design and construction professions [3-7]. This led to an ongoing
research program exploring theoretical and applied methods to understand the problems and
recommend better design and construction practices [8-10]. The climates within Victoria range
from somewhat milder than Tasmania through to Alpine climates, and there is a growing voice
from homeowners, and the design and construction professions regarding concerning amounts of
condensation and mould within Class 1 and Class 2 buildings.
Within this context, the aim of this research was to ascertain if an external wall system was
constructed as described and specified with the current national building regulations (the National
Construction Code), would these 6 Star external wall systems support or promote:
1. Moisture accumulation, that can lead to structural failure, and/or
2. Mould growth leading to unhealthy interior environments and decay of structural
elements.
To explore these questions, this research conducted 880 hygrothermal, and 880 bio-hygrothermal
simulations, of nine contemporary residential wall systems, within eight Victorian NatHERS climate
zones, with three NCC 2019 climate zone insulation variables (climate zone 4, climate zones 6 and
7, and climate zone 8), and with an Air Change Rate of 10 (the maximum allowable for a 2019 6
Star home) and an Air Change Rate of 5 (as many homes are more airtight than an ACR of 10).
The simulation results, as shown in Section 5 and within the Appendices, show that aside from a
few outliers there is no concerning amount of moisture accumulation with the analysed nine
external wall systems.
It is important to note two key principles:
1. Moisture will form within external walls quite often, when the dew point temperature
condition is achieved in each component (i.e., less than 12°C). The aim of modern façade
design is to acknowledge this wetting condition, to minimise how much moisture may
form and to promote a wall’s ability to dry and remove the moisture via drainage and
ventilation. A traditional brick veneer wall, which includes a vented cavity between the
Moisture accumulation and mould growth for 6 Star housing
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clay brick and the timber frame, has applied these principles since the 1920s.
2. Whereas dew point temperature requires a relative humidity condition of 100%, mould
will grow in ventilated spaces when the relative humidity is above 70% and in still air
spaces when the relative humidity is above 60%. This indicates that you do not need to
have a ‘wet’ building to have a ‘mouldy’ building.
The capacity for a building to be mouldy but not wet is exemplified by the results from the biohygrothermal
simulations which were completed to ascertain if there is a risk of unacceptable
mould growth. Mould growth on interior and interstitial surfaces leads to building decay, structural
failure and mould spores significantly impact human health. The very small size of mould spores
allows them to freely enter our blood stream via our lungs and the process of breathing. The World
Health Organisation has advised that there should be no visible presence of mould within the built
environment. An unacceptable mould growth index is any simulation with an MI of 3 or more. An
MI of 3 or more is defined as having a visible presence of mould. Of the 532 walls simulated with
an Air Change Rate of 10, the maximum permissible building leakage (infiltration and exfiltration)
rate within NCC 2019, 230 (43%) showed the need for a more detailed analysis or an unacceptable
mould growth index. This indicates that nearly half of these external walls systems may currently
be constructed in a manner that poses a high risk of an unacceptable level of mould growth to
construction requirements and climatic conditions.
Of the 348 walls simulated with an Air Change Rate of 5 (the minimum permissible building leakage
rate (infiltration and exfiltration before mechanical ventilation is required), 327 (94%) showed the
need for a more detailed analysis or an unacceptable mould growth index. This indicates that many
of these external wall systems may currently be constructed in a manner that poses a high risk of
an unacceptable level of mould growth due to construction requirements and climatic conditions.
In both scenarios described above, there is reference to walls requiring further analysis. This
indicates that the bio-hygrothermal simulation calculated a Mould Index greater than 1 but less
than 3. The software is simulating a ‘Perfect Wall’. However, walls are rarely made perfectly and
the environmental conditions inside and outside the wall system may vary, leading to an
unacceptable amount of mould growth. Within this context, further analysis requires a deeper
consideration of all environmental factors that may impact the wall system’s long-term durability.
In all scenarios, the southern orientated external wall systems showed the greatest mould growth
index values. Due to the solar radiation induced drying potential, the northern facing walls often
performed much better, with a nil or lower mould growth index value. However, if one considers
external wall shading from nearby buildings, eaves or landscaping, the minimum performance
requirement should use the material arrangements such that the southern wall has a nil, or
acceptable mould growth index. Within this context the recommendations of this research report
are:
1. Each of the 6 Star wall systems needs to be modified such that the southern orientation
shows either a nil, or an acceptable long-term mould growth index. This would include
the specification of pliable building membrane water vapour diffusion resistance
properties and the inclusion of a vented cavity for most external wall systems.
2. The next stage of the research includes 7 Star wall systems. The Council of Australian
Governments (COAG) approved the NCC 2022 7 Star requirement for Class 1 and Class 2
residential buildings for adoption from 1 January 2023, (subject to Jurisdiction based
adoption processes). When compared to a 6 Star home, a 7 Star home will have a
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NatHERS simulation based 25% reduction in energy needed to heat or cool the habitable
rooms. This will further increase the challenges of water vapour diffusion through the
built fabric. The findings from the hygrothermal and bio-hygrothermal simulations of the
6 Star wall systems will need to be applied to the 7 Star wall systems.
3. A significant education program needs to be undertaken to inform design and
construction professions of the risks evident in the simulation results. Since 2014, the
Tasmanian government has actively engaged with the design and construction
professions via training activities and two editions of the Tasmanian condensation design
guide.
4. Further research must consider various aspects of airtightness and the climatically
appropriate envelope elements to control water vapour diffusion such that concerning
amounts of, or unacceptable amounts of mould growth do not occur.
5. There needs to be a coordinated approach by the ABCB, NatHERS Administrator,
appropriate researchers, and industry-based representatives, for the co-development of
energy efficiency and hygrothermal regulations.
6. This research is limited to the wall systems and inputs, as shown and discussed in the
report body and Appendices. Changes to these inputs will change the results.