Moisture accumulation and mould growth for 7-Star housing in Victoria (Research Report 2)
In 2008, the first meeting in response to New Zealand's 'wet-buildings' syndrome phenomenon [7-11] and the potential for parallel risks in Australia occurred between the University of Tasmania architectural science 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. Research Report 1 highlighted research showing that 40% of 2,662 survey responses from Australia’s design and construction professions [5] found a concerning presence of condensation and/or mould in Class 1 and Class 2 buildings
constructed between 2006 and 2016. The research report for that project included the statistical analysis of survey responses and a review of international experiences and regulatory development in other OECD nations. This report provided the Australian Building Codes Board with initial principles for regulatory consideration. This included a cost-benefit
analysis and prioritisation of report recommendations [5].
As Tasmania has some of Australia’s coolest climates, condensation and mould problems started appearing from 2010. Informal communications with members of the design and construction community of Victoria indicated that similar condensation and mould problems were becoming evident in Victoria. In Tasmania, the State’s building regulator identified the need for ‘better-than-code’ design and construction guidance for the design and construction professions [12-16]. This led to an ongoing research program exploring theoretical and applied methods to understand the problems and recommend better design and construction practices [17-19]. The climates within Victoria range from somewhat milder than Tasmania through to Alpine climates. Where condensation and mould problems do already exist in 6 Star housing, it is likely that if building practices do not change, these problems will increase as houses become warmer, for longer periods of time, to meet the heating and cooling energy requirements of 7 Star Class 1 and Class 2 buildings.
Within this context, the aim of this research was to ascertain if an external wall system was constructed to achieve a 7 Star NatHERS simulation result, would the external wall system
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 594 hygrothermal, and 594 biohygrothermal simulations, of nine contemporary residential wall systems, within eleven Victorian NatHERS climates, with two NCC 2022 variations in pliable wall membrane variable (NCC 5 to 8, and NCC 2 to 4), and with an Air Change Rate of 10 (the maximum allowable for a 7 Star home), an Air Change Rate of 7.5 (as many homes are more airtight than an ACR of 10) and an Air Change Rate of 5 (which is the minimum permitted before mechanical ventilation should be installed).
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. 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 water vapour diffusion and ventilation and water drainage.
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 bio-hygrothermal 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 that significantly impact human health [1,
2, 20]. 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 3.0 or more [21, 22]. An MI of 3.0 or more is defined as having a visible presence of mould [23].
Of the 396 walls simulated with an Air Change Rate of 10, the maximum permissible building leakage (infiltration and exfiltration) rate within NCC 2022, 120 (30%) showed the need for a more detailed analysis or an unacceptable mould growth index. This indicates that nearly 30% of these external walls systems may be constructed as per the NCC 2022 requirements for Energy Efficiency (insulation & airtightness) and Health and Amenity (condensation) for Class 1 and 2 buildings [24-26]and may pose a high risk of an unacceptable level of mould growth.
Of the 99 walls simulated with an Air Change Rate of 7.5, which may better represent the airtightness of many Class 1 and Class 2 buildings, 79 (80%) showed the need for a more detailed analysis or an unacceptable mould growth index. This indicates that more than three quarters 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.
Of the 99 walls simulated with an Air Change Rate of 5 (the minimum permissible building leakage rate for infiltration and exfiltration before mechanical ventilation is required), 97
(98%) showed the need for a more detailed analysis or an unacceptable mould growth index.
This indicates that nearly all 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 the three scenarios described above, there is reference to walls requiring further analysis. This indicates that the bio-hygrothermal simulation calculated a Mould Index greater than 1.0 but less than 3.0. 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, the hygrothermal and bio-hygrothermal simulation results may be quite conservative. This is the reasoning behind the DIN4108 [21] and ASHRAE Standard 160 [22] recommendations that further analysis is required, which would explore in greater detail the exterior and interior environmental factors and construction material properties.
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 7 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. 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.
3. 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.
4. A coordinated approach by the ABCB, NatHERS Administrator, appropriate researchers, and industry-based representatives, for the co-development of energy efficiency and hygrothermal regulations.
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.
Funding
Commissioned by: Victorian Building Authority
Transient hygrothermal and bio-hygrothermal risk analysis for housing in temperate and cool temperate Australia : Victorian Building Authority
History
Publication title
Assessment of mould growth risk in new Victorian homes (Research Report 2)Confidential
- No
Commissioning body
Victorian Building AuthorityDepartment/School
Architecture and DesignPublisher
Victorian Building AuthorityPublication status
- Published