Enhancements to 6 Star and 7 Star external wall systems for Class 1 and Class 2 housing in Victoria
This research explores via simulation the hygrothermal (heat and moisture) and bio-hygrothermal (mould growth) actions that occur within selected external wall systems, located within Victoria. The use of hygrothermal and bio-hygrothermal simulation to inform building envelope design is quite common in North America, Japan, Korea, the United Kingdom, and many nations within Europe. Recently, Australia has adopted a similar regime with the first ‘condensation’ regulations included within the Health and Amenity clauses of NCC 2019, followed by some enhancements in NCC 2022. Whereas NCC 2019 focussed on condensation, NCC 2022 has moved the focus to a simulated Mould Index (MI) value. Internationally a building envelope system that has a simulated MI of 3, or more, should not be constructed. In some nations, an envelope system with a simulated interstitial MI of 2, or more, should not be constructed. Envelope systems with a simulated MI <1.0 and <3 generally require further analysis and investigation. Generally, a
Mould Index of 3 or more, indicates the visible presence of mould to the unassisted human eye. This scale relates to the World Health Organisation recommendations that describe any visible presence of mould as a risk to human health.
Research Report 1 and Research Report 2 identified mould growth risks associated with the nine selected wall systems within the eight selected climate types. Research Report 1 explored these issues applying the principles of NCC 2019 and typical 6 Star external wall system construction. That report highlighted a significant number of wall system scenarios with a MI of ≥3 and many with a MI of ≥1 and <3. Research Report 2 explored these issues applying the principles of NCC 2022 and typical 7 Star external wall system constructions. That report highlighted an improvement due to the new requirement of a Class 3 vapour permeable weather control layer in NCC Climate Zone 4, and a Class 4 vapour permeable weather control layer in NCC Climate Zones 6-8. However, a significant number of southern orientated external wall system scenarios still documented a simulated MI of ≥3 and with many others showing a MI of ≥1 and <3.
The function of Research Report 3 was to explore built fabric assemblages that may reduce the simulated mould growth risks from Research Report 1 and Research Report 2. A parallel activity included the Victorian adoption of NCC 2022, which included a revised set of NatHERS RMY climate files. As a result, the climate data adopted for Research Report 3 included the NCC 2022 NatHERS RMY climate data. However, Research Report 1 and Research Report 1 also highlighted the conservative nature of the results as the NatHERS RMY climate data did not include any hourly data for precipitation (rainfall) which has been internationally identified as a critical hygrothermal and biohygrothermal simulation input. Recognising this limitation, hourly precipitation data was added to the NatHERS 2022 RMY climate data for the simulations completed in this report.
Research Report 1 and Research Report 2 identified that the wall orientation that posed the greatest risk was the southern oriented wall (non-equatorial facing). All simulations for this report only considered the non-equatorial southern oriented wall.
To ascertain if changes to the NatHERS RMY climate impact simulation results the results in this report include key results from Research Report 1 and Research Report 2. In all cases, the 2022 NatHERS RMY climate data, with hourly precipitation data added, provided a higher simulated Mould Index value.
As a key focus of this report was to explore actions that may reduce the simulated Mould Index value, enhancements were made to each wall system, as shown in Table 1. Generally, the enhancements included increasing the water vapour diffusion properties of the exterior weather resistive pliable membrane, adding a vented and drained cavity between the weather resistive pliable membrane and the cladding system, and
adding a water vapour control membrane to the interior side of the insulated timber framed wall system.
Enhancement 1 Research Report 1 and Research Report 2 versions of the nine external wall systems with the weather resistive pliable membrane modified to the minimum requirements of NCC 2022 (Class 3 for NCC climate zone 4 and Class 4 for NCC climate zone 6)
Enhancement 2 The weather resistive pliable membrane was then simulated with increasing water vapour diffusion permeance properties.
Enhancement 3 The external wall systems from Research Report 1 and Research Report 2 that did not have a vented and drained cavity, had a vented and drained cavity added to the simulated wall systems.
Enhancement 4 In a similar pattern to Enhancement 2, the external wall systems were simulated with a vented and drained cavity and weather resistive pliable membranes with increased water vapour diffusion permeance properties.
Enhancement 5 A water vapour diffusion control layer was added to the interior side of the external wall insulation layer. This type of product controls how and when water vapour may diffuse into the timber framed wall system.
The results of 4,310 hygrothermal and 4,310 bio-hygrothermal simulations of nine external wall systems, within eight of the NatHERS climates, with NCC Climate Zone 4 and Climate Zone 6 & 7 variables applied for Victoria identified several trends, namely:
- The increasing of the water vapour permeability of the weather resistive pliable membrane always reduced the simulated MI value.
- The addition of a vented and drained cavity in most cases significantly reduced the simulated MI value.
- The combining of an increased vapour permeable weather resistive pliable membrane with a vented and drained cavity provided reduced simulated MI values.
- The incorporation of an interior vapour control membrane, paired with a minimum Class 4 weather resistive pliable membrane and a vented and drained cavity system often provided the most robust external wall system with the lowest simulated MI values.
- Many code compliant wall systems failed to provide a Mould Index of <3, as prescribed within the verification method H4V5 of NCC 2022.
The climate type that each wall system was located within also played a significant role. In the warmer climate types for Victoria, many external wall variations provided suitable results with a simulated MI of ≤1. However, in the cooler climate types for Victoria, the external wall systems that provided suitable results increasingly required a vented and drained cavity, a vapour permeable external membrane and an interior vapour control membrane.
Many wall systems performed suitably in most climate types when the hourly air exchange rate was set to 10, which is the maximum air exchange rate allowed in a modern 6 Star or 7 Star home. When the hourly air change rate was reduced to 7.5@n50, wall systems generally required a vented and drained cavity and a greater water vapour permeance properties for the weather resistive pliable membrane. When the hourly air change rate was reduced to 5.0@n50, the most suitable wall systems often included a vented and drained cavity, a vapour permeable weather resistive pliable membrane and an interior vapour control membrane. Habitable spaces with an air change rate of <5@n50 are expected to include mechanical ventilation.
Table 170 and Table 171, within the results section, summarises the bio-hygrothermal simulation results for the NCC climate zone 4 and the NCC climate zone 6-7 respectively. Surprisingly, many southern oriented 7 Star external wall systems required the trio of an interior vapour control membrane, paired with a
minimum Class 4 weather resistive pliable membrane and a vented and drained cavity system.
Contrary to expectations, in some climate types, some wall systems struggled to achieve a suitable MI. This has been linked to the adoption of the new NatHERS RMY data with hourly precipitation data added. A sample analysis identified that most climatic variables were very similar except for radiation data and the additional precipitation data. Subject to the external wall systems ability to adsorb moisture, combined
with the addition of precipitation data, some external wall systems did increase in moisture content. The reduced amount of annual solar radiation then compromised the wall system’s ability to dry.
The research report findings raise a few areas of immediate concern, namely:
- For the external wall systems that did not achieve an appropriate simulated MI value, further simulation-based research needs to occur to establish likely built fabric arrangements that do provide an appropriate simulated MI. This may include adding an insulation layer on the outside of the timber frame.
- This research has utilized the NatHERS 2022 RMY data, with hourly precipitation data added. At this stage there is no regulatory guidance regarding the choice, or source, of climate data for hygrothermal and bio-hygrothermal simulation purposes. As the results from this report and Research Report 1 and Research Report 2 document, the selection of climate data plays a pivotal role in the simulation process and the resultant MI values. To ensure all simulations are comparable, the climate data used for this type of analysis should be regulated.
- The construction material physical properties selected for the simulations completed in this report included measured values for a few products and estimated values for others. At this stage, the only construction materials that require a water vapour diffusion permeance classification are pliable membranes. Aside from all materials used to construct an external wall needing to have a published water vapour diffusion permeance value, these
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 3)Confidential
- No
Commissioning body
Victorian Building AuthorityDepartment/School
Architecture and DesignPublisher
Victorian Building AuthorityPublication status
- Published