Navigating Energy Efficiency and Mould Risk in Australian Low-Rise Homes: A Comparative Analysis of Nine External Wall Systems in Southeast Australia

Abstract

As energy-efficient buildings become central to climate change mitigation, the opportunity for interior and interstitial moisture accumulation and mould growth can increase. This study investigated the potential simulation-based mould growth risks associated with the current generation of insulated low-rise timber framed external wall systems within southeastern Australia. More than 8000 hygrothermal and bio-hygrothermal simulations were completed to evaluate seasonal moisture patterns and calculate mould growth potential for nine typical external wall systems. Results reveal that the combination of increased thermal insulation and air-tightness measures between the 2010 and 2022 specified building envelope energy efficiency regulations further increased predicted Mould Index values, particularly in cool-temperate climates. This was in part due to insufficient moisture management requirements, like an air space between the cladding and the weather resistive layer and/or the low-water vapour permeability of exterior weather resistive pliable membranes. By contrast, warmer temperate climates and drier cool-temperate climates exhibit consistently lower calculated Mould Index values. Despite the 2022 requirement for a greater water vapour-permeance of exterior pliable membranes, the external walls systems explored in this research had a higher calculated Mould Index than the 2010 regulatory compliant external wall systems. Lower air change rates significantly increased calculated interstitial mould growth risk, while the use of interior vapour control membranes proved effective in its mitigation for most external wall systems. The addition of ventilated cavity in combination with either or both an interior vapour control membrane and a highly vapour-permeable exterior pliable membranes further reduced risk. The findings underscore the need for tailored, climate-responsive design interventions to minimise surface and interstitial mould growth risk and building durability, whilst achieving high performance external wall systems.

1. Introduction

This research investigated the growing concern of moisture accumulation and indoor and interstitial mould growth risk in Australian homes built to meet enhanced energy efficiency standards. Research conducted since 2010 has increasingly observed concerns from design and construction professionals and building occupants regarding an increase presence of interior an interstitial mould and excessive moisture in Australian dwellings. Since the 1990s, as the world grapples with the urgent need to mitigate climate change, energy-efficient buildings have been increasingly viewed as a cornerstone of a sustainable, zero-carbon future [1,2,3]. The building sector is a significant contributor to global energy consumption, accounting for more than 40% of overall energy use and 33% of greenhouse gas emissions [4]. To address these challenges, energy-efficient building design focuses on reducing energy demands for heating, cooling, and appliances [5], thereby minimising the environmental footprint of residential and commercial structures [6].

Many countries, particularly in the northern hemisphere, have a long history of implementing energy efficiency for human health, thermal comfort, and energy source management. Austin, Texas, pioneered a comprehensive voluntary energy efficiency programme in 1970, France introduced its first thermal regulations (Réglementation Thermique) in 1974. Germany followed suit with the Energy Saving Ordinance (EnEV) in 1977 [7,8]. These regulations have continuously evolved, setting higher benchmarks for energy performance in new construction and major renovations, and they have proven instrumental in reducing energy consumption and enhancing building sustainability [9]. Australia’s first buildings focused energy efficiency requirements were introduced in 2003 [10,11,12].

Recognising the large landmass of Australia, with significant areas of coastal and non-coastal areas, eight general climate zones were established, see Figure 1, Figure 2, Figure 3 and Figure 4. Figure 1 shows the National Construction Code (NCC) climate zone map for Australia [13] which includes eight generic climate types, from hot and humid to cool-temperate, the black coded area, representing “cool-temperate” zones, highlights southeast Australia. This classification defines built fabric requirements including the vapour permeance properties of building membranes.

Figure 2 shows a recent Köppen–Geiger climate classification map for Australia which includes twenty-seven climate classifications, with ten climate zones in this research’s focus area of southeast Australia (highlighted). Recognising some of these limitations, a more detailed climate segregation was established by the Nationwide House Energy Rating Scheme, which established 69 broad climate zones for Australia [15], with 10 located within the research focus area (see black coded area). Since those first energy efficiency regulatory requirements in 2003, Australia has also progressively strengthened the thermal performance of residential building envelopes in 2004, 2007, 2010 and 2023 [10,16,17]. Since 2016, more than 90% of standalone residential dwellings have used the NatHERS simulation pathway to establish insulation requirements and energy efficiency compliance.

The NatHERS scheme was initially developed in 1993, as a non-regulatory software-based tool, to inform the solar passive design of new residential buildings [18,19]. The combination of improvements in desktop computing capacity and improved algorithms and calculation methods allowed for the expansion of the government developed software and private sector-based software products with Australia. The software development included establishing a scale, which was initially from zero stars to five stars [20], with zero stars indicating a house that required the maximum amount of energy to maintain intermittent comfort (a house which was principally the same temperature as the outdoor environment, in terms of temperature and relative humidity). Each star in improvement would indicate a 25% reduction in simulation-based heating and cooling energy demand. In essence, if a 0-Star house in location needed potentially 1000 MJ/m², a 1-Star house would be 25% less, nominally 750 MJ/m², see

Table 1. NatHERS Star-bands.

Climate No.	Location	NatHERS Stars
Star-Band		0.5	1	2	3	4	5	6	7	8	9	10
1	Darwin (NT)	809	741	626	535	462	399	340	298	257	219	190
21	Melbourne	523	430	290	199	143	107	81	62	44	25	11
22	East Sale	808	655	441	310	228	172	131	100	69	40	15
27	Mildura	616	530	379	263	185	136	103	79	55	32	14
33	Broome (WA)	711	636	519	438	378	328	280	250	219	191	170
60	Tullamarine	790	628	405	279	210	167	131	99	69	41	17
61	Mt Gambier	860	695	461	319	234	180	140	106	73	41	15
62	Moorabbin	668	548	373	262	192	146	112	85	59	34	13
63	Warrnambool	880	688	433	296	226	184	147	112	78	45	19
64	Cape Otway	680	570	401	285	207	155	116	88	62	37	16
66	Ballarat	1111	909	622	446	338	266	207	161	117	75	41

NatHERS Star Band Criteria (Energy Loads [thermal] in MJ/m².annum).

below.

In 2004, when the NatHERS system was adopted within the national building regulations to demonstrate building envelope thermal performance [21], the scale was modified from 0-Stars to 10-Stars, where a 10-Star dwelling would require literally nil energy to intermittently heat or cool a home. Recognising simulation software algorithm improvements and changes to climate data inputs, the MJ values from 0-Stars to 10-Stars for each location has been massaged over time [22,23,24]. An example of star-band values is shown in Table 1 [11]. The cells with no highlight relate to this research and includes temperate and cool temperate locations within southeastern Australia. The two highlighted rows show data for hot and humid climates in northern Australia. These are included to highlight the differences in energy efficiency expectations based on climate type. In a hot and humid climate, cooling can only be achieved via mechanical means, whereas in temperate and cool-temperate climates, heating and cooling needs can be minimised via appropriate solar, thermal, and airtightness control measures.

Research in the mid-1990s, which involved the building energy rating analysis of new and existing dwellings, established benchmark data regarding the potential energy needed to heat and cool typical low-rise timber framed residential buildings to meet intermittent thermal comfort expectations. This research established that many new dwellings were between 0.5-Stars and 2.0 Stars [19,25] and that the first step (2004) for building thermal performance focused on energy rating should be 4.0-Stars. This building envelope energy efficiency requirement was increased to 5-Stars in 2007, 6-Stars in 2010, and 7-Stars in 2023 [26]. For most locations in southeastern Australia (highlighted in red in Figure 3, the move from 6-Stars to 7-Stars would expect a 25% reduction in simulation-based heating and cooling energy and a corresponding increase in building envelope insulation, airtightness, and glazing performance.

While enhancing energy efficiency, the 7-Star rating can present challenges to the indoor environmental quality [27]. Tighter building envelopes and increased insulation can trap moisture, increasing the risk of surface and interstitial mould growth, with the added overlay of Australia’s diverse climate zones [28,29,30,31]. Mould growth threatens structural integrity and occupant health (respiratory problems, asthma, and allergies), with health risks occurring even before visible signs appear [32,33,34,35]. Many countries have faced significant issues with interstitial condensation, poor indoor air quality, and costly remediation. In the United States, it is estimated that mould-related infections cost approximately $5.6 billion annually, while asthma adds another $16.8 billion in healthcare costs [36]. Similar economic burdens are reported in other countries, such as New Zealand, where mould-related issues since the 1990s have cost an estimated 52.3 billion NZ dollars [37]. Australia has also faced significant challenges related to interstitial condensation within building envelopes, leading to poor indoor air quality and costly remediation efforts [27,38]. It is important to note that all construction materials can be affected by moisture and mould. However, renewable and natural materials like solid and engineered wood products will be affected sooner than materials like concrete (concrete cancer) and steel (corrosion) [39]. For these natural materials to provide long-term sustainability, carbon sequestration, and durability, the hygrothermal design considerations and detailing must be more comprehensive.

Given concerns about moisture and mould in energy-efficient buildings, this study explores hygrothermal (moisture) and bio-hydrothermal (mould growth) behaviours in nine typical Australian low-rise timber-framed external wall systems across various climate zones in southeastern Australia. Using the WUFI Pro.6 hygrothermal simulation software and the WUFI VTT mould growth prediction software, the research aims to inform the design of energy-efficient buildings that also maintain healthy indoor environments. By examining 6-star and 7-Star scenarios, this study seeks to determine how different wall systems respond to moisture and mould risks in various climates, ensuring Australia’s energy-efficient building practises protect occupant health and building longevity.

2. Research Method

This study investigates the hygrothermal and bio-hygrothermal behaviour of commonly used Australian low-rise timber-framed external wall systems to identify design mitigations that minimise condensation and mould growth risks across wall section. A three-step simulation-based approach is employed to efficiently assess results and test potential solutions. First, external wall systems were established based on the Australian NCC specifications regarding layer composition, cross-sectional dimensions, and material properties. Second, these models are adapted to various climate zones, as per NCC requirements. Third, viable mitigation strategies are designed and tested. The process is illustrated in Figure 5 below.

Whilst acknowledging the prevalence of Australian homes lacking structured mechanical ventilation, this study adheres to the 70% relative humidity threshold prescribed by Australian Construction Code (NCC) DA-07 [40], ASHRAE 160, and ISO 13788 [41].

The hygrothermal behaviour of building components significantly influences their performance and service life. Given the limitations of experimental investigations, which are often costly and can only address specific aspects of real-world phenomena, the utilisation of computational tools has become increasingly essential. In contrast to earlier moisture calculations that relied on complex and often immeasurable material parameters, contemporary models offer simplicity and accuracy, primarily requiring standard material properties. The validity of these models, based on physical sound formulations [42], has been demonstrated in numerous applications.

The WUFI Pro and WUFI VTT simulation software was employed to analyse the performance of nine typical low-rise timber-framed external wall systems over a ten-year period within different climates in southeastern Australia. This approach provides a comprehensive evaluation of simulation-based condensation risk and predicted indoor and interstitial mould growth risks in new Australian homes.

2.1. Timber-Framed External Wall Envelop Assemblies

In Australia, low-rise framed buildings are predominantly constructed using either solid wood or lightweight steel. Timber-framed external wall systems represent the most prevalent construction method for low-rise buildings in Australia and exhibit a higher susceptibility to mould growth. The Australian Forest Products Association (AFPA) indicates that approximately 70–80% of new Australian houses utilise timber framing. These timber-framed wall envelopes align with the increasing emphasis on zero-carbon building practises, as timber is a renewable resource with inherent carbon sequestration properties. Consequently, through the application of appropriate design and construction techniques, timber-framed wall envelopes remain a cornerstone of sustainable building practice globally, effectively integrating functionality, aesthetics, and environmental considerations. This widespread adoption is attributed to their advantages in sustainability, cost-effectiveness, and design flexibility [43]. Typically comprising a frame constructed from plantation-grown softwood timber, these structures offer a lightweight yet durable solution for buildings [44]. Therefore, timber framing constitutes a significant choice in the residential construction sector, aligning with the prevailing economic and environmental considerations in Australian residential building practises.

Low-rise metal framing was not included in this stage of the research due to its the much lower prevalence and its resistance to mould growth [45,46]. Future research will explore mould and corrosion risks associated with low-rise steel-framed external wall systems include mould and corrosion. The influence of these factors is examined through a series of cases adapted to individual climate zones. The external wall types assessed are detailed in

Table 2. Exterior wall types.

No.	The Exterior Wall Types Examined
1	Timber cladding, with glass-wool batt insulated timber structural frame
2	Compressed fibre cement sheet (CFCS)cladding, with glass-wool batt insulated timber structural frame
3	Clay masonry veneer, with glass-wool batt insulated timber structural frame
4	Concrete blockwork masonry, with glass-wool batt insulated timber structural frame
5	Externally insulated clay masonry, with glass-wool batt insulated timber structural frame
6	Extruded polystyrene (XPS) cladding, with glass-wool batt insulated timber structural frame
7	Expanded polystyrene (EPS) cladding, with glass-wool batt insulated timber structural frame
8	Autoclaved aerated concrete (AAC) masonry cladding, with glass-wool batt insulated timber structural frame
9	Flat sheet-metal cladding, with glass-wool batt insulated timber structural frame

. Examples of the timber framed construction methods for the light weight clad and massive clad external wall systems are shown in Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10.

There are eight monitor points that were positioned across four key layers within each wall, namely:

-

inside and outside surface of plaster board;

-

interior and exterior surface of the glass-wool batt insulation;

-

interior and exterior surfaces of pliable membranes;

-

interior and exterior surface of clay brick.

The monitor points track how indoor and outdoor conditions affect the flow of heat and moisture through the wall system. Each layer is assessed for temperature, moisture content, and moisture accumulation. Additionally, the monitor points in the ventilated and drained cavity include air movement with the intent of promoting drying potential and resisting moisture accumulation between the exterior weather resistive pliable membrane and external cladding layers. The results from the hygrothermal simulation are then processed by the WUFI VTT mould growth calculation software to establish a Mould Index value.

The physical properties of the materials constituting the nine selected external wall systems were sourced from a combination of manufacturer’s data, the NatHERS material properties database, the AIRAH Handbook, and national databases within the WUFI Pro software [47]. These sources provided established values for thermal conductivity, density, and specific heat. However, due to inconsistencies in the availability of water vapour diffusion resistance values, these values were obtained from the WUFI material database, based on materials exhibiting similar thermal conductivity (W/(m⋅K)) and density (kg/m³) to the Australian products under consideration.

2.2. Outdoor Climate Data

Internationally, various climate datasets are available for use as input data in WUFI simulations. Hygrothermal and bio-hygrothermal simulation software utilises climate data to model heat and moisture transfer, and vapour pressure within building envelopes. WUFI requires input parameters including air temperature, relative humidity, solar radiation, rainfall, wind speed, and wind direction. In this research the hygrothermal and bio-hygrothermal performance were assessed using the hourly NatHERS reference meteorological year (RMY) climate datasets, commonly employed in energy modelling, building design, and environmental analysis. However, standard RMY data does not include rainfall data. Rainfall data, which provides information on precipitation levels, is crucial for applications such as hygrothermal modelling and water resource management [48].

This study employed a climate dataset combining RMY data with hourly rainfall data. This combination enables a more comprehensive analysis of weather patterns and their impact on the hygrothermal performance of building envelopes, specifically in relation to condensation and mould growth. This integrated approach facilitates more informed decision-making. The NatHERS climate types, which account for most areas of Class 1 and Class 2 building (single residential and some apartments and units) development in Australia [29], are presented in

Table 3. NatHERS climate zones selected for simulation.

NatHERS Climate Zone	NCC Climate Zone	Location of Weather Station	New Houses Built in This Climate (Percentage)
21	6	Melbourne	6%
22	6	East Sale	12%
27	4 and 6	Mildura	9%
60	6	Tullamarine	12%
61	4 and 6	Mt Gambier	2%
63	6	Warrnambool	4%
64 (+62)	6	Cape Otway (+Moorabbin)	25%
66	4 and 6	Ballarat	30%
TOTAL			100%

.

In addition to the influence of climatic conditions, these classifications also prescribe minimum water vapour permeance properties for weather-resistive pliable membranes [49,50]. Specifically, NCC 2019 required a minimum Class 3 membrane for NCC climate zones 6, 7, and 8, while NCC 2022 requires a minimum Class 3 membrane for NCC climate zone 4, and a minimum Class 4 membrane for NCC climate zones 6, 7, and 8 (see

Table 4. Australian regulatory water vapour diffusion requirements of exterior membranes.

NCC	Energy Rating	NCC Climate Zone 1–3	NCC Climate Zone 4–5	NCC Climate Zone 6–8
NCC 2019	6-Star	No requirements	No requirements (Class 1 applied in this study)	Min Class 3, AS4200 (Class 3 applied in this study)
NCC 2022	7-Star	No requirements	Min Class 3, AS4200 (Class 3 applied in this study)	Min Class 4, AS4200 (Class 4 applied in this study)

for water vapour diffusion property details).

2.3. The Indoor Climate: ASHRAE160 Adjusted with NatHERS

Indoor climate data are a critical input for hygrothermal and bio-hygrothermal simulation models, particularly in cold climates, where wintertime wall moisture conditions are highly dependent on indoor humidity levels. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards are widely used internationally, including in Australia, as guidelines for indoor environmental quality and energy efficiency in buildings. However, while ASHRAE standards are frequently referenced, they do not always align perfectly with the specific climatic conditions and building practises prevalent in Australia. Therefore, while this study is primarily based on ASHRAE standards, it adopts the NatHERS standard of 20 °C for indoor temperature, instead of the ASHRAE recommendation of 21 °C. In addition to indoor temperature, this study examines the influence of varying rates of Air Exchanges per Hour Rates (AEH), including the ASHRAE-recommended 0.25 AEH, and additional rates of 0.375 AEH and 0.5 AEH, to further explore design solutions.

The interior vapour load has been applied, as per ASHRAE160, which includes a moisture generation rate of 0.000126 kg/s. This method continuously adds moisture to the interior air increasing relative humidity. However, the software applies a cap to the interior relative humidity of 70% (presuming some form of humidity control).

2.4. Hygrothermal and Bio-Hydrothermal Analysis

The simulation-based assessment comprised the evaluation of the probability of surface and interstitial moisture accumulation and mould growth for different wall types in south eastern Australian NatHER’s climate zones 21, 22, 27, 60, 61, 63, 64 (+62), and 66. The ASHRAE160 [40] standard prescribes criteria for moisture control and analysis of buildings. There are three performance evaluation criteria that were set to minimise the potential issue associated with indoor and interstitial moisture and mould growth risks in the building envelope:

• To minimise mould growth, limit indoor relative humidity to 70%.
• The initial value of mould index shall be zero, the predicted mould growth and mould index shall be accumulated for each hour such that the Mould Index shall not exceed a value of three (3.0).
• The analysis should be predicted on loads that exceeded 90% of the time.

These criteria must be satisfied at every surface of the wall which is from the inside surface of the plasterboard to the inside surface of the cladding system. The performance criteria were assessed through the completion of a two stage simulations process, firstly hygrothermal simulations were completed using the WUFI Pro software that was developed by the Fraunhofer Institute of Building Physics laboratory (IBP). The resultant data were then post processed by the WUFI VTT mould growth prediction software. Both models have been developed and enhanced based on empirical data and consider transient ambient conditions [51]. The result is a multi-year predicted rate of mould growth named as the mould index (MI). The MI value indicates the predicted rate of mould growth. The higher the MI value the greater the chance of surface mould growth. Many nations, including Australia, specify that the MI should not be equal to or greater than three (≥3.0) [44]. Generally, the MI values have been categorised as the following:

• MI <1 is suitable to construct if the MI does not increase.
• MI 1≤ and <3 is not ideal and should be investigated further.
• MI ≥ 3 does not achieve the minimum performance requirements.

Recognising Australia’s location within the southern hemisphere, the simulations discussed in this article focus only on the non-equatorial facing southern external wall. This follows experiences in both the northern and southern hemispheres, where the non-equatorial facing external walls normally present the worst case scenarios regarding condensation and mould growth [52]. The non-equatorial facing wall often poses the greatest risk, whilst the equatorial facing wall often poses the lowest risk due to solar radiation driven drying potential. The simulations were completed for a period of ten calendar years, commencing in April, which represents the southern hemisphere’s onset of Autumn.

Whilst the 2016 National Construction Code had nil requirements regarding condensation and mould, the 2019 edition only described requirements in NCC Climate zones 6, 7 and 8 (See Figure 1). For these three climate zones, a weather-resistive pliable membrane, when installed externally to the insulation layer, was required to meet a minimum water vapour permeability standard of AS/NZS 4200.1 Class 3 (as detailed in

Table 5. AS4200.1 water vapour diffusion classifications.

	Min ≥ (µg/N. s)	Max < (µg/N. s)	Corresponding Water Vapour Diffusion Resistance Factor
Class 1	0.0000	0.0022	≥90,909.0909 to infinite
Class 2	0.0022	0.1429	≥1399.5801 to <90,909.0909
Class 3	0.1429	1.1403	≥175.4078 to <1399.5801
Class 4	1.1403	No Max	0.0000 to <175.4078

). Subsequently, the more stringent building envelope thermal performance criteria (7-Stars) of the 2022 National Construction Code (NCC) stipulated a minimum AS/NZS 4200.1 Class 3 pliable membrane for climate zones 4 and 5, and a Class 4 pliable membrane for climate zones 6, 7, and 8. Therefore, simulation models for 6-Star and 7-Star external wall systems needed to align with the NCC climate zone specific requirements, as shown in Table 4 and Table 5.

Building Energy Rating (BER) simulations of sample house plans established that the R-value of external wall systems for the 6-Star and 7-Star variations was the same (R2.5 to R2.7) for the brick veneer, timber, compressed fibre cement sheet, and sheet-metal cladding systems. Minor variations in exterior wall insulation were implemented across the EPS, XPS, concrete block, and AAC external wall systems. Concurrently, additional efficiency gains were realised through a marginal reduction in glass-to-floor area ratios and the integration of double glazing. This mirrored experiences from combined with advice from representative design and construction bodies who were project collaborators.

The NCC only specifies the minimum requirements. However, if a code compliant wall system does not meet the minimum performance requirement, there is a need to better understand the risks and potential solutions. Therefore, this research also explored ‘better’ than code enhancements, like, the addition and ventilated and drained cavities, a greater water vapour permeability of the exterior membrane and the addition of interior vapour control membranes.

3. Results of 6-and 7-Star External Wall Systems

The study examined the effects of moisture accumulation and mould growth in national regulation complaint (NCC) timber-framed external wall systems of modern energy-efficient homes (NatHERS 6-Star to 7-Star). The simulation results reveal seasonal patterns where cooler months create moist conditions that promote mould growth, while warmer periods see a reduction or stagnation in mould activity. This highlights the dynamic nature of relative humidity (RH) and dewpoint temperature within external wall systems, which can lead to conditions fostering mould growth. The key concern is to ensure that these conditions do not result in year-on-year increases in predicted mould growth, which could lead to structural failures and serious health risks.

The results summarise the hygrothermal and bio-hygrothermal simulations of the low-rise timber-framed external wall systems within eight of the NatHERS climates of southeastern Australia. The base cases for each of the nine external wall systems applied the built fabric as described in the paper for the 6-Star systems and 7-Star systems, include further details in the VBA report [47]. Each wall system was then subject to the addition of an increasingly water vapour permeable external membrane, a vented and drained cavity between the cladding system and the weather resistive pliable membrane, and an interior vapour control membrane. Many wall systems performed suitably in most climate types when the Air Exchange per Hour (AEH) was set to 0.5. When the AEH was reduced to 0.375, wall systems generally required a vented and drained cavity and a more water vapour permeable external membrane. When the AEH was set to 0.25, the most suitable wall systems often included a vented and drained cavity, a vapour permeable external membrane, and an interior vapour control membrane. The climate that each wall system was located within also played a significant role. In the warmer cool temperate climate types (NCC climate zone 4), many external wall variations provided suitable results that mould index greater than 0 but less than 3, especially for AEH0.5 and AEH0.375. However, in the cooler climate types (for NCC climate zone 6 and 7), 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.

The study delved into the analysis of the nine wall types, that include 11 climate zones as defined by NCC and NatHERS climate zones. Various combinations of external membranes, interior membranes, and cavity solutions were tested to evaluate their efficacy in mitigating moisture accumulation and indoor and or interstitial mould growth risks of 6-Star and 7-Star new houses.

3.1. Hygrothermal Assessment: Moisture Accumulation and Condensation

When condensation occurs, the Relative Humidity (RH) is 100%. Condensation happens when air cools down, and its temperature reaches a dew point. The dew point is the temperature at which air becomes saturated, causing water vapour to condense into liquid form.

The findings revealed that most simulated wall types within different southeastern Australian climate zones exhibited no moisture accumulation based on hygrothermal simulations. However, only the glass-wool batt insulated timber-framed clay masonry veneer external wall system exhibited significantly high predicted moisture accumulation, across a wide spectrum of southeastern Australian climate zones, as detailed in

Table 6. Hygrothermal simulation results from non-equatorial facing 6-Star and 7-Star glass-wool insulated, timber-framed clay masonry veneer clads external wall.

NatHERS Climate Zones	NCC Climate Zones	Moisture Accumulation Results
27	4–5	Nil
61		Moisture accumulation
66		Nil
21	6, 7 and 8 4.6	Moisture accumulation
22		Nil
27		Nil
60		Moisture accumulation
61		Moisture accumulation
63		Moisture accumulation
64		Moisture accumulation
66		Moisture accumulation

below. It is worth mentioning that this research made the initial moisture content within the wall system components 80%, as per national and international guidelines, which allows for the post construction drying of the built fabric over a 2–5-year period (DA07, NCC 2022).

Notably, in seven out of the eleven climates examined, moisture accumulation was identified. This is significant, as >33% of low-rise glass-wool insulated timber-framed dwellings in Australia, have a clay masonry veneer external wall system [53]. Clay masonry veneer accounts for $6.35b in low-rise timber and steel framed residential buildings [54]. This calculated moisture accumulation requires further investigation.

When relative humidity exceeds 60–70%, there is a higher probability of mould growth risks occurring. From the hygrothermal results we could predict the mould growth risks of the climate zones with significant moisture accumulation could have high MI exceeding the MI ≥ 3.0 threshold.

3.2. Bio-Hygrothermal Assessment Results

Whilst elevated relative humidity conditions can lead to condensation, relative humidity conditions above 60% can support mould growth. The mould growth risk significantly increases when the RH surpasses 70%, see Figure 10. The condensation requirements within the Health and Amenity section of the Australian building regulations (NCC) includes a simulation-based verification method as specified in AIRAH DA07. Even though there is no numeric requirement to manage the interior relative humidity in Australian homes, the principles of ASHRAE120, DIN4108, and BS5250 [55] were adopted, where the interior RH was capped at 70%.

A comparison of the results from the 6-Star (Figure 11 and Figure 12) and 7-Star (Figure 13 and Figure 14) scenarios for the nine external wall types and the mix of three NCC climate zones (4, 6, and 7) and the eight NatHERS climate types, Figure 11 provides a detailed visualisation of the Mould Index (MI) results for the nine external wall systems, with a non-equatorial orientation, constructed to the 6Star standard within NCC climate zone 4 and NatHERS climate zones 27, 61, and 66. Figure 12 provides a detailed visualisation of the Mould Index results for the nine external wall systems, with a non-equatorial orientation, constructed to the 6-Star standard within NCC Climate zones 6 and 7 and NatHERS climate zones 21, 22, 60, 61, 63, 64, and 66.

In each graph, the top axis shows nine panels representing the nine external wall systems as listed in Table 1. Each wall type, ranging from timber cladding to flat sheet-metal cladding, is evaluated under three different AEH’s (0.25, 0.375, and 0.5) indicated in blue, yellow and red, respectively. The bottom horizontal axis represents climate zones. The vertical axis is mould index which ranges from 0.0 to 6.0. The dark red dotted line indicates the maximum mould index accepted in the NCC 2022 of 3.0. For the 6-Star results, coloured dots are used to illustrate the predicted MI.

For the 7-Star scenarios coloured triangles are used to illustrate the predicted MI. The results demonstrate a clear pattern in which cooler climates, particularly those associated with CZ61 to CZ66, exhibit significantly higher predicted MI values, often reaching the maximum possible value of 6.0. This indicates a severe risk of mould growth in these climates, especially for wall systems that lack sufficient moisture management features, such as a vented and drained cavity or an interior vapour control membrane.

Focusing on Figure 11a which shows the results for NatHERS CZ27, the first panel represents the MI predictions for a timber clad, glass-wool batt insulated timber-framed external wall system. The red dots, which correspond to an AEH of 0.5, indicate a Mould Index (MI) of 5, exceeding the critical threshold represented by the dark red dotted line. This result is deemed unacceptable and warrants further investigation. In contrast, the yellow dots (AEH 0.375) and blue dots (AEH 0.25) demonstrate MI values close to zero, which are well below the MI benchmark of 3, indicating acceptable outcomes.

The second panel depicts a compressed fibre cement sheet (CFCS) clad glass-wool batt insulated timber-framed external wall system. The red dot (AEH 0.5) shows an MI of 2, while both the yellow (AEH 0.375) and blue dots (AEH 0.25) display MI values of zero. All three scenarios in CZ27 meet the acceptability criterion of MI ≤ 3.

Examining panels 3 through 9 reveals a higher frequency of dots exceeding the MI benchmark, particularly for the yellow (AEH 0.375) and blue (AEH 0.25) scenarios. summarising the findings for CZ27, as shown in Figure 11a, panel 5—externally insulated clay masonry with an insulated timber structural frame—exhibited the worst performance, with MI values ranging from 4.5 to 6 for all three AEH scenarios. These results are unacceptable, and this construction approach is not recommended in this climate. By contrast, panel 2 (CFCS) demonstrated the best performance, with MI values ranging from 0 to 2 across all three AEH scenarios, making it the most suitable option for this climate zone.

In Figure 11b,c, more of the external wall systems show a predicted Mi higher than 3.0. Panel 4 (concrete blockwork masonry), panel 5 (externally insulated clay masonry), and panel 8 (AAC) show for all three AEH scenarios a predicted MI between 5.5 and 6.0.

For the results of NCC climate zone 4, Figure 12 demonstrates significant variations in the predicted MI value based on wall types, climate zones, and AEH scenarios. In warmer climate zones (CZ 21 to CZ 27), MI values generally remain below the benchmark of 3.0, indicating an acceptable risk of mould growth. For example, in CZ 27 (Figure 12, second row, first column), panel 2 (compressed fibre cement sheet cladding with an insulated timber structural frame consistently performs well across all AEH scenarios, with MI values ranging from 0 to 2. In contrast, panel 5 (externally insulated clay masonry with an insulated timber structural frame) shows MI values between 4.5 and 6.0 across all AEH levels, exceeding the acceptable threshold and indicating a significant risk for mould growth.

In cooler climates (CZ 61 to CZ 66), the results highlight severe mould growth risks, with many scenarios exceeding the maximum MI of 6.0. Wall systems like concrete blockwork masonry (panel 4), externally insulated clay masonry (panel 5), and autoclaved aerated concrete masonry (panel 8) show consistently high MI values across all AEH scenarios, indicating a critical vulnerability to mould growth. These findings align with the understanding that cooler climates, with their higher relative humidity levels, promote conditions conducive to mould growth.

The data also reveal the importance of ventilation in mitigating mould risks. While higher AEH values (such as AEH 0.5) generally correspond to lower MI values due to increased airflow, some scenarios with AEH 0.25 exhibit unexpectedly high MI values, warranting further investigation into the influence of other factors such as wall design and material properties. For the 7-Star scenarios, the greater water vapour permeance requirements of NCC 2022 were applied.

For 7-Star scenarios, Figure 13 and Figure 14 show a summary of the predicted MI values for the same nine external wall systems within the three NCC climate zones and the eight NatHERS climate types.

The results illustrate the interplay between ventilation, material properties, and climate-specific conditions in influencing the predicted mould growth risk these 7-Star external wall systems.

In Figure 13, the analysis focuses on NCC climate zone 4 and NatHERS climate zones CZ27, CZ61, and CZ66. For CZ27, the results indicate that wall systems with greater moisture absorption properties, such as concrete blockwork masonry, exhibited elevated predicted MI values, often exceeding the critical threshold of 3.0, particularly under lower AEH’s (i.e., AEH 0.25).

Lightweight systems like CFCS cladding and externally insulated lightweight walls generally had lower predicted MI values for all through AEH scenarios. However, this performance disparity becomes more pronounced in cooler climates like CZ61 and CZ66. In CZ61, wall systems such as EPS cladding and clay masonry veneer displayed MI values exceeding 4.5 under AEH 0.25 and 0.375, demonstrating a heightened vulnerability to moisture accumulation.

Even with AEH 0.50, some systems remained at or above the MI 3.0 threshold. CZ66 posed the greatest risks, with most wall systems exceeding showing a MI > 5.5 when the AEH was <0.375. High moisture absorbent systems, such as AAC and clay masonry veneer, always had a predicted MI ≥ 3.0, even at AEH 0.50, highlighting the critical need for targeted moisture management strategies in cooler regions. Compare the results with NCC climate zone 6 in Figure 14 below.

Figure 14, which shows the simulation results for the 7 Star external wall scenarios, expands the scope of analysis to include eight NatHERS climate zones (CZ21, CZ22, CZ27, CZ60, CZ61, CZ63, CZ64, and CZ66), providing a broader perspective on predicted mould growth risks. Warmer zones, such as CZ21 (Figure 14a), showed lower risks for AEH 0.5 and AEH 0.375 where except for flat-sheet-metal cladding, most wall systems consistently achieved MI values below 3.0. However, with the exception of clay masonry veneer, when the AEH was reduced to 0.25, most wall systems had a predicted MI > 3.0. Cooler zones like NatHERS CZ22, CZ61, CZ63, and CZ66 consistently demonstrated higher MI values, with predicted MI values >3.0 for AEH 0.5, 0.375, and 0.25.

The findings highlight that the focus on energy efficiency, with limited changes to the vapour diffusion properties of the exterior weather resistive pliable membrane, has not adequately considered water vapour diffusion risks. The increased interior vapour pressure, caused by a greater number of hours where the interior climate is warmer than the exterior climate, has not adequately addressed the water vapour diffusion management needs for the external wall systems. In a similar pattern to the results for the 6-Star external wall systems the AEH can play a critical role in mitigating mould growth risks. While an AEH of 0.5 showed better predicted MI values, as the AEH was reduced to 0.375 and 0.25, external wall systems increasingly had MI values greater than 3.0. This underscores the need to explore region-specific design strategies that incorporate effective moisture control measures, such as ventilated and drained cavities, vapour-permeable external membranes, and interior vapour control layers.

The variability of predicted MI values also underscores the importance of tailoring wall system designs to specific climatic conditions. Whereas the areas included in this research include three NCC climate zones; they encompass six Köppen–Geiger climate zones and nine NatHERS climate zones, all with differing exterior temperature, relative humidity, solar radiation, and seasonal precipitation conditions.

3.2.1. NCC Climate Zone 4

The second stage of the research aimed to explore enhancements that could reduce the predicted MI values to an acceptable value of <3.0 and <1.0. This is divided into two sub-groups: NCC climate zone 4 (discussed here), and NCC CZ6-7 (see 3.2.2 Figure 15). The left side of Figure 15 shows a summary of the simulation results for the nine external wall systems located within NatHERS climate zones 27, 61, and 66 and applying the principles of NCC climate zone 4. Exploring this in a vertical fashion, if the water vapour diffusion properties are increased from the NCC 2022 requirement to a Class 4-1 membrane, as shown in the second and third rows from the bottom in the leftmost column of Figure 15, the number of wall systems that have a MI of <3 increases from 43% to 57%, and if a cavity is added, this increases to 67%. To exceed 90%, the wall systems include a Class 3 (1398) exterior weather resistive pliable membrane, a ventilated and drained cavity, and an interior vapour control membrane.

This figure demonstrates that further research into the NCC CZ4 external wall systems needs to explore what benefits may occur if a Class 4, rather than a Class 3, weather resistive pliable membrane is applied. Additionally, a Class 3 membrane has a water vapour diffusion resistivity range from 176.0 to 1398 [49]. A very water vapour-permeable Class 3 pliable membrane with a water vapour diffusion resistance factor of 176.0ay have a very similar result to a Class 4 water vapour-permeable pliable membrane with a with a water vapour diffusion resistance factor of 175.4.

International policy makers and researchers are increasingly exploring the need to a maximum MI value within interstitial spaces of 2.0, and as noted above, and simulation with a simulated MI > 2.0 and <3.0 normally warrants further investigation. The right side of Figure 15 shows a comparison of the results of MI ≤ 3.0 and ≤1.0. This figure includes a summary of all simulation results for the 6-Star and 7-Star insulated wall systems. The bottom two rows represent the minimum built fabric requirement for NCC2019 and NCC2022. The remaining twelve rows show the results from the mould risk mitigation enhancements that were explored. The figure continues to illustrate the need for the combination of interior vapour control, ventilated and drained cavities, and vapour open exterior weather resistive pliable membranes.

3.2.2. NCC Climate Zone 6 and 7

Figure 16 presents a comprehensive summary of the bio-hygrothermal simulation results for the enhancements to the nine external wall systems with NCC climate zones 6 and 7, considering different AEHs. This shows a summary of all simulation results for the 6-Star and 7-Star insulated wall systems. The bottom two rows indicate the minimum built fabric requirement for NCC2019 and NCC2022 construction code. The remaining twelve rows show the results from the mould risk mitigation enhancements that were explored. The vertical axis lists the external wall system configurations, including the type of pliable membrane used (Class 4-3, Class 4-1, or nil), whether the wall includes a ventilated and drained cavity, and the inclusion of an interior vapour control membrane.

The horizontal axis shows the percentage of simulations that achieved specific Mould Index (MI) thresholds under various AEH conditions (AEH 0.50, AEH 0.375, and AEH 0.25). The dark green, mid green, and light green bars represent simulations where the MI is ≤3.0, when the AEH was 0.5 (representing the NCC 2022 requirements), 0.375, and 0.25, respectively. Similarly, the yellow, peach, and pink bars represent simulations where the MI is ≤1.0, when the AEH was 0.5 (representing the NCC 2022 requirements), 0.375, and 0.25, respectively. The analyses reveals that the highest performance in terms of moisture control and mould growth mitigation is achieved when all three components are combined, namely;—an AZ/NZS 4200 Class 4 exterior weather resistive pliable membrane, a ventilated and drained cavity, between the cladding and the weather resistance membrane, and an interior vapour control membrane. Specifically, an external wall system that combined the AS/NZS4200.1 Class 4 pliable membrane with both a ventilated and drained cavity and an interior vapour control membrane achieved a 96% success rate for predicted MI’s of <3.0 when the AEH was 0.50, 0.375, or 0.25. This indicates a robust solution across different ventilation scenarios.

In contrast, wall systems without a ventilated and drained cavity but with a Class 4 pliable membrane (row 6 from the top of the figure) show a significant drop in performance, with only 69% of the external wall systems having a predicted MI of ≤3.0, when the AEH was 0.50, decreasing further when the AEH was 0.375 (52%), or 0.25 (24%). The results suggest that while increasing the water vapour diffusion properties of the exterior membrane slightly improves performance, the absence of a vented and drained cavity severely limits the wall’s ability to manage moisture and prevent mould growth.

Figure 17, which compares the results for NCC CZ4 and NCC CZ 6-7, further underscores the similar needs of the external wall systems in these different climate types, especially as the AEH decreases. The findings clearly indicate that a combination of high-quality pliable membranes, ventilated and drained cavities, and interior vapour control membranes is essential to achieving acceptable predicted MI values in the temperate and cool temperate climates explored in this research.

3.3. Air Change Rates: Impact on Bio-Hygrothermal Performance

Figure 18 further illustrates the impact of AEH on the bio-hygrothermal performance of the external wall systems analysed in this research. The analysis reveals that as AEHs decrease, the risk of simulation-based moisture accumulation and mould growth risk increases. The findings underscore the necessity for wall systems with lower AEHs to incorporate additional moisture management strategies. This graph can provide critical insights into optimising wall system designs across different AEHs, aiming to strike a balance between energy efficiency and moisture control in various climate zones.

However, the most newly constructed homes in these climate zones are demonstrating AEHs between 0.375 and 0.25 [56]. These figures show that as AEHs decrease, the percentage of cases achieving acceptable MI results also declines. Specifically, the optimal wall system with a Class 3 pliable membrane only achieves a 61% success rate in keeping the MI ≤ 3.0 and a 50% success rate for an MI ≤ 1.0. To achieve over 90% of wall systems with an MI below 3.0 at an AEH of 0.375 or higher, the wall system must include a Class 3 or Class 4 exterior weather-resistant pliable membrane, a ventilated and drained cavity, and an interior vapour control membrane.

This analysis highlights a critical concern for energy-efficient homes, particularly in the transition of less insulation to more insulation, the ability for occupants to economically condition the interior temperature and the improvements in building envelop airtightness from AEH 0.50 to AEH 0.25. It emphasises the need for well-designed wall systems that integrate robust moisture control mechanisms to maintain indoor environmental quality and prevent mould growth while meeting stringent energy efficiency standards.

4. Discussion

This study simulated the long-term hygrothermal and bio-hygrothermal performance of nine “perfectly constructed” external wall systems over a ten-year period. While such an idealised model provides useful baseline insights, it does not fully reflect the complexity of real-world construction, where imperfections in workmanship, material interfaces, and assembly practises often occur. Localised moisture accumulation, particularly at thermal bridges such as timber column junctions, connection interfaces, and open joints can significantly influence potential mould development and is not captured in simplified 1D simulations. These localised phenomena such as moisture accumulation are often multidimensional in nature, involving complex heat and moisture transfer, and warrant further investigation.

Furthermore, this study is subject to several limitations. The accuracy of the simulation outcomes depends heavily on reliable input data, including interior and exterior climatic conditions, material properties, and the exclusion of human behavioural factors. Moreover, most simulation tools were originally developed for cold climates with continuous conditioning, whereas many Australian regions experience temperate or humid conditions with requirements for only intermittent heating and/or cooling. These climatic differences may intensify transient hygrothermal and mould risks, underscoring the need for simulation models tailored to Australia’s building context.

Additionally, the research reinforces that many external cladding systems in developed countries are designed as rain control layers rather than impermeable barriers. Effective drainage and ventilation behind cladding systems are essential for long-term durability, particularly in managing moisture ingress from wind-driven rain and internal condensation.

This research adhered to the international requirement that capped the relative humidity within building interior at 70%. However, previous studies have identified that relative humidity levels in many Australian residential buildings often exceeds 70%. Consequently, further hygrothermal simulations that explore the impact of uncapped interior relative humidity within the Australian temperate and cool temperate climate context are needed.

Future studies should examine the effects of construction imperfections, variable workmanship, and localised moisture retention at interfaces and junctions. Integrating advanced methods such as multidimensional simulations, predictive modelling, and real-time building monitoring can provide a more holistic understanding of mould risk in energy-efficient housing. Ultimately, the challenge lies in balancing health, durability, and energy efficiency through design strategies and regulatory frameworks that address both environmental sustainability and occupant well-being.

5. Conclusions

The primary focus of this research was to understand if ‘code compliant’ timber-framed external walls systems commonly constructed in southeastern Australia had a simulated surface or interstitial mould growth risk, and if so, explore enhancements to achieve a Mould Index (MI) value of <3.0, or better. The research analysed nine glass-wool batt insulated timber-framed external wall systems, located within three NCC climate zones (4, 6, and 7) and eight NatHERS climate types (21, 22, 27, 60, 60, 61, 63, and 64). The first stage of the research explored simulation-based mould growth risks in ‘code’ compliant commonly constructed timber-framed 6-Star and 7-Star external wall systems. The second stage then explored interventions that could be made to reduce predicted mould growth risks.

The first stage of the research demonstrated that most 6- and 7-Star code compliant external wall systems had predicted mould index values > 3.0. One of the key insights from this research is the critical role of climate in determining mould growth risks. Cooler climates were found to be particularly vulnerable, with MI values remaining elevated even with the introduction of more permeable membranes as prescribed by the NCC 2022 standards. This suggests that a more holistic approach that incorporates effective moisture control mechanisms—such as ventilated and drained cavities and interior vapour-control membranes—may be essential to maintain a durable built fabric and healthy indoor environments. The research also revealed inconsistencies regarding the current NCC climate zone classifications and the non-correlating relationship to some NatHERS climate zones. For instance, NCC CZ4, categorised as temperate and dry, includes NatHERS CZ66, one of the coolest and most humid climates. This overlap suggests the need for more precise hygrothermal climate classifications to guide wall design requirements and reduce potential mould growth risks.

Notably, differences in moisture retention among wall systems further influence mould growth risks. Materials like unpainted clay masonry veneer showed significant moisture accumulation due to exposure to site-specific conditions, such as rain and high relative humidity, as well as internal moisture diffusion. While painted surfaces like concrete block masonry showed a lesser external moisture accumulation, indicating that moisture management strategies must address material-specific vulnerabilities.

The first stage of the research emphasises the need for integrating robust moisture management strategies into the design of 7-Star energy-efficient homes. Addressing region-specific vulnerabilities through optimised material selection, improved ventilation, and tailored moisture control measures can balance energy efficiency with enhanced indoor health and durability.

The second stage of the research explored enhancements to the timber framed external wall systems, including increasing the water vapour permeability of the exterior weather resistive pliable membrane, adding a ventilated and drained cavity between the weather resistive layer and the cladding system, and adding an interior vapour control pliable membrane. The research demonstrated that increasing the water vapour permeability of the exterior weather resistive pliable membrane significantly improved the number of scenarios when the external wall system simulations achieved an MI of 1.0 or less when the AEH was 0.5. However, as the AEH was reduced to 0.375 and 0.25, more enhancements were required. The results indicated that an external wall system combined AS/NZS 4200.1 Class 4 (175.4) weather resistive exterior pliable membrane with a ventilated and drained cavity, and an interior vapour control layer, provided the most consistent outcomes with a predicted MI of ≤1.0 for AEH scenarios of 0.5, 0.375, and 0.25. Recognising other research that has documented many news homes with AEH values between 0.25 and 0.375 indicates that new external wall systems should include these three enhancements.

The findings of this study underscore the complex interplay between energy efficiency and indoor environmental quality, particularly concerning moisture management and mould growth in Australian homes. While the transition from a 6-Star to a 7-Star energy efficiency rating represents significant progress in reducing heating and cooling energy consumption, it also introduces new challenges, particularly in cooler climates where the risk of moisture accumulation and mould growth is heightened. The bio-hygrothermal simulations conducted over a decade revealed that, despite an overall reduction in MI values with the 2022 requirement for a more vapour permeable exterior weather resistive pliable membrane, the potential for mould growth remains substantial, especially without additional moisture management strategies.

The implications for building design and policy development are significant. Enhanced insulation and airtightness in 7-Star homes can exacerbate moisture risks. Updates to the National Construction Code (NCC) should include detailed guidance on wall system configurations and mandatory ventilation standards tailored to regional climate conditions.

Furthermore, the study highlights the significant impact of air exchange rates per hour (AEH) on mould growth risks. Lower AEHs were consistently associated with higher predicted Mould Index values for all nine external wall types and in all three NCC climates and eight NatHERS climates, indicating that ventilation plays a crucial role in managing indoor moisture levels. As Australia continues to push towards more stringent energy efficiency standards, it is imperative that these standards do not inadvertently compromise indoor air quality or occupant health.

In conclusion, this research emphasises the need for a balanced approach to designing energy-efficient homes. While achieving higher energy ratings is crucial for sustainability, it must be complemented by robust moisture management strategies to prevent the adverse effects of mould growth. Future building codes and regulations should consider these findings to ensure that energy-efficient homes are not only environmentally sustainable but also safe and healthy for their occupants. This study provides a foundation for further research and policy development aimed at integrating energy efficiency with comprehensive indoor environmental quality standards. Future research should explore the potential impacts of climate change on mould risks and utilise dynamic simulations to refine these findings further.