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Hygrothermal Performance of Engineered Wood Multi-Story Buildings in Australian Tropical and Sub-Tropical Climates


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Hygrothermal performance of engineered wood multi-storey buildings in Australian tropical and subtropical climates highlighting a framework for modelling durable assembly designs.
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Hygrothermal Performance of Engineered Wood Multi-Story Buildings in
Australian Tropical and Sub-Tropical Climates
Marcus Strang*1 and Paola Leardini2
1, 2 School of Architecture, University of Queensland, Australia
A landscape ripe for Engineered Wood (EW) construction to displace conventional practice in the
Australian context has emerged since the release of the Australian National Construction Code (NCC)
Volume 1 fire-protected concessions for timber constructions in 2016. These concessions are to be
extended to all class types for the 2019 NCC changes. In 2018 alone, this has led to more than 23,500m3 of
EW in 35 projects, making-up half of Australia’s total 71 EW construction projects (“Timber Architecture
& Constructions News - Edition 1 - 2019,” n.d.). EW construction also provides a pathway for achieving
the 21st Conference of the Parties (COP21) Paris agreement to limit global warming to 1.5°C above pre-
industrial levels, as utilising structural EW construction may, in some cases, save 159.5% of embodied
carbon Tonnes (tC) when compared to the base-case conventional reinforced concrete building (Li,
Rismanchi, & Ngo, 2019). Despite the acceleration of EW uptake in Australia, there is still a significant
education gap in the construction industry. This is especially the case in Australian tropical and sub-
tropical climates, with high relative humidity, where the hygrothermal behaviour of EW products remains
unknown. Typical EW construction details have evolved in cold temperate to deal with heat retention,
cavity condensation and moisture removal during long, cold, wet winters. In contrast, buildings in
Australia, especially in sub-tropical and tropical climates, typically face the opposite conditions.
Addressing these knowledge gaps is essential to inform correct construction details appropriate to humid
Australian climates: optimised design solutions help avoid interstitial condensation, which may result in
mould growth and respiratory reactions in occupants, in addition to a reduction in thermal performance
of insulation materials and timber rot – with potential structural ramifications.
This paper summarises results of a preliminary study aimed at setting out the methodological framework
of on-going research carried out at The University of Queensland to assess hygrothermal performance of
EW construction assemblies for tall timber buildings. The preliminary study utilises simulation tools to
compare alternative construction solutions and identify the assembly showing the best overall drying
capacity and humidity performance. With a focus on hygrothermal behaviour and airtightness, future
research will employ evidence-based analysis of existing case studies, whole building simulation,
experimental testing and post-construction monitoring to develop optimised Cross Laminated Timber (CLT)
construction systems and details that are optimised for tropical and subtropical Australian climate. Ultimately,
it aims to demonstrate the superior holistic performance, economic advantages and reduced environmental
impact of net zero energy buildings that employ optimised CLT solutions. This is expected to accelerate the
uptake of new Engineered Wood Products and timber technologies by the Australian construction market as
an effective means to increase the overall hygrothermal performance and energy efficiency of the growing
multi-storey building stock.
The aim of this paper is to discuss a set of solutions that optimise energy performance, freedom from
structural damage and healthy indoor environments for buildings that employ CLT construction in tropical
and sub-tropical Australian climates.
The study presented in this paper analyses conventional CLT assemblies and investigates optimized
solutions that can provide superior hygrothermal performance for multi-story buildings in sub-tropical
and tropical climates. It is essential to understand how the layering of the construction assembly affects
moisture control and which assemblies are most effective at improving hygrothermal performance by
decreasing interstitial and surface condensation risk in sub-tropical and tropical Australian climates.
Material and Methods:
Preliminary analyse of CLT construction solutions in sub-tropical and tropical climates was carried out
through computer simulation using WUFI Pro. This suite of software products allows realistic calculation of
the dynamic coupled one- and two-dimensional heat and moisture transport in walls and other multi-layer
building components exposed to natural weather by simulating built-in moisture, driving rain, solar
radiation, long-wave radiation, capillary transport, and summer condensation of a construction assembly.
The preliminary hygrothermal analysis (interstitial condensation, surface condensation and mould risk) of 30
construction assemblies for two Australian climate zones was commissioned to eZED Limited (2017) as part
of this study. The assembly build-up consisted of a 100mm structural CLT panel with insulation (rigid and
fibrous) and vapour control membrane. Generic values have been adopted in lieu of proprietary values. The
position in the assembly of these layers was iteratively altered to parametrically study the hygrothermal
performance of the CLT wall assembly construction.
The simulated external climatic conditions were based on the sub-tropical and tropical climate zones for
Cairns and Brisbane and were simulated over a duration of 3 years. Internal conditions of 20ºC with 1ºC
amplitudes and 50% RH with 5% RH amplitude were based on the assumption that simple sine curves with
a yearly period may be used to describe the ambient temperature and humidity. The simulations used an
initial relative humidity of 80% constant across all layers to account for high initial relative humidity due to
moisture exposure of the materials during construction. The wall orientation was analysed for worst-case
performance to determine the most critical condition based on lack of drying potential and solar shading. For
Brisbane, the South-East orientation was analysed, for Cairns, the South orientation was analysed. For this
study it was assumed that the cladding material performed a perfect driving rain protection and weather tight
The ventilation level of the external cavity has been calculated using an air change of 1 ACH, as there are
several parts in a building that obstruct effective air flow and may have long term implications on the
construction moisture performance. Whilst external cavity ventilation can be significantly higher, this
assumption is conservative in nature.
Analysis and Discussion:
The effects of the vapour control layer within the construction was analysed using high and low vapour
resistance. A vapour barrier with SD = 1,500m positioned between CLT and fibreglass insulation showed a
steady increase in predicted mould growth for both climates, Brisbane and Cairns. Using a layer with SD =
0.1m, however, led to acceptable results for both climates.
Expanded Polystyrene has a higher vapour resistivity than fibreglass, which makes it less dependent on a
separate vapour control layer. However, if placed in the wrong location of the construction, it can block
vapour movements. It is therefore important to choose the correct position in the construction during the
planning and design phase to prevent long-term issues. Both insulative materials are also unique in their
infiltration properties. Fibreglass alone will allow more air movement than extruded Polystyrene and should
therefore always be used in combination with a separate construction layer for airtightness to ensure
insulation performance is retained. Extruded Polystyrene, on the other hand, can be sufficiently airtight
when connection details are either taped, or glued.
For both Brisbane and Cairns climate, it was determined that the construction showing the best overall
drying capacity and humidity performance is for CLT with internal insulation and a vapour resistive layer
placed on the outside of the CLT as shown in Figure 1. In these case, the vapour resistivity of the rigid
insulation or standard fibreglass insulation had no significant effect on the construction. However,
reversing the construction as shown in Figure 2, by placing the vapour resistive layer on the inside and
insulation on the outside of the CLT, showed critical moisture levels in all simulation iterations and is not
recommended. For both climates, this presented the worst results. This preliminary assessment
demonstrates that the proposed construction can therefore be used for tropical and subtropical climates
when the insulation is placed on the inside of the high-performance building employing CLT structures.
The effectiveness of the vapour control layer varies with its location in the construction. When placed on
the external side of the CLT, a larger range of vapour resistivity values is possible whilst retaining the
membranes vapour resistance performance.
Figure 1. Preliminary WUFI results for optimal CLT assembly with internal insulation and external
vapour resistive layer
Figure 2. Critical moisture levels with external insulation and internal vapour resistive layer
For this study it was assumed that the cladding material performed a perfect driving rain protection and
weather tight boundary; further research is required to investigate driving rain infiltrating through the cladding
and into the assembly, additional insulation types such as rockwool, membrane selections, ventilation system
types, additional climates zones and building typologies. It is also important to note that these WUFI models
do not account for construction discontinuities in two or three dimensions. Therefore, as it is more likely that
moisture (in liquid or vapour form) will penetrate in to the assemble at these construction discontinuities,
assessment of this condition should also form a part of the future research.
Preliminary results of this study demonstrate that in the climates of Brisbane and Cairns, the best overall
drying capacity and overall humidity performance is for CLT with internal insulation and a vapour
resistive layer placed on the outside of the CLT. Reverse assemblies or assemblies in other build-ups
showed critical levels of moisture accumulation. These preliminary findings, however, will be expanded
and verified in future research to assess the impact of additional variables such as driving rain infiltrating
through the cladding and into the assembly, additional insulation types such as rockwool, membrane
selections, ventilation system types, additional climates zones and building typologies. Comprehensive
simulation coupled with sample testing of optimised construction details will lead to the identification of
CLT construction solutions that are suitable for net zero energy buildings in tropical and subtropical
Australian climates.
Keywords: CLT, Hygrothermal performance, Timber, WUFI, Moisture
eZED Limited. (2017, November 27). CLT Hygrothermal Review.
Li, J., Rismanchi, B., & Ngo, T. (2019). Feasibility study to estimate the environmental benefits of utilising timber to construct
high-rise buildings in Australia. Building and Environment, 147, 108–120.
Timber Architecture & Constructions News - Edition 1 - 2019. (n.d.). Retrieved February 26, 2019, from
ResearchGate has not been able to resolve any citations for this publication.
Reinforced concrete has played a significant role in the construction industry and is one of the most popular construction materials. However, different studies suggest that reinforced concrete is not environmentally friendly with a significant adverse environmental impact during its production, usage and end of life. Therefore, a more sustainable material that could perform as well as reinforced concrete can overcome this limitation. During the past decade, a number of studies on timber construction have shown the potential to replace concrete with timber in parts of a building without compromising the resilience of the structure. Most of them focused on greenhouse gas emissions rather than embodied energy and structural requirements. Therefore, this research has focused on the feasibility of timber and its potential benefits as a construction material in parts of a high-rise building located in Australia. The potential benefits and limitations of utilising timber to construct a high-rise building in Australia were investigated. For this purpose, a hypothetical 43-storey building is considered, to make it comparable with the existing studies in the literature; the baseline model is designed according to the study by Kuilen et al. (2011). Three scenarios were considered with different proportion of timber. Parametric studies were subsequently conducted on the effects of materials, size and shape of the structural elements on the performance of the building. It was found that in the selected site, using timber to construct internal parts of high-rise buildings would provide the best solution in terms of structural and environmental benefits.