An empirical evaluation of the hygrothermal water vapour diffusion resistivity testing methodology for construction materials

The last five decades has seen substantial success in the development of hygrothermal simulation programs to inform design decisions about moisture control in building envelopes. Despite these international developments, Australian buildings are experiencing condensation issues, visible and interstitial mould growth, and poorer occupant health outcomes. This is in sharp contrast to other developed nations where the implementation of hygrothermal assessment has become a key component to regulatory implementation for envelope moisture and mould mitigation. However, Australia’s diverse climate types, from hot and humid to cool temperate, requires climatically specific hygrothermal design responses. Due to Australia’s slow adoption of hygrothermal building physics, an additional challenge is the lack of construction material properties data. Construction material physical properties that have been established in other developed nations includes water vapour diffusion resistivity. However, the water vapour diffusion resistivity values that have been established and used for some years may be either insufficient to provide realistic results or already outdated due to the method of quantification available at the time. Therefore, this research sought to evaluate existing methods and explore more appropriate methods to better establish the dynamics construction material water vapour diffusion resistivity properties for long-term transient hygrothermal calculation.
Transient hygrothermal analysis is the most appropriate method to ascertain the climatic appropriateness of envelope systems to support or mitigate moisture accumulation and mould growth. To complete hygrothermal analysis, requires the water vapour diffusion resistance properties of construction materials. In Australia, requirements to consider risks associated with moisture, moisture accumulation and mould growth that results from water vapour pressure differentials in new buildings are relatively recent (2019). As a result, the water vapour diffusion resistivity properties of most Australian manufactured and imported materials has not been quantified, tested, or published. Early adopters of hygrothermal simulation in Australia commonly use material data from an international material database. However, these types of databases may not accurately represent how Australian construction materials behave in various climate regions. Without empirical testing of these materials, there is the potential that inappropriate design or forensic guidance may be provided. After consulting relevant stakeholders in the construction industry, product manufacturers and state and federal government agencies, it was agreed that a robust hygrothermal database should be created for hygrothermal simulation. It was recommended that this should begin by quantifying the water vapour diffusion resistivity properties of common construction materials in Australia. This research was established within these regulatory and national standards vacuum.
The most appropriate method to source and validate a construction material’s water vapor diffusion resistivity properties is to conduct laboratory measurement. Preliminary studies into appropriate methods indicated that the existing international testing methodology may be outdated and insufficient to provide accurate data for modern transient hygrothermal simulation. This research developed a more appropriate testing methodology to establish the water vapour diffusion resistivity properties of construction materials. As a consequence, this research both questions existing ASTM E96m (ASTM, 2010, 2016; Bomberg, 1989) methodology used in the United States of America and the ISO 12572 (ISO, 2016) used in European union. And even though these standards have been regularly revised, dynamic measurement under varying environmental conditions is yet to be addressed, leading to this research to proposing a new dynamic testing methodology. This exploration fits within the international discussion which has raised concern that the current single point water vapour diffusion resistivity test method may not adequately represent the temperature and relative humidity conditions that each construction material experience within an external envelope of a building, and how individual materials respond these variabilities. This concern identifies the need for more detailed understanding of the water vapour diffusion resistivity properties for construction materials in different relative humidity and temperature conditions.
To answer the hypothesis involved establishing a temperature and relative humidity-controlled test room, (hereafter referred to as the test room), to measure the water vapour diffusion resistance properties of construction materials that better reflect the diversity of different climatic conditions experienced by building envelope components. Five pliable building membranes were tested in this research including vapour permeable, vapour impermeable and variable permeance products. These are typically placed on the outer side of exterior walls in Australia. These pliable building membranes are classified subject to their water vapour diffusion resistance properties within Australian Standard 4200:1 (2016). Both the international and Australian standards apply a single value to water vapour diffusion resistivity value based on the products performance at 23 °C and 50% RH. As the temperature and relative humidity conditions within walls may only occasionally be at 23 °C and 50% RH and to establish if materials have a singular or variable water vapour diffusion resistivity value, the membranes were tested whilst the test room was maintained under four different environmental conditions, namely at 23 °C and relative humidity values of 35%, 50%, 65%, and 80%. If the results demonstrated a non-singular water vapour diffusion resistivity value, these could be used to provide a more realistic, and accurate, high quality transient hygrothermal simulation.
Four key findings are discussed from the results:
1. All five of the pliable building membrane products evaluated in this research, regardless of their water vapour diffusion resistivity class, behaved in a non-linear manner when exposed to different relative humidity conditions and water vapour pressure gradients within the test room.
2. The hygrothermal boundary curve for these materials was plotted using a new harmonic adjustment method. This new method was adopted as the non-linear results from the gravimetric cup measurements are not enough to determine the effectiveness of a materials’ moisture (or water vapour diffusion resistance) behaviour through its cross-section, which is needed for hygrothermal modelling.
3. The water vapour diffusion resistivity measurement and the harmonic adjustment method are novel, as the single-point value, from material testing at 23 °C and relative humidity value of 50%, has been the standard method to obtain input values for hygrothermal simulation for several decades.
4. Finally, the hygrothermal simulation results showed a significant difference in the moisture and mould growth performance of a typical low-rise residential external wall system when the measured single-point data or the measured and harmonically adjusted multi-point values of water vapour diffusion resistivity values were used. In conclusion, this research demonstrates that the current single-point water vapour diffusion resistivity test method, as described in ASTM E96m (2016) and ISO 12572 (2016) is inadequate
Therefore, the methodology employed in this research is recommended for establishing a multivariable relative humidity water vapour diffusion resistivity properties of construction materials, which better represent the conditions experienced within a building’s external envelope, and for these values to be used as input to long term transient hygrothermal simulation