Laboratory Measurement and Boundary Conditions for the Water Vapour Resistivity Properties of Typical Australian Impermeable and Smart Pliable Membranes

Abstract

The duo of better insulated and more air-tight envelopes without appropriate consideration of water vapour diffusion and envelope moisture management has often demonstrated an increased potential of moisture accumulation, interstitial condensation, and mould growth within the building envelope. To inform a resilient, energy efficient, and healthy building design, long-term transient hygrothermal modelling are required. Since 2008, concern has been raised to the Australian building regulators regarding the need to establish the vapour diffusion properties of construction materials, in order to develop a hygrothermal regulatory framework. This paper discusses the results from laboratory testing of the vapour diffusion properties of two common reflective pliable membranes, and one smart pliable membrane. The two reflective pliable membranes are often used within the exterior walls of Australian buildings. The smart pliable membrane is a relatively new, internationally available product. The three membranes were tested as per ISO 12,572 at 23 °C and 50% RH. To establish if the vapour resistivity properties were constant, under different relative humidity conditions, the membranes were further tested at 23 °C and relative humidity values of 35%, 65%, and 80%. The results of the three pliable membranes show that the vapour resistivity properties varied in a non-linear (dynamic) manner subject to relative humidity. In conclusion, this research demonstrates that regardless of the class, each of the tested membrane types behaved differently under varying relative humidity and pressure gradients within the testing laboratory.

Measuring water vapour permeability using remote-reading humidity sensors

“…These values are typical of conditions within construction elements and adjacent spaces. Other humidistats can be selected to suit the needs of the user [31]. All tests were run at 25 ± 0.2 • C. Test samples were conditioned at the Outer Box RH in order that the sorbed water content should be the same at the beginning and the end of a test run.…”

Hygrothermal Modelling of the Differences between Single versus Variable Relative Humidity Vapour Diffusion Resistivity Properties of Pliable Membranes

“…To establish the likelihood of moisture accumulation and mould growth, hygrothermal and bio-hygrothermal simulation tools are used to inform the design and construction processes [1][2][3][4]. Pliable building membranes are used to prevent the presence of unmanaged and excess indoor moisture from water vapour, which can lead to moisture accumulation, support mould growth, and affect the durability of a building [5,6]. Apart from this, unplanned moisture accumulation within the built fabric of an insulated building can significantly reduce envelope thermal efficiency, which is an important issue when considering the world's collective goal of reducing carbon emission from the operation of buildings [7,8].…”

“…Pliable building membranes are used to prevent the presence of unmanaged and excess indoor moisture from water vapour, which can lead to moisture accumulation, support mould growth, and affect the durability of a building [5,6]. Apart from this, unplanned moisture accumulation within the built fabric of an insulated building can significantly reduce envelope thermal efficiency, which is an important issue when considering the world's collective goal of reducing carbon emission from the operation of buildings [7,8]. In addition to building durability, the issue of unmanaged moisture in buildings has been identified as a significant health risk to building occupants, where mould and other pathogens thrive, which are detrimental to human health [9].…”

“…Consequently, more attention is being focused on the published physical properties of materials that are included in the hygrothermal software data libraries. Some studies have critiqued the methodology for sourcing input data and provided new innovative methods for establishing some critical hygrothermal properties such as water vapour diffusion resistivity of construction materials [6,16,29,30].…”

“…That research paper recommended that the quantification of construction material water vapour diffusion properties should be completed for various agreed steps between 30% and 80% relative humidity, the measured gravimetric values undergo harmonic mean adjustment, and the muti-point values be applied to the material physical properties within hygrothermal simulation tools. However, the previous paper did not include the results of the differences that may occur when the hygrothermal simulation uses either a single point or multipoint water vapour diffusion values [1,6]. This is an important matter, as all simulation results are the sum of their inputs, and if the inputs do not correctly reflect the true physical situation, the simulation is likely to provide inadequate or even incorrect, guidance.…”