Hygrothermal modelling is increasingly used to inform building envelope design. A key input for these calculations is the material’s vapour diffusion properties. Respecting a growing international concern, this research has questioned the appropriateness of the current test method to establish construction material for vapour diffusion properties. This article reports on the empirical measurement of the vapour diffusion properties of two vapour-permeable building membranes commonly used in Australia residential systems when subjected to variable relative humidity conditions. The method involved completing dry cup and wet cup standard tests as specified in ISO 12572, (23 °C and 50% relative humidity RH). Further tests were then conducted as temperature remained at 23 °C but the relative humidity changed to 35%, 65% and 80%, respectively, in order to know if the diffusion properties are the same or change with varying relative humidity. The results from the wet cup and dry cup tests under different relative humidity conditions were non-linear and different. These results indicate vapour-permeable membranes behave differently when exposed to different relative humidity conditions. In conclusion, this research demonstrates that the current vapour resistivity test method is inadequate, hence the need to establish more detailed diffusion resistivity properties in different humidity ranges that represent conditions experienced within a building’s external envelope.
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.
Hygrothermal assessment is essential to the production of healthy and energy efficient buildings. This has given rise to the demand for the development of a hygrothermal laboratory, as input data to hygrothermal modeling tools can only be sourced and validated through appropriate empirical measurements in a laboratory. These data are then used to quantify a building’s dynamic characteristic moisture transport vis-a-vis a much more comprehensive energy performance analysis through simulation. This paper discusses the methods used to establish Australia’s first hygrothermal laboratory for testing the water vapor resistivity properties of construction materials. The approach included establishing a climatically controlled hygrothermal test room with an automatic integrated system which controls heating, cooling, humidifying, and de-humidifying as required. The data acquisition for this hygrothermal test room operates with the installation of environmental sensors connected to specific and responsive programming codes. The room was successfully controlled to deliver a relative humidity of 50% with ±1%RH deviation and at 23 °C temperature with ±1 °C fluctuation during the testing of the water vapor diffusion properties of a pliable membrane common in Australian residential construction. To validate the potential of this testing facility, an independent measurement was also conducted at the Fraunhofer Institute of Building Physics laboratory (IBP) Holzkirchen, Germany for the diffusion properties of the same pliable membrane. The inter-laboratory testing results were subjected to statistical analysis of variance, this indicates that there is no significant difference between the result obtained in both laboratories. In conclusion, this paper demonstrates that a low-cost hygrothermally controlled test room can successfully replace the more expensive climatic chamber.
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