Water-vapour permeability of inorganic construction materials

Christopher Hall, Gloria J. Lo, Andrea Hamilton

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Abstract

Water vapour permeability (WVP) data on brick, stone, plaster and cement-based materials from some seventy publications are reviewed and assessed. Almost all sources use standard cup-test methods or close variants. Comparisons of WVP values from different sources on similar materials confirm that reproducibility between different laboratories is poor. Some deficiencies of cup-test methods are discussed, including uncertainties arising from the use of saturated-salt humidistats and desiccants. There is evidence that the water vapour resistance factor decreases as volume-fraction porosity increases, and data support a simple porosity–tortuosity relation. Data also show that the resistance factor decreases with increasing mean relative humidity across the test specimen, with the wet-cup value consistently lower than the dry-cup values for a given material. The contribution of liquid film flow to mass transfer is discussed. It is shown how film thickness and film-flow permeability may be estimated from the water-vapour sorption isotherm; and a related regression equation is developed It is concluded that available data are inadequate to establish the fundamental physics of WVP; vapour-only permeability data for engineering purposes should be obtained in dry-cup tests at low humidity; and research studies should aim to integrate the WVP into the framework of unsaturated flow theory.
Original languageEnglish
Article number39
Number of pages20
JournalMaterials and Structures
Volume57
Issue number2
DOIs
Publication statusPublished - 26 Feb 2024

Keywords

  • water vapour permeability
  • water vapour diffusivity
  • water vapour resistance factor
  • cup test
  • humidity
  • porosity
  • film flow
  • sorption isotherm
  • Schirmer equation

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