Measurement of the water vapour permeability of earth plasters using small-scale wind tunnels under variable air flow regimes

• Published in: Materials and structures Vol. 55; no. 4
• Main Authors: McGregor, Fionn, Mauffré, T., Force, M.-S., Contraires, E., Fabbri, A.
• Format: Journal Article
• Language: English
• Published: Springer Verlag 01.05.2022
• Subjects: Civil Engineering; Engineering Sciences; Earthen materials Water vapour permeability Small low speed wind tunnels Surface resistance factor Wet-cup experiments Earth plasters; Earthen materials; Water vapour permeability; Surface resistance factor; Earth plasters; Small low speed wind tunnels; Wet-cup experiments
• DOI: 10.1617/s11527-022-01950-8
• ISSN: 1359-5997

Abstract To determine the water vapour permeability of porous building materials, the wet cup and dry cup tests are frequently performed. Those tests have shown to present high discrepancy. The water vapour permeability of building materials is an essential parameter to determine the hygrothermal behaviour of the material and its impact on indoor comfort. Several previous studies have aimed to improve the reproducibility of the tests, by improving the protocol, the analysis of the results, notably by taking into account the surface film resistance. Yet, it is commonly accepted with no evidence that this surface film resistance can be neglected for an air velocity above 2 m/s over the cup. This study aims at experimentally testing the influence of either the flow regime or the flow velocity on the robustness of the measured water vapour permeability. For this purpose, two mini wind tunnels were designed to produce a laminar or a turbulent flow above the cups with variable air velocity. Water vapour permeability tests were performed in the tunnels with varying air velocity and flow regime on earth plasters with different compositions. The results have shown that regardless of the air velocity and flow regime, the surface film resistance should not be neglected. Based on the presented results, to reach an optimal repeatability, the use of wind tunnels should be considered as they allow to precisely control the air flow above the samples.