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

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.

Automated summary

The water vapour permeability of porous building materials is an important parameter to determine the hygrothermal behaviour and indoor comfort. However, there is a high discrepancy between the wet cup and dry cup tests frequently used. Previous studies aimed to improve the reproducibility of the tests, but the commonly accepted belief that surface film resistance can be neglected above 2 m/s airflow has no evidence. This study experimentally tested the influence of flow regime and velocity, finding that surface film resistance should not be neglected regardless of airflow conditions. Wind tunnels should be considered for optimal repeatability.