RILEM TC 275-HDB: results of round-robin testing for the vapor permeability of hemp concrete

Abstract Determining the hygrothermal properties of bio-based construction products on a material scale is a major challenge in order to highlight and guarantee their performances. These data are also essential to feed models at wall and building scales for prediction and optimization of their contribution to energy consumption and user’s comfort. Among these properties, water vapor permeability is commonly measured with the dry cup method. Despite the good repeatability and reproducibility of this test when performed in one lab, one can observe a high level of discrepancy in the literature values. To address the origin of these differences and establish recommendations for the characterization of highly hygroscopic and permeable bio-based materials, an interlaboratory test is therefore necessary. In the context of the activities of the RILEM TC 275-HDB, two interlaboratory campaigns have been launched to measure the water vapor permeability of bio-aggregate based building materials by means of cup test. The analysis of the results from six participating laboratories from different countries highlighted the main sources of discrepancy, in particular the progressive saturation of the desiccant during the experiment and the incompatibility of the “steady state” criterion and the air velocity recommended by the standard ISO 12572 in the case of highly permeable materials. Despite the many sources of uncertainties, recommendations are proposed to improve the accuracy of this measurement for permeable materials such as a continuous measurement of the vapor pressure within the air layer and the measurement of the air velocity above each cup to evaluate the local interface resistance.