Characterization of a precast hemp concrete block. Part II: Hygric properties

International audienceThe hygroscopic, insulating and agronomic properties of hemp concrete make it a promising building material for addressing the sustainable building challenge. Part I of this series of two papers described a characterization of the thermal properties of a prefabricated hemp concrete block, confronting the impact of different methods of measurement. Part II focuses on the hygric properties, which describe moisture transfer and storage. The sorption isotherm was measured at 23 °C and 45 °C, considering distinct experimental procedures (saturation salt solution (SSS) methods with single relative humidity step or stepwise equilibrium, and Dynamic Vapour Sorption (DVS)). Differences were observed in the results and hypotheses are formulated considering the distinct drying procedures between SSS and DVS methods or the specific water vapour sorption process on cellulose-based materials. With the measurements at two temperatures, the isosteric heat of sorption is evaluated which allows an evaluation of the sorption at any temperature. The water vapour permeability was measured with dry and wet cups. Different parameters were tested: thickness (5 and 8 cm), air velocity (0.5 and 2 m s−1) and calculation method (standard calculations, consideration of the water vapour interface resistances). Each parameter, with the chosen values, had an impact of around 10% on the final water vapour permeability calculated. The capillary absorption coefficient was evaluated through capillary uptake measurements, leading to an estimation of the liquid transfer coefficient, albeit 12 times lower than the one evaluated thanks to the dry and wet water vapour permeabilities. All these observations underline the significant impact of experimental parameters and methods on the determination of hemp concrete hygric properties and support the necessity to develop more specific measurement methods for such permeable and hygroscopic bio-based building materials