Effect of Rendering on Moisture Buffering of Hemp Concrete
Résumé
In a context of sustainable development, the choice of materials for building envelop shall take into account the environmental impact of the material, the energy consumption of building (for heating, cooling), the indoor environmental quality (safety and comfort), etc... The so-called "environmentally friendly" materials are more and more used. Hemp concrete is told to have a low impact on environment as it is made of hemp shiv (renewable raw) and of lime (lower embedded energy than cement). This material is not used as load-bearing due to low mechanical properties. Hemp concrete is thus mainly used as filling material associated to a wooden structure. On hygrothermal point of view, hemp concrete shows low thermal conductivity, about 0.1 W/m/K, and can be used without added insulation layer in buildings. The hydric characterization of hemp concrete shows high transfer and storage capacities. Numerical studies have shown that the transient hygrothermal behavior allows reducing energy demand of building while maintaining indoor relative humidity. However, these studies are based on experimental data of sorption curve and water vapour permeability; it was shown that the uncertainty on this kind of data hampers reliable simulations of hygroscopic buffering. Thus, this study deals with hydric characterization of hemp concrete under dynamic conditions looking for more realistic characterization. The experimental method is based on the NORDTEST protocol where specimens are exposed to daily cyclic variation of relative humidity. This measure leads to moisture buffer value. The study is held, firstly, on uncoated specimens of hemp concrete and then on coated ones. Six specimens are tested. Only one side is exposed to the ambient condition while the other sides are sealed with aluminum tape. Their thickness is chosen to be higher than the predicted penetration depth. The steady state is reached after two days and the measures are performed during five steady cycles. The mean value of moisture buffer value of uncoated hemp concrete is 2.14 g/(m².%RH). This induces that hemp concrete is an excellent hydric regulator according to the MBV classification given in the NORDTEST project. By comparison, the moisture buffer value given in the NORDTEST project are : 1.04 g/(m².%RH) for cellular concrete (good hydric regulator), 0.64 g/(m².%RH) for gypsum (moderate hydric regulator) and 0.38 g/(m².%RH) for concrete (limited hydric regulator). The coating reduces moisture buffer value of hemp concrete. For closed coating (sand-lime coating), the value is nearly half the previous one (1.08 g/(m².%RH)). For open coating (hemp-lime coating), the value is much less reduced (1.82 g/(m².%RH)). This capacity would allow energy saving while maintaining adequate air quality and hygrothermal comfort. Also, this experimental method is very relevant to characterize dynamic behavior of building material. The experimental data set can be used for benchmarking buffering models.