Moisture content influence on the thermal conductivity of insulating building materials made from date palm fibers mesh

•Thermal conductivity and density of DPF-B and composite increase with water content.•Lightening the mortar by DPF mesh increases its thermal insulation capacity.•DPF-B and composite MDPF51% could be used as insulating materials in buildings.•Measured DPF-B and composite thermal conductivities are best predicted by two models. Vegetal fibers used alone or as additive in composite construction materials bear a growing interest owing to their several advantages such as their lightening and insulation effects. However, they are very hydrophilic. An experimental study was carried out on the water absorption and effect of humidity on thermal conductivity and density of binderless board of date palm fibers (DPF) mesh and a composite based on mortar reinforced with different percentage of DPF mesh varying from 0% to 51%. The obtained results show that the thermal conductivity and density of binderless board and composite increase significantly with volumetric water content. The incorporation of DPF mesh in the mortar has a positive effect, regardless of the water content of the composite, as it lowers its density (lightening effect) and decreases its thermal conductivity (insulation effect) owing to the increase of its total porosity. The addition of DPF to mortar lowers its water retention capacity related to the open porosity which is a positive effect when considering that trapped water increases the thermal conductivity of the insulating materials. The experimental findings on the evolution of thermal conductivity with water content of DPF mesh boards and MDPF composites were fitted to the theoretical models available in the literature. The best fitting models were used to identify the values of the intrinsic thermal conductivities of DPF and mortar.