The major objective of this study is the determination of the physical and mechanical properties of building materials lightened by vegetable fibers, used as heat insulator. An experimental work was set up in order to include/understand and to evaluate the thermophysical and mechanical characteristics of these materials according to the water content. The experimental results show a significant decrease in the material density and in the thermal parameters with the increase in the percentage of vegetable fibers in the concrete. The results also show the effect of humidity on the thermal conductivity. The composite material studied offers interesting thermal insulation, and consequently an optimal gain of energy in comparison to common construction materials. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.