Biosourced materials derived from agricultural by-products hold significant promise in both enhancing the hygrothermal performance of buildings and reducing their environmental footprint. We investigate the hygrothermal properties of green composites based on date palm fibers and lime. The eco-friendly building composites explored are manufactured using either trunk fiber (surface fiber) or rachi and petiole fiber (wood fiber). Through an experimental approach, for different fiber percentages, the hygrothermal characteristics of the explored composites were experimentally determined in terms of thermal conductivity, diffusivity, porosity, water vapor permeability, and sorption scanning isotherms. The results show that date palm fiber has a positive effect on the thermal properties of the composite material. Indeed, it significantly enhances the insulating capacity of the composites. Water vapor permeability exhibits significant variation depending on the fiber content, with samples containing higher fiber content exhibiting greater permeability. Furthermore, the effective moisture penetration depth (EMPD) model proved effective describing the experimental adsorption scanning isotherm curves. It was observed that the sorption process is significantly influenced by both the type and percentage of fibers. Finally, the observed results demonstrate that date palm fiber and lime composites could offer notable advantages for construction applications and serve as an effective, cost-effective insulation.