This work investigates both the moisture dependence of thermal properties and the hygric behavior of cement mortars incorporating different proportions of micronized miscanthus fibers up to ~7 wt%. The experimental program encompasses thermal characterization at dry (10 % RH), moderate (50 % RH), and saturated (100 % RH) states of samples, along with hygric characterization of the various mortar specimens through sorption-desorption tests, water vapor permeability assessment, Moisture Buffer value (MBV) determination, and capillary absorption tests. Thermal measurements showed a significant decrease in thermal conductivity with the addition of fibers. For a given biobased mortar composition, conductivity values were almost identical at dry/moderate RH level but exhibited an increase at saturation. This shift was attributed to the fibers' absorptive properties, which lead to a higher water content within the samples in saturated humidity environments. Collectively, the moisture sorption, moisture buffering capacities, water vapor permeability, and capillary absorption properties demonstrated consistent enhancement with rising fiber content, confirming the significant impact of plant fibers on the material's hygrothermal properties. In addition, the GAB model (Guggenheim-Andersonde Boer) was used to fit the sorption and desorption isotherms, yielding a good correspondence with experimental data. Finally, mortars with the higher fiber contents (M7.5 F and M10F with 5.70 wt% and 6.94 wt% of fibers, respectively) combined high hygroscopicity and low thermal conductivity values (even under moisture-saturated conditions), making them promising candidates for applications requiring both good hygric performance and effective insulation properties.