This study investigates the improvement of thermal and hygric performances of cement mortars through the introduction of micronized miscanthus fibers. Micronized fibers were chosen because they are expected to promote fine and homogeneous fiber distribution within the cementitious matrix, hence opening possibilities for the design of 3D printable mortar mixes including vegetal fibers. An experimental protocol was first developed for preparing five mortar mixes with miscanthus fiber contents up to 7 wt.%. These bio-based mortars were then characterized at the age of 28 days, in terms of mechanical strength (under flexural and compression tests), thermophysical properties (determination of the thermal conductivity/diffusivity and the volumetric Heat Capacity by the Hot-Disk method), and in terms of hygric properties as well (evaluation of the Moisture Buffer Value MBV according to the Nordtest method). The results of this experimental campaign showed that increasing the miscanthus fiber content leads to large improvements in both thermal resistance (up to 87%) and moisture buffer capacity of mortars (with MBV values up to 2.05), suggesting that such bio-based mortar is a good insulating material and has an excellent ability to mitigate external moisture variations. On the other hand, the introduction of vegetal fibers was also found to decrease very significantly the mechanical strength of the modified mortars, making these latter incompatibles with structural applications. Nevertheless, the developed bio-based mortars retain sufficient mechanical properties for handling and are suitable for building insulation.