Bio-based materials are increasingly used in construction due to their excellent hygrothermal performance. They help manage energy consumption in buildings by contributing to thermal insulation, which reduces reliance on heating and cooling systems. This study aims to explore the potential of using an alkali-activated binder (AAB) to develop Compacted Bio-based Blocks (CBB) made from flax shives for thermal insulation in buildings. Four AABs were used, combining metakaolin (MK) and granulated blast furnace slag (GBFS) in different proportions (100% MK, 70% MK - 30% GBFS, 30% MK - 70% GBFS and 100% GBFS), and activated by a sodium silicate solution (SiO₂/Na₂O ratio of 1.5 and 35% by mass). These binders enabled the manufacture of two block types, L-BBC and M-BBC, with bulk densities of 360 kg/m3 and 500 kg/m3, respectively. The parameters evaluated included apparent bulk density, thermal conductivity, and moisture buffer value (MBV). The results show that the blocks manufactured have an apparent density of between 360 and 520 kg/m3, a thermal conductivity ranging from 0.11 to 0.13 W/(m.K), and a moisture regulation capacity (MBV) greater than 2 g/(m2.%RH). The hygrothermal properties of the blocks are mainly influenced by the amount of flax shives in the mix. Increasing the Shives/AAB ratio leads to a rise in intergranular porosity, resulting in lower bulk density and thermal conductivity, and improved MBV values. Furthermore, the type of binder had minimal impact on the overall hygrothermal performance of the blocks. In conclusion, bio-based materials with alkali-activated binders exhibit promising hygrothermal properties, making them a viable solution for addressing the energy and environmental challenges in the construction industry.