In the face of current environmental challenges and the ongoing increase in energy consumption, the use of locally available bio-based materials emerges as a key solution to achieve environmental objectives [1]. The transportation of these materials constitutes a significant portion of their environmental impact [2], making their local availability essential to reduce the environmental impact associated with transportation. Plant aggregates stand out due to their high hygroscopicity, allowing them to absorb significant amounts of water. However, this adsorption phenomenon within porous networks can generate internal stresses on the solid skeleton, leading to volumetric deformations. Moreover, adsorption/desorption cycles can negatively affect the structural integrity of materials, causing cracking, degradation, or plastic deformations. These interactions also depend on the size and shape of individual pores, generating pore-scale forces that impact the overall adsorption mechanisms. Theoretical studies, such as those addressing the impact of pore size distribution on deformation induced by sorption in porous materials [3], have helped to better understand these mechanisms. This experimental work fits into this framework by exploring the interactions between the microstructure and the hygroscopic performance of bio-based materials. It specifically focuses on miscanthus, a resource available in the Hauts-de-France region, and aims to fill the gaps identified in the literature. The study relies on a thorough analysis of the microstructure of miscanthus aggregates using high-resolution tomography and characterization of their hygroscopic properties. The results allow to understand the interaction phenomena between microstructure and hygrothermal properties, focusing especially the poral network of the skeleton of bio-based miscanthus building materials.