<p>This study aims to clarify the respective roles of porosity and fiber orientation in controlling the effective thermal conductivity of fibrous bio-based materials, with emphasis on compression-induced structural evolution. Systematic measurements were performed on model cellulose fiber assemblies over a wide range of porosities in the dry state, using steady-state thermal conductivity measurements in axial and transverse configurations. The results were interpreted using simple effective-medium concepts and a refined structural model accounting for partial solid connectivity. The approach was further tested on hemp fibers. The measurements reveal a pronounced anisotropy induced by compression, with thermal conductivity increasing moderately with density in the axial direction and more strongly in the transverse direction. Classical bounds and isotropic effective-medium models do not capture this behavior. In contrast, a simplified layered representation, combined with a refined structural description, reproduces the observed trends and highlights the dominant role of compression-induced fiber reorientation. The results show that the effective thermal conductivity of fibrous cellulosic bio-based materials is primarily controlled by structural organization rather than intrinsic material properties. The proposed framework provides a physically grounded interpretation of the variability reported in the literature and offers guidance for optimizing thermal performance through control of processing-induced microstructure.</p>