Fibrous materials are structures whose complexity depends on the size and geometry of the fibres and on their arrangement induced by the manufacturing process. We interest here in the permeability for air of wood-based fibrous materials with high porosity on which experimental measurements are difficult to be implemented without structural modifications. In this study, we show the usefulness of 3D X-ray tomography imaging in both experimental and numerical permeability measurements. Image analysis tools derived from mathematical morphology are used to access quantitative structural parameters of the interconnected pores (porosities, pore size distributions and correlation lengths), as well as needed information on the experimental samples (cross-section area and length). A specific experimental setup and methodology is proposed to assess the permeability derived from Darcy's law. Results are then compared to direct numerical simulation of Stokes flow carried out on 3D representative volumes of the fibrous materials.