Porous carbon fiber materials are used as effective insulators in many applications where high temperatures are involved. In particular, they are used as the substrate of ablative thermal protection materials for atmospheric entry systems. In this application and in many other industrial uses, quantifying the permeability of porous materials is needed to compute the flow rate of gases through them, under certain environmental conditions. In this work, direct simulation Monte Carlo (DSMC) simulations are used to compute permeability of several fibrous substrates to high temperature gases. The actual porous geometry of the materials is digitized using X-ray microtomography. Numerical results at various pressures and Knudsen numbers are compared with experimental data published in the literature. The method confirms that the pressure dependence of effective gas permeability is well represented by the Klinkenberg formulation. The method is validated by showing close agreement between measurements of permeability from simulations and experimental investigations. Four carbon fiber materials with different microstructures are investigated. We show that the permeability strongly depends on the pore size distribution, as well as on the porosity of the material. (C) 2016 Elsevier Ltd. All rights reserved.