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The paper presents the results of an experimental investigation of air pressure drop while flowing through wheat straw beds. According to Darcy's law, the smaller the porosity of the bed, the bigger the pressure drop. The investigation was conducted using three different porosities (or three bed densities), and for two different air flow rates. After determining porosity (which is directly measur-able), the permeability of straw could be found. For high flow velocities, such as the velocity of air flowing through a straw bale, the Forchheimer equation be-comes more relevant as a correction of Darcy's law with inertial effects included. Otherwise, the permeability tensor depends only on the geometry of the porous medium. When permeability is established, the Forchheimer equation coefficients can be easily estimated. These results may be important for the future develop-ment of efficient biomass combustion facilities. The measurement methods and facility characteristics are described in more detail.