Abstract: The invention concerns a method for treating a porous material based on cellulose and/or lignin which consists in impregnating said porous material with at least a monomer comprising at least an irradiation-polymerisable vinyl group, then in irradiating the impregnated material with high-energy ionising radiation. The impregnating monomer comprises a grafting monomer having at least a function capable of reacting with the hydroxyl groups of the cellulose and/or the lignin and/or at least a cross-linking monomer, and the impregnation is carried out under conditions suitable for limiting the thickness of the impregnation layer to a thickness of 0.1 to 5 mm beneath the surface of the porous material. The invention is applicable to a material whereof the moisture impermeability coefficient does not exceed 55.