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The effective thermal conductivity of a porous material is due to both conduction and radiation processes. A theory is presented relating the effective conductivity to the conductivity of the solid material, to the emissivity of the surface of the pores, and to the size, shape, and distribution of the pores. By means of an anisotropic distribution and orientation of pores, materials can be prepared having different thermal conductivities in different directions.
Arthur L. Loeb (1954) studied this question.