Following the analogy between the laws of heat conduction and vapor diffusion, two theoretical expressions for the thermal conductivity of a composite medium are applied to the water vapor permeability coefficient of certain porous media. It is shown that both expressions reduce to a form very similar to the empirical relationships found by Penman and Edenholm for soils, glass spheres, charcoal, and cellular concrete. The calculation of the variation of water vapor permeability with density for a cellular lightweight concrete is illustrated.