The stabilization of a mesoporous structure formed during the deformation of isotactic polypropylene (PP) in physically active liquid media (PALM) via the crazing mechanism is studied. The influence of PALM, tensile strain, and annealing conditions on the volume porosity and vapor permeability were determined. It was shown that annealing in the presence of PALM is an effective way to obtain PP matrices of nanocomposites with a porosity of about 30% and “breathing” materials with vapor permeability of up to 890 g m−2 day−1 in combination with good waterproofing properties.