Water vapour permeability, often expressed as evaporation resistance Ret, belongs to the most important thermal comfort parameters of protective, sport and other functional clothing (garments) and related textile fabrics. For outdoor applications, these garments should exhibit the so-called semi-permeability: the textile laminates creating the outside garment fabrics contain micro- or nano-porous membranes, which prevent the penetration of outside liquid water into the laminates, but simultaneously allow the passage of water vapour from the body through the whole fabric system. In nano-porous membranes, when entering the laminate, water vapor molecules first condensate into single water molecules, which then travel through the macro-molecular structure of the polymer fibres. The more water molecules are present in the amorphous phase of the polymer, the higher is the flux of moisture. The average moisture level inside the fibre structure is then proportional to the moisture level on both sides (surfaces) of the nano-porous membranes. The moisture level is in this case expressed (proportional) to the level of water vapour partial pressure on both surfaces of the nano-porous membrane. 
In the study, a new observation based on the requirements of the highest level of water vapour partial pressure on the membrane surface is presented.