How is the waterproof and breathable membrane breathable?

1. Physical: Hydrophobic (afraid of water) is waterproof, but vapor permeable - the pores of a microporous coating or laminate are so small that liquid water cannot pass through. However, vapor water molecules are many times smaller than liquid molecules and can pass through these "micropores".
2. Chemicals: Hydrophilic/non-porous laminates or coatings move moisture by chemical diffusion. Water molecules are positively charged, and hydrophilic PU is negatively charged, attracting water through the intermolecular gap of PU. Because ionic bonds are relatively weak, water is pushed through these gaps until the water vapor has passed completely.
3. Differential pressure is the difference in heat and humidity levels between one side of a material and the other. According to the second law of thermodynamics, moisture will tend to dry out. Thus, the warm, moist air will flow towards the cold, dry air until an equilibrium is reached. There is almost always a high level of heat and humidity inside the clothing system due to body heat and moisture. This creates a pressure differential that forces heat and moisture outward. The greater the difference between the heat and humidity inside and outside the laundry system, the greater the pressure differential that will force the heat and humidity out.