A fabric that stops rain from getting in but allows perspiration out may sound impossible, but the principle is quite simple. Raindrops are made up of many molecules of water, while sweat is in the form of vapour, which is usually a single molecule. This evaporates from the surface of your skin, through a base layer then out through the holes in the fabric which are big enough to let the vapour out, but too small to let raindrops in. Of course, the technology required to create such precisely engineered fabric is far from simple. Breathability is reduced if there is a lot of water clinging to the outside of the fabric, so their water repellent finish makes the water run off the outside of the fabric so the vapour can escape ... More info
