Breathable waterproof fabrics have been around since the 1970s. These technologies, whether using polyurethanes or other substances, work in a similar way, repelling moisture on the outside, while actually attracting moisture (from sweat) on the inside. The inside moisture seeks to find equilibrium from the warm, humid interior to the cooler area outside, where it can evaporate.
Unfortunately, when your body is under greater exertion, you can create more sweat than the material can successfully remove. This causes moisture to build up inside the clothing and can lead to either overheating as your activity continues or chilling once you stop....Athletics