Useless Information Dept. All breathable/water repellent coats rely on the fact that liquid water, because of its high surface tension, cannot form droplets below a certain diameter. Water in vapour form has no such problems. Thus, if it is possible to provide a membrane with a porosity that is small enough to prevent the entry of liquid water droplets, your problem is solved. Providing the membrane was no problem, providing it in a durable form was.
W.L. Gore worked for DuPont, a big manufacturer of fluorocarbons, including PTFE (polytetrafluoroethane). He discovered how to make the stuff porous (the original method involved one guy holding one side of the sheet and standing still and another guy holding the other side and running away from him, stretching the sheet!). DuPont wasn't interested, so Gore left and set up on his own to develop the idea. PTFE is mechanically weak, so it had to be laminated on to a fabric to protect it. This took a long time to achieve, and the early "Gore-tex" fabrics had constant delaminating problems. In addition, the pore size had to be optimised - human perspiration has fatty compounds and can block pores that are too small. This still happens to some degree and the coats should be washed regularly to keep them working at their best.
"Gore-tex" itself is NOT patented, to the best of my knowledge. However, the best laminating techniques probably are. Moreover, it's an expensive technology. There are a number of cheaper alternatives. One of the most popular is to apply a porous polyurethane coating directly to the fabric (generally a woven nylon). This is much cheaper, but it's not as effective and it will crack with repeated folding.
