Waterproof Breathable Fabrics: Fact Or Fiction?
First some history: the original fabric advertised as waterproof and breathable was Gore-Tex, developed by an ex-DuPont chemist called Wilbur Gore back in 1969 after a serendipitous discovery that stretching polytetrafluoroethylene resulted in a material full of tiny holes that were so small they would cause rainwater to bead on the surface rather than soak into the material. Conversely, these tiny holes would enable water vapor (aka sweat) to pass out, thus avoiding the condensation that results from using impermeable barriers like plastic or rubberized canvas, which were the previous options if you wanted garments that would keep rain on the outside and not soak into your clothing.
Gore-Tex no longer uses the original material as it’s toxic and polluting; today, every “waterproof & breathable” garment from every manufacturer uses the same (different) chemical structure including Gore-Tex, which is now a brand rather than a patent-defended unique proprietary fabric. So you can buy “waterproof & breathable” clothing cheaply today provided you’re not captivated by the Gore-Tex label.
There is, however, a problem with the “waterproof & breathable” claim. Yes, the fabric is waterproof (except for certain conditions) and yes, it is breathable — but not at the same time.
The reason for this is basic physics. In order for water vapor (sweat) to transit through the micro pores in the fabric there has to be a differential between the outside of the garment and the warm moist inside that’s next to the human body. Just as the NaCl in salty water poured into a bucket of distilled water will diffuse so as to reach equilibrium, so too does water vapor move toward establishing an equilibrium when passing through a permeable membrane. This means that when the air on the outside of the garment is relatively moisture-free, it’s easy for sweat to transit through the micro pores in the fabric. But when it’s raining, this difference between the moisture on the outside and the moisture on the inside vanishes. With no positive gradient between moist inside and relatively dry outside, perspiration has no ability to transit the tiny holes. Condensation therefore begins to accumulate inside the garment even…