What Does Waterproof Breathable Mean? 

"Waterproof Breathable"

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Is that Waterproof AND Breathable or is it Waterproof OR Breathable?

Gore-Tex, Neo Shell, eVent and other waterproof membranes are funny things. For many people it's the layer they reach for anytime the weather isn't perfect. The thing is that those waterproof breathable membranes just don't seem to breathe very well when it's wet out and as far as I can tell, nothing does. The problem is physics.

Okay, so I talked about membranes, what are those? There's two classes of waterproof jacket out there - membranes and coatings. Membranes work by bonding a membrane to an outer material (and often another layer or two) while coatings are just that - something basically slathered onto the outer material with maybe a protective layer or raised profile on the inside. Membranes - like Gore-Tex and eVent tend to breathe better, but the jackets also tend to be heavier, less packable and more expensive. Coatings - like Marmot's NanoPro or Pertex Shield+ are lighter, less expensive, but less breathable and often less durable that their membrane counterparts.

The Waterproof Breathable Problem

Waterproof breathable membranes and coatings work on the principle of diffusion. Diffusion needs a difference in humidity on each side of the membrane in order for your jacket to accomplish anything. The bigger the difference, the higher the rate of diffusion. When you are working hard, it's wet inside your jacket, so it's best at moving that sweat to outside your jacket when it's dry out. 

Pissing rain means it's pretty damn humid out - which means the driving pressure for diffusion is low or straight up zero which means maybe your jacket is waterproof, but it just isn't very breathable. If you are sweating while it's pouring raining, you're getting wet. If the humidity outside your jacket is the same as the humidity inside your jacket - then there's just nowhere for your sweat to go, so unless there's some crazy voodoo going on that I don't know about, I don't know how a breathable jacket could keep you dry while you are working hard in a humid environment. It's certainly what I've experienced during my time in the super wet coast mountains of BC. 

If you want more info on how waterproof breathable membranes and coatings work, REI has a really good article on it.

What it really comes down to, is if it's raining out, you have three realistic options: 

1. Say screw it to a shell, get wet from rain. Works when it's warm.
2. Wear a waterproof shell, work hard and get wet from sweat which is generally warmer than getting wet from the rain, but you freeze when you stop.
3. Wear a waterproof shell , realize that it's going to breathe like shit, even if you have the fanciest thing out there because the humidity inside and outside the jacket are so similar and restrict yourself to only going as fast as you can go without seriously sweating. If you start getting wet, slow down. 

Notice that there's no solution for 'stay dry while working hard in the rain'. There just isn't anything out there that can do that at this point.