Waterproof breathable fabric - what it really means

Imagine this (which happened to me): You're in the mountains enjoying glacial travel by sky with a number of your friends. You're wearing your new three-ply Gore-Tex jacket and feeling great. The effort you are giving to enjoy this experience requires you to unzip your jacket to cool down and once it is open...you see a frosting on the inside of the jacket.

What the...

Is the fabric defective? No. Let's not go down that path. Outfitters get that all too much simply because breathability is a complicated issue.

What gives? Quite simply, the temperature at the jacket surface is below freezing thus causing your perspiration to freeze.

But if I wear my wind jacket (uncoated), it doesn't frost up in the same situation? This is because uncoated jackets are very breathable in comparison such that the escaping body heat keeps the jacket temperature above freezing. This helps us understand why a waterproof breathable jacket is like wearing insulation, or an additional layer -- it keeps the heat in.

"Why are you telling me this, tell me about exposure to rain." Okay, I'll keep it simple. I use a winter scenario because frost cannot be mistaken as a leak. If we were discussing rain, then I'd end up telling you how to take care of your jacket, and that you can find with a simple web search.

How breathable is a waterproof breathable jacket?

It seems that most people think that a waterproof breathable jacket is as breathable as a non-waterproof jacket. Which is fantastic when compared to a waterproof non-breathable jacket. The reality is that waterproof breathable jackets are really much closer to waterproof non-breathable end of the spectrum than we realize.

Get ready to get your socks knocked off.

The level of breathability for waterproof breathable fabric

Check out the breathability of a number of fabrics here: http://www.verber.com/mark/outdoors/gear/breathability.pdf. A very breathable fabric that is waterproof breathes at a rate of 6500 g per meter squared per day. Since 1 gram of water is basically 1 milliliter of water, this is the same as 6500 ml per meter squared per day.

Comparing apples to apples

An average person's body surface area is 1.7 meters squared, or almost twice the area in the data above. Since the jacket doesn't cover the legs and face, lets say the surface area covered is one meter squared. That helps makes the calculation very simple.

Lets convert the 6500 ml per meter squared per day to "per hour" (because we can imagine an outdoor activity lasting 1 hour). 270ml per meter squared per hour. Done.

Referring to "Coated Textiles" By A. K. Sen, John Damewood, a person walking perspires at a rate of 200-500 ml per hour. Well, it seems like for a leisurely walk, this fabric breathes just the right amount. Any vigourous walk might be pushing this fabric. But wait, Gore-Tex XCR breathes at about half of this level, which isn't enough for leisurely walking. What's going on here? It is clever marketing or is this article bogus? (It is one of the two -- you decide.)

One last thing you should know

You've done it yourself (or your mom did it to you when you were a kid--kids just love it), when you raise a bed sheet into the air, it floats down off to the side -- the crazy sheet just does not go where you want it to go. This is because air is not flowing through the sheet but out at the sides.

Now consider this: a bed sheet is very breathable and still, air flows out at the sides of the sheet. I'd even hazard a guess that 99% of the air does not flow through the fabric, but flows out at the sides. What makes us believe that a waterproof breathable fabric could possibly perform better?

What makes a waterproof breathable jacket breath at a desired level is the air flow pumping action of movement. As we move, we are pumping air to flow inside the jacket such that perspiration escapes through the neck, cuffs and bottom edge at a significant level.

Part of breathability is perception. If a waterproof breathable fabric did not have a liner, which is often a mesh or uncoated nylon, we would perceive the moisture between our skin and the fabric. The mesh or uncoated nylon disperses this moisture through capillary action. If you now start thinking about three-ply fabric (which does not have a liner), the inner surface is actually a little furry, which performs the capillary action and takes away the damp feeling.

Conclusion

Waterproof breathable jackets are well designed. The fabric of the jacket itself does contribute to the dry feeling inside, however the breathability is mostly achieved through air flowing through the cuffs, neck and bottom edge of the fabric. (Give some thought to those pit-zips.)

For the record, I own a couple of waterproof breathable jackets

I've more or less hacked away at the credibility of these fabrics, but really, they are an engineering marvel. They work, but not as well as they are marketed.

I do own a couple of waterproof breathable jackets. "Why?", you might ask. Quite frankly, companies have done a great job at designing these jackets. If, however, a company were to sell a nicely styled non-breathable waterproof jacket with an advanced air flow system, I'd be giving that jacket careful consideration.