Understanding Breathability

Or why waterproof-breathable rain jackets get an unfair bad rap for not being breathable enough

The truth is, that when exercising hard, you’ll get significantly wet-from-the-inside wearing ANY wind-resistant jacket or shell—even a light supposedly “breathable” wind shirt! Put the fabric up to your mouth. If you can’t easily breathe through it then it will trap a lot of moisture, even if it is non-waterproof wind shirt.

The problem has more to do with having any wind-resistant fabric covering you than whether it’s a plain “breathable” nylon fabric, or a waterproof-breathable fabric for a rain jacket. And the difference in wetness-from-the-inside for a wind shirt vs. waterproof/breathable rain jacket may be less than you think.

Why any shell increases moisture retention

Any wind-resistant shell, hiking rain jacket or wind shirt, blocks air movement to and from your base layer and the outside environment. This has the following consequences:

• The shell retains heat, trapping warm air against your skin, and making you significantly warmer and sweat more. (In technical terms your shell creates a “boundary layer” of non-moving moist air.)
• Moist air from your skin/base-layer can’t move freely to drier and possibly cooler outside air (as it would if you were just wearing a T-shirt exposed to the outside air). This essentially traps most of your body moisture inside the shell —thus the term wet-from-the inside.

From the above, you can see why even a “breathable,” non-waterproof wind shirt can cause moisture to build up rapidly. And the harder you exercise, the worse it gets. This is supported by a lot of research by the Armed forces clothing test labs and other PhD professionals in the field. And I did some controlled testing with runs in both a wind shirt and a WP/B rain jacket. In an hour the rain jacket only accumulated 20% more sweat on my base-layer.

Bottom line, even a wind shirt is going to trap a lot of moisture when you exercise. Thus rainwear truly goes get an unwarranted bad rap for not being breathable. Or at least people should also start complaining about how unbreathable wind shirts are.

This is the reason I no longer carry a wind shirt. Instead, many times I use a fleece shirt/jacket for warmth in moderate wind. While nowhere near windproof, it does a decent job of blocking enough of the wind to keep me warm. The benefit is that I don’t end up getting sweaty chilled out. Finally, when it does get cold enough and/or windy enough to chill me, I use my rain jacket as a “wind-shell.” By this point it’s cold enough that moisture accumulation in the rain jacket is not a big a deal.