When my girlfriend was getting together gear for an Outward Bound instructor training course in the Pacific Northwest, she had a tough time with one item on the gear list: “waterproof jacket.” Every time she asked someone at a gear store or an online retailer, “Is this jacket waterproof?” they talked for minutes without ever saying “yes.” I told her if she wanted something “waterproof,” she should get one of those jackets like the guys on The Deadliest Catch wear.

But, if you’re moving in anything but the chilly ocean waters off the Alaskan coasts, that Deadliest Catch rain gear is going to get pretty hot, and you’ll be soaked in sweat in minutes, and when you stop, you might be looking at a fun case of hypothermia.

Is there a truly “breathable waterproof” material out there? No. “Waterproof” is your Nalgene waterbottle. Going for a jog with your shirt off is “breathable.” Making a rain jacket with both these qualities is a pretty tall order. Measuring it is also complicated, and finding out how your rain jacket measures up definitely takes some digging.

Your rain gear’s breathability is often measured in RET, which stands for “Resistance to Evaporative Transfer” or “Resistance of Evaporation of a Textile,” depending on who you ask. It can also be referred to as MVT (Moisture Vapor Transmission) or MVTR (Moisture Vapor Transmission Rate), but it all means the same thing: How much am I going to sweat when I’m wearing this rain jacket? The lower the RET or MVTR, the less the fabric resists evaporation – sweat can more easily evaporate and move to the outside of the fabric.

If you ask a gear shop employee, “How breathable is this?” You'll probably get a pretty nebulous answer. There's a reason for that: Unless they work in a testing lab themselves, they probably don't know. Try finding some RET figures yourself. Poke around the Internet, and you won't find much hard information anywhere. Retailers put things like “guaranteed waterproof” or “guaranteed breathable” on their web sites. Gore-Tex and eVent, the two bigger players in the waterproof-breathable front, don’t even put RET ratings on their web sites. Here are a few figures you can find though, with some highly motivated search engine detective work:

• Gore-Tex Active Shell: 3
• Gore-Tex Performance Shell: <13


• Gore-Tex Pro Shell: <6
• Gore-Tex Paclite: <4


• Marmot MemBrain 6
• Marmot PreCip: 6.5
• Marmot Membrain 10: 6.5-7


• Marmot MemBrain Strata: 6-6.5


• eVent 2-layer: 2.7
• eVent 3-layer: 4.5

Why can't retailers just tell us a number – Jacket X has a rating of 3.5, and Jacket Y has a rating of 6, and so on? Well, for a couple of reasons:

1. The testing methods don't accurately replicate human sweating during physical exertion: one common test, ASTM F1868 – 09, uses a "sweating hot plate," but the designers of the test themselves say: “While a possible indicator of clothing performance, measurements produced by the testing of fabrics have no proven correlation to the performance of clothing systems worn by people. Clothing weight, drape, tightness of fit, and so forth, can minimize or even neutralize the apparent differences between fabrics or fabric assemblies measured by this test method.”

Another test, ASTM F2370 – 10, uses a sweating mannequin and comes with a few caveats, one of which is: “The evaporative resistance values obtained apply only to the particular ensembles evaluated and for the specified environmental conditions of each test, particularly with respect to air movement and sweating simulations." In other words, a stationary mannequin in a room with no breeze isn't going to replicate a human being hiking uphill in a windy rainstorm.

2. Obtaining an accurate figure is tough. Take Patagonia's experience: "When Patagonia first decided to test the breathability of our fabrics, we chose six different fabric labs around the world and sent off a piece of fabric cut from the same bolt to each lab. When the results came back it was both disappointing and enlightening. All six results were substantially different, so much so that you’d have sworn they tested different fabrics."

One study readily available on the Internet comes somewhat close to a comparison of waterproof fabrics: A U.S. Army study published in the Journal of Engineered Fibers and Fabrics in 2009, “Influence of Hydration State on Permeation Testing and Vapor Transport Properties of Protective Clothing Layers.”

Lots of folks in the outdoor industry blogging world latched on to that study, pulling one graph out of it and saying that it showed that eVent performed better than Gore-Tex or any other fabric. It did, but what the study actually showed was that some fabrics' breathability increased when they were “well hydrated.” The study concluded that “standard test methods that only use a single humidity condition to evaluate or rank materials may completely reverse material rankings depending on whether the test method produces low or high mean relative humidity test conditions,” and that “a polymer membrane swollen with water is usually much more permeable to vapor than a dry polymer membrane.”

Rating a jacket’s breathability isn’t as simple as, say, figuring out how much force a carabiner can withstand before it breaks. Explaining it to consumers is a bit like explaining love to a 4-year-old: It’s complicated.

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