Outerwear manufacturers gratuitously toss around the terms “windproof,” “wind-resistant,” and “breathable.” And yes, every garment registers somewhere between zero and awesome on the wind-resistance and breathability scales. But it can be hard to pin manufacturers down on an actual number rating. 
And usually when you do see a rating, all you can guess is that higher numbers are better, because, to us Americans, bigger is better (duh).
We’d like to shed a little light on this whole ratings thing and how manufacturers come up with those numbers—and what those numbers actually reflect in terms of your comfort.
First, breathability and air permeability (a.k.a. wind resistance) are not the same thing. Breathability represents how much perspiration vapor can escape through a fabric from the inside out, whereas air permeability measures how easily the breeze passes from the outside in. Since these are two totally different properties, it’s possible for a garment to be very air permeable but not very breathable—air could pass right through, while sweat could still bead up inside, giving you a wicked case of the clammies. And the opposite is also possible—for example, a WINDSTOPPER® jacket blocks wind completely (zero air permeability) but will still let perspiration vapor pass through at a decent rate.
BREATHING = SWEAT-THROUGH RATE, WHATEVER THAT MEANS
There’s no standardized test to measure breathability, and one wonders if most manufacturers just use whatever number makes their products look the best. There are two basic tests to measure breathability: MVTR (Moisture Vapor Transfer Rate) tests and Sweating Hot Plate tests. MVTR tests are good for comparing one fabric against another in a controlled laboratory environment. The more complex Sweating Hot Plate test is performed in only a few labs (including GORE-TEX®); its results indicate moisture vapor transfer regardless of climactic conditions.
Breathability is usually indicated in terms of g/m2/24h—i.e., how many grams of moisture can pass through a square meter of fabric in a 24 hour period. Since different manufacturers test their fabrics’ breathability differently, these ratings aren’t of much use unless you’re comparing fabrics within one brand. If a Marmot rain jacket says it rates at 20,000 g/m2/24h and a The North Face rain jacket breathes at 625 g/m2/24h, the Marmot jacket isn’t necessarily 32 times as breathable as The North Face one. They could’ve used totally different tests in totally different lab conditions—so you have to take these numbers with a grain heap of salt.
However, you can compare Marmot jackets to Marmot jackets and The North Face jackets to The North Face jackets, since they probably use the same tests for each of their fabrics. And many different brands use fabrics produced by manufacturers like GORE-TEX®, Schoeller®, or Polartec®, which lends some cross-brand consistency to the playing field.
AIR PERMEABILITY: FEEL THE WIND, OR NOT—YOUR CHOICE
Fortunately, there is an internationally standardized scientific test for air permeability, so if a manufacturer provides a number, you can know across the board what that rating means. Air permeability is rated in CFMs—cubic feet of wind that can pass through a fabric in a minute. Carl Moriarty, Arc’teryx designer, says that “Generally, if a garment lets less than 40-60CFM through, you’re not going to feel the breeze. In a really stiff wind [read: typhoon-ish], you may need a jacket that lets less than 5 or 10 CFM through to be impervious.”
The North Face Habachi Jacket, a wind-resistant softshell fleece
A typical fluffy fleece with no special wind-blocking properties, like a 200-weight Polartec®, will let about 200CFM of wind through. By comparison, a more softshell-y Polartec® Powershield fabric will only let around 4-5CFM through. A The North Face WindWall jacket rates around 20CFM, and WINDSTOPPER® softshells rate 2CFM. Many manufacturers supply this information if you dig around for it, but not all of them do, which can cause the consumer a bit of head-scratching when trying to interpret what a “wind-resistant” jacket will really feel like.
It’s tempting to think air permeability ratings are like golf scores—fewer CFMs are better, right? Only if you really do want to conserve as much heat as possible by blocking the wind altogether. However, maybe you want to block most of the high-alpine gale, but you know you’ll get hot hiking uphill and you’d like to let a bit of cool air blow into your jacket to make things more comfortable. In such a situation, you’d want to lean toward something that is “wind-resistant” rather than something in the single-digit CFMs.
THE BOTTOM LINE
… On breathability? More is better for high-exertion activities—it’s never ideal to have a sweat lodge inside your jacket. And for low-perspiration activities like belaying your climbing partner on a cold day, you could compromise breathability for a big ol’ puffy jacket. But only compare within a particular brand or within a particular fabric. Going cross-brand/fabric will be irrelevant.
… And when it comes to air permeability, it also depends on the activity. Letting a small dose of wind into your jacket sounds really nice when you’re skinning up 3,000 feet to earn some ski turns. But if the day’s activities call for total windblocking, shoot for something rated closer to 0CFM for optimal contentment.
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Tags: fabric tech, technology, waterproof breathable, windproof
[...] Breathability ratings are far more subjective. First off, there’s no standard test like that for waterproof fabric. More importantly, not everyone sweats the same. But breathability is still very important. A plastic bag is waterproof, but since it doesn’t breathe, it will collect moisture on the inside that will make you cold and uncomfortable. [...]
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