Breathe Easy: Unraveling the Hype of Waterproof-Breathable Materials

AMC Outdoors, January/February 2006

There’s nothing quite like exercising inside a steam room, which is exactly the environment created inside your waterproof outerwear any time you strenuously exert yourself. With all the marketing hype surrounding waterproof-breathable (WP/B) technology—and the astonishing amount of money you can spend on a high-end piece of outerwear—surely there’s a miracle shell that keeps you dry, inside and out, in any conditions. Right?

Not exactly. Despite what the outdoor industry would have you believe, no WP/B fabric exists that can effectively move enough moisture to keep you dry during episodes of peak sweat. So what gives? Wading through the marketing deluge to get even a modicum of actual science can be virtually impossible. So here’s a quick primer to help you keep your head above water next time you shop for a WP/B shell.

Barriers to entry

For a piece of outerwear to be waterproof, it must contain a barrier that blocks moisture from entering from the outside. This comes in two forms, a laminate or a coating. A laminate is a solid layer of material that is glued to another fabric, usually nylon (Gore-Tex is the best known example), whereas a coating is applied (think painted on) to the inner surface of the shell. Laminates are generally more durable over time, but heavier than coatings.

In addition to this barrier, or membrane, the outer surface of a garment is almost always coated with a “durable water repellent,” or DWR. This is what causes water droplets to bead up rather than be absorbed by the fabric.

Materials vary in their degree of waterproofness, as measured by the amount of pressure it takes to force water through it. In general, the pressure generated by even a heavy rain is insufficient to cause a waterproof garment to leak, though pressure points such as your knees (when kneeling) or shoulders (under the weight of heavy pack straps) may cause problems with some fabrics. Beware any product that claims to be “water-resistant”—it will inevitably start to leak.

No sweat

For a material to be breathable, there must be a way for moisture generated inside the jacket (your sweat) to pass through the fabric and escape into the environment. This is accomplished in two principal ways.

In the first method, the waterproof membrane is full of continuous pores large enough for water vapor to physically pass through. Despite the marketing material you may read, the pores in such a membrane are also large enough to permit the passage of liquid water.

To combat this, the outer surface of these so-called microporous membranes (typically laminates) are chemically treated to be strongly hydrophobic, repelling water before it has a chance to move through the material. This is the basic principle behind Gore-Tex.

Alternatively, the waterproof membrane is solid—there are no physical pores for water movement. Known as monolithic membranes, these barriers move moisture through the process of solid-state diffusion. Moisture collects on the surface of the membrane, and is transported through the material from areas of high water concentration (the sweaty inside) to areas of low water concentration (the dry outer layer). Most coatings fall into this category, and are usually slightly less breathable than microporous membranes.

In both instances, the primary driving force for moving moisture is the difference in humidity between the inside of the jacket and the outside environment. This means that the higher the relative humidity is inside your jacket and the lower the humidity is outside, the greater the breathability. WP/B jackets will breathe best in a cold, dry environment, and perform poorly in hot, humid conditions.

Drier standards

There is no industry standard for measuring a garment’s breathability. “Testing breathability requires a lot of expensive and precise equipment,” explains Alan Dixon, product review program director for Backpackinglight.com. “It’s a fairly complex physical process that’s difficult to measure, and often the results can’t be reproduced in other labs.”

Given this lack of standard, manufacturers tend to focus on whichever results put their WP/B system in the best light. Add to this confusion the fact that tests are usually performed only on small pieces of fabric. “Equating what a swatch of material does in the lab to the performance of a garment in the field is a stretch,” says Dixon. “The results can be used as a guideline, but it’s not exactly clear what these values mean in the real world.”

A few years ago, the U.S. Army’s Soldier System Center in Natick, Mass., performed breathability tests on some common outdoor fabrics. Leading the pack of tested materials was the eVent laminate which, depending on relative humidity, was between 1.3 and 3 times more breathable than the next best material, Gore-Tex XCR. Following this, the results clustered together, with the best-performing materials approximately twice as breathable as the least. In descending order of breathability, the results were: Gore-Tex XCR, Gore-Tex (standard), HydroSeal (The North Face), Membrain (Marmot), Sympatex, Conduit (Mountain Hardwear), and OmniTech (Columbia).

But no matter how good these WP/B materials are, they will never be able to transport all of the moisture you produce during periods of peak sweat, which can approach eight ounces per hour. To minimize moisture build-up, vent your outerwear by opening pit zips, loosening any elastic around your waist, and cracking your front zipper. “How you use your WP/B outerwear is often more important than the material itself,” Dixon concludes.

—Matt Heid is Senior Editor of AMC Outdoors.