What Exactly is Waterproof Breathable Fabric?


Before waterproof breathable fabrics, there were fabrics that were waterproof but did not breath (rubberized rain wear) or breathable but not waterproof (the 60/40 parka anyone?). By now, most people know something about the waterproof breathable (WPB) concept when it comes to performance outerwear– namely that water from the outside is kept out but water vapor from the inside (in the form of sweat) is allowed to pass through. This innovation provides superior comfort, mainly in the area of moisture management (although lighter weight garments are another benefit).

Gore-Tex was the first to market with their WPB technology in the late 1970’s. Since then, there have been other branded WPB players plus a huge proliferation of generic WPB technology that is branded privately by outdoor brands with names like Hyvent, Omni-Tech, and Conduit.

The trouble for the consumer in knowing what you are getting when you buy a WPB garment. Everyone makes claims that attempt to put their WPB brand in the best possible light. Gore-Tex’s claim for example is simply “guaranteed to keep you dry”.

How Much Waterproofness is Enough?

Waterproofness we intuitively understand. But when testing fabric for waterproofness, we see numbers like 3,000mm, 10,000mm or 20,000mm. These numbers refer to a pressure test that says (for example) a fabric with a 3,000mm rating can withstand the pressure of a column of water 3,000 millimeters high. Likewise, a 20,000mm rating would be waterproof under a column of water 20,000 millimeters high.

So how much water pressure does your jacket encounter in a rainstorm or out on the ski slopes? Actually much less than you might think. For example wind driven rain is equal to about 1,400mm of pressure. Hurricane driven rain is about 7,000mm.

And if you are in an environment where snow is present, then waterproofing is usually overkill unless you are sitting on a snow covered chair lift. Even then, an 180lb person is estimated to produce only 5,632mm worth of pressure when sitting. So unless you are being hit with a fire hose on a regular basis, a 5,000mm rating is probably good for most situations. And if you like to drive over the speed limit, a 10,000mm rating should handle everything short of that fire hose.

Why Breathability is Important

But breathability is the thing that keeps us comfortable when we are very active and generating a lot of heat. Breathability ratings are measured in grams of moisture vapor transferred per square meter of fabric over a 24hr period (mvtr). A higher number means the more moisture that can be released through the fabric. But all things are relative right?

Even the most breathable item of clothing can be overwhelmed if you are working hard enough. Plus, the breathability works best if there is a pressure difference (higher to lower) between the inner of the garment and the outer and if the air humidity is low. Skiing on a dry, cold Colorado day should allow for great fabric breathability conditions. Wearing a hardshell in Thailand during a rainstorm with the humidity between inner and outer about the same allows for basically no breathability.

Polartec NeoShell

How it Works

Waterproofness and breathability in fabrics is a function of a coating or lamination. This film layer (usually polyurethane) is responsible for keeping the water out and allowing the vapor to escape. The “breathing” takes place by adsorption or direct venting.

In the absorption method, the film layer adsorbs your sweat vapor and then transpires it to the exterior of the fabric layer where it evaporates. The direct venting method allows vapor to escape through tiny pores without adsorbing the moisture first. The absorption method film is less expensive so you can probably guess what most jackets use.

It is almost impossible, however, to know what type of breathability method is being used in your garment as brands don’t normally explain the differences. As you can see from this very brief article, it is a complicated process that is not easy to explain in marketing sound bites. That is why you get generic slogans like “guaranteed to keep you dry”.

Bottom Line

If you are overheating, then you are probably not wearing what is appropriate for the conditions or activity and your waterproof breathable layer is probably not going to save you from being uncomfortable. We have been told through decades of marketing to expect amazing results from our waterproof yet breathable jackets but what you wear under it will have a big affect on your working comfort.

The good thing about breathable fabrics is they will eventually dry after being sweat out, but only if you stop what you are doing for awhile. And remember, whether skiing, biking, hiking or climbing, removing a hat is the quickest way to cool without removing your jacket. After that, try removing your gloves and exposing your wrist (the wrist area carries a lot of blood close to the surface and cooling that area works wonders), unzip pit zips, and as heat rises, unzipping the center front at the neck is an obvious choice.

Dan Tiegs is the founder of WILD Outdoor Apparel and has specialized in performance apparel for over 15 years. 

The company is currently running a crowd funding campaign for their new Burnside Alpha Jacketthe perfect fusion of a lifestyle jacket with technical performance, boasting WILD's signature "Mountain to Bar" styling to create technical style with street personality. True to the WILD brand, it uses unique, high quality fabrics (waterproof breathable fabric + Polartec Alpha) to ensure you stand out from the crowd. The American made, Burnside Alpha will be a limited edition with only 120 numbered pieces made per color (Denim Blue, Deep Space Green and Drifter Brown)

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