Adapted from Waterproof Breathable Fabric Technologies: A Comprehensive Primer and State of the Market Technology Review, by Alan Dixon, Backpacking Light Magazine (ISSN 1537-0364), http://www.backpackinglight.com/cgi-bin/backpackinglight/00316.html, February 10, 2004. This excerpt is used with permission from BackpackingLight.com publisher Beartooth Mountain Press under a nonexclusive license to ProLiteGEAR.com. No part of this excerpt may be reproduced or distributed without express written permission by the publisher. Copyright 2004 BackpackingLight.com. All Rights Reserved.
Consider Gore-Tex: A nylon face fabric laminated to a fragile expanded PTFE membrane and coated with a breathability-squelching layer of polyurethane. What if you could use the original expanded PTFE membrane and not have to cover it with PU? You’d have a waterproof breathable membrane that would move moisture approximately 30% to 200% faster than Gore-Tex XCR*. BHA Technologies has developed such a membrane, branded as eVENT. Sounds like first generation Gore-Tex, doesn't it (which did not have a PU layer)? Here's the difference: BHA engineered the PTFE membrane to be oleophobic, and thus, highly resistant to contaminant fouling. The eVENT PTFE membrane repels most of the common oily contaminates that originally plagued the first generation Gore-Tex membrane. Thus, eVENT does not need a breathability-inhibiting PU membrane to protect it from contaminants.
* Results from independent testing: US ARMY Natick Labs, Dynamic Moisture Permeation Cell (DMPC), ASTM F2298 approved June 2003.
Figure 1 - Breathability performance of some common outdoor fabrics. Higher values indicate better performance: Test results from US Army’s Soldier Systems Center, Natick, MA using a Dynamic Moisture Permeation Cell (DMPC), ASTM F2298. Courtesy of Phillip W. Gibson.
With unimpeded spaces between fibers (a micro-porous membrane), eVENT’s moisture transmission behavior is more like an uncoated woven or knit fabric. From Figure 1, you can see how closely eVENT matches the performance of Schoeller’s Dryskin Extreme, a stretch woven fabric that has no WP/B membrane (note that the air permeability effects of a fabric on moisture transport are eliminated in this test; if air permeability is considered, much higher moisture transmission rates would be measured for Schoeller Dryskin Extreme).
eVENT’s membrane structure is important in two ways. First, molecules in their gaseous state (including perspiration vapor) can pass through the pore channels between the membrane’s fibers, a process that is capable of moving significantly higher fluxes of moisture than what is possible for by a solid layer of PU (as in Gore-Tex). Second, and more important, eVENT passes moisture equally well at both low and high humidity levels, evidenced by the independence of eVENT’s moisture transmission rate on the humidity level (see Figure 1). At a 70% mean humidity level, eVENT transports moisture about 30% faster than XCR and about 70% faster than standard Gore-Tex and the best PU membranes. But at a 30% mean humidity level, eVENT transports moisture about 200% faster than XCR and about 250% faster than standard Gore-Tex and the best PU membranes.
Advantages of eVENT Garments
Why is faster moisture transport at low humidity important? If moisture can be moved out of a clothing system soon after humidity begins to build (low humidity levels), the risk of moisture vapor accumulation, condensation, and wetting of clothing worn underneath the shell is minimized. The ability of a shell fabric to pass moisture at a high rate at low humidity levels, then, increases the time at which the shell can be worn during periods of moderate exertion levels.
But now consider sustained or higher exertion levels, resulting in perspiration that overwhelms the ability of a garment to breathe, and your clothing starts to get wet from the accumulation of perspiration. Since eVENT moved moisture faster and started moving it earlier, you’ll maintain drier clothing longer than if you wore a PU garment. More important, when your exertion level lowers and the internal humidity level in the garment decreases, eVENT will continue to move moisture at a higher rate than PU, which ultimately contributes to faster drying times for your clothing.
In cool and dry environments, such as winter camping or high altitude mountaineering, humidity levels can be fairly low - a situation that would favor eVENT. This is an environment where getting your clothing damp may have serious consequences for staying warm. This may be even more important for non-clothing applications like tents and bivies. Here, relative humidity may remain low but the need to move moisture out and reduce condensation is high. Some manufacturers are starting to use eVENT in their single wall tents and bivy sacks, including Integral Designs, Exped, and Bozeman Mountain Works.
One final advantage for eVENT garments is that they are easy to clean. Gore recommends that you wash Gore-Tex with special (and expensive) cleaning products (if you don't, you run the risk of reducing the hydrophilicity of the PU membrane - which could compromise its ability to transport moisture). You can wash eVENT pretty much the same way you wash your regular clothes - use a mild liquid soap (detergent-free) and a 2x rinse cycle.