Rainwear: How It Works
How Waterproof/Breathable Fabrics Work
Waterproof/breathable (WP/BR) fabrics made outdoor headlines in 1978 when outerwear designed with a Gore-Tex laminate was introduced. Since then many other WP/BR laminates have been created (eVent, Sympatex, MemBrain Strata, many generics), yet people commonly refer to the entire WP/BR category as "Gore-Tex." Of course, not all waterproof/breathable rainwear is Gore-Tex—just like not all facial tissue is Kleenex and not all gelatin is Jell-O. Gore-Tex, due to its huge presence within its product category, is simply a deeply ingrained brand name.
WP/BR fabrics are engineered to juggle 2 tasks:
- Repel precipitation (to keep you and your clothing layers dry).
- Provide an escape route for perspiration vapor (to accommodate evaporative cooling and maintain a comfortable body temperature during exertion in rainy conditions).
How is this accomplished? It requires some inside-outside work on the main fabric.
The interior (underside) of WP/BR rainwear uses one of the following technologies to become waterproof and breathable:
- Laminates (which include Gore-Tex and eVent) are formed when a WP/BR membrane is bonded to the underside of a garment's face fabric, as if wallpaper was applied to a wall—in other words, membrane (wallpaper) + fabric (wall) = a laminate.
- Coatings are liquid solutions that provide WP/BR characteristics when spread across the interior of a garment—like applying a superthin coat of paint to a wall.
All rainwear exteriors (also known as face fabrics) are treated with a durable water repellent (DWR) finish. Even rainwear classified as water-resistant (which includes soft shells) carries a DWR finish. Here are some DWR fast facts:
- A DWR affects only the exterior of rainwear and is separate from a laminate or coating.
- Its purpose is to protect the face fabric from becoming saturated, weighing it down and causing any sensation of dampness.
- A DWR accomplishes this by causing water to bead up and roll off the garment's exterior.
- DWRs do not inhibit fabric breathability.
- Abrasion, grime and, to a lesser degree, laundering reduce DWR performance.
- To remain optimally effective, DWRs must be regularly cleaned and periodically renewed using spray-on or wash-in products.
More information on DWRs appears later in this article. A more detailed explanation for how to care for DWRs is found in our companion article, Rainwear: DWR Care.
Next: a closer look at the inner workings of rainwear.
The core of a laminate is its membrane. Membranes are made from:
- Expanded (i.e., stretched) polytetrafluoroethylene, or ePTFE (sometimes also referred to as PTFE). Most know it by its DuPont brand name: Teflon.
- Polyurethane (PU) films.
- Polyester films.
Gore-Tex and eVent, 2 widely recognized laminates, use membranes formed from ePTFE. An ePTFE membrane has a microscopic web-like structure that is amazingly thin—about 10 microns thick. (One micron equals one-millionth of a meter; the period at the end of this sentence measures about 500 microns.) W.L. Gore, the maker of Gore-Tex, estimates ePTFE contains 1.4 billion pores per square centimeter, or about 9 billion per square inch.
In spite all these microscopic holes, ePTFE is extremely resistant to water (hydrophobic, to use the technical term). Why? The reason most commonly cited is that pores in ePTFE are much smaller than the smallest raindrop (20,000 times smaller, according to W.L. Gore), yet large enough to allow water vapor molecules to pass through.
Yet the most scientifically sound reason is the fact that an ePTFE membrane is a solid that possesses what scientists call a low "surface energy" or "surface tension." In this state it cannot be wetted unless it is contacted by a liquid with a correspondingly low surface energy (isopropyl alcohol, for example).
Plain water, though, has a high "surface energy." This means that water molecules are strongly attracted to each other compared to other surfaces. Thus they always want to pull together into a shape that occupies the least amount of space on other surfaces, such as spherical drops. When water (high surface energy) contacts ePTFE (low surface energy), it quickly consolidates into rounded beads or droplets and slides off.
Place a drop of water on a Teflon cooking surface. It will not flatten and flow in all directions because the attraction between the molecules pulls them into the rounded shape of a drop. Meanwhile, the attraction between water and Teflon (ePTFE) is so weak that no force pulls the water toward the Teflon.
So water can only penetrate ePTFE in 2 ways:
- If water is applied with tremendous force. Wind-driven rain exerts a force of about 2 pounds per square inch (2 psi)—not nearly enough to penetrate ePTFE.
- If the ePTFE's low surface energy is altered due to contamination.
Contamination (caused by dirt, body oils, sweat, sunscreen, insect repellent or similar foreign matter) became the unforeseen foe of the original Gore-Tex laminate of the late 1970s. Those early laminates were designed with plain ePTFE membranes, unshielded against contaminants. The membranes worked splendidly—until they collected dirt and oils, which possess a high surface energy. Any time water makes contact with dirty, oily ePTFE, it now sees what has become a water-attracting high-energy surface and thus wets that surface. The dreaded result: leakage.
The way to protect ePTFE from contamination is to make it oleophobic—resistant to oils. In today's laminates, Gore-Tex and eVent take different approaches to achieving this objective:
Gore-Tex takes a plain (unguarded) ePTFE membrane and attaches it to a superthin protective polyurethane (PU) film, creating what is known in the rainwear industry as a bicomponent laminate. The PU layer is solid (technically speaking, monolithic) and shields the ePTFE from body oils and other contaminants.
Yet if the PU film is solid, it raises an inevitable question: How can such a combo breathe—or, to use more technical language, permit water vapor transfer?
Gore-Tex accomplishes this by formulating the polyurethane film to make it water-attracting (hydrophilic). This is commonly done by incorporating functional chemical groups that "like" water, or by combining the PU with other polymer materials that are hydrophilic, often polyethlyene oxide.
As a human perspires, sweat molecules are drawn to the water-loving PU film and adhere to its inner side, a process known as adsorption. These moisture molecules gradually seep through the solid PU film via diffusion. What drives their movement? The variance in concentration (also known as gradient, or differential pressure) on the 2 sides of the film.
Everything in nature moves toward equilibrium. Hot air moves toward cooler regions; moisture moves toward drier areas. So moisture with a higher concentration of heat and humidity (as generated inside a jacket worn by a vigorously exercising person) will move toward an area of lower concentration/lower heat (outside the jacket).
The difference in concentrations drives water molecules, with their positive charges, from one hydrophilic polyurethane molecule (negatively charged) to the next. The movement can be likened to child on a set of monkey bars, progressively swinging from one bar to the next.
Once on the outer side of the PU film, the molecules evaporate and escape through the ePTFE membrane as a gas, a final step called desorption. This adsorption-diffusion-desorption process delivers the magic of breathability (technically quantified as "water vapor transfer rate," WVTR, or "moisture vapor transfer rate," MTVR).
How does this process compare to the performance of an unguarded ePTFE membrane? Alas, it is slower. On occasion moisture can collect and condense on the inside of the PU film, creating a sensation of dampness inside a garment even though the garment is not leaking.
Created by a company called BHA Technologies (and now owned by General Electric), eVent was originally engineered for use in industrial air filters, then later discovered to be an effective WP/BR material. Though formed from ePTFE like Gore-Tex, the eVent membrane is protected from contaminants without an added PU film. In industry jargon, Gore-Tex uses a hydrophilic monolithic membrane (water-attracting and solid); eVent uses a hydrophobic microporous membrane (water-resisting and equipped with tiny, tiny holes).
So how does eVent shield ePTFE from contaminants? The process is proprietary (a trade secret). One textile scientist has described it as an "oleophobic coating applied in a supercritical fluid process."
Steve Nagode, a member of the REI Quality Assurance Lab and a big fan of eVent rainwear, explains it a little more visually—eVent's developers, he says, figured a way to somehow slather individual ePTFE filaments with a protective "secret sauce" without clogging the infinitesimally small open spaces around the nano-size filaments.
Dr. Alfred Lo, an engineer on the eVent development team, puts it this way: "It is accomplished with a molecular surface coating of the individual fibrils that comprise the open-pore structure of the ePTFE material," he says. Without a PU film involved, he adds, perspiration vapor produced inside a jacket will be vented directly to the outside of the garment without first making the inside wet.
Gore-Tex vs. eVent
Some advantages of eVent:
- The eVent membrane mimics the performance of an unguarded ePTFE membrane. Its 1-step direct venting approach produces speedier moisture-vapor transport than the 3-stage adsorption-diffusion (plus desorption) process required by an ePTFE-PU combo. While no universally accepted lab tests for fabric breathability exist (discussed later in this article), claims by media reviewers, customer reviews at REI.com and anecdotal comments from REI staffers suggest eVent enjoys an edge in breathability.
- eVent performs with similar efficiency in both low and high humidity levels; Gore-Tex tends to favor higher-humidity environments and performs better after its PU film has collected some moisture. During rest stops in REI field tests that involved eVent jackets in cold, dry conditions, some testers were observed standing in steam-like vapor clouds due to the volume of moisture vapor escaping through the fabric.
- Steve Nagode of REI's Quality Assurance Lab offers a cold-weather tip for eVent wearers: "Because the eVent laminate transfers vapor quicker than other waterproof/breathables, active users have discovered that sweat evaporates faster. So wearing eVent in cold weather for the first few times can chill users accustomed to less breathable materials. The solution: Wear another layer or a heavier layer of insulation with eVent shells."
- No WP/BR fabric can keep any high-exertion wearer totally dry, yet eVent's high moisture vapor transfer rate means it is less likely to collect water on the interior of a garment than other WP/BR technologies.
Some advantages of Gore-Tex:
- The bicomponent ePTFE-PU (hydrophobic-hydrophilic) combo has been used and refined for 30-plus years. Its ability to resist contamination has established a strong track record of durability. Meanwhile, the long-term durability of eVent's approach to shielding ePTFE (individually coating microscopic filaments) is still being evaluated.
- Dr. Lo, part of the eVent development team, says the long-term performance of eVent ePTFE will remain high as long as the garment is regularly cleaned. Dr. Lo recommends cleaning eVent garments with "reasonable frequency" to prevent dirt and oil from lingering in the pore structure of eVent's ePTFE. Over time, dirt and oil could permanently plug eVent's pores, impacting breathability.
- The maker of Gore-Tex also recommends regular cleanings for garments that use Gore-Tex laminates.
- DEET, an ingredient in many insect repellents, is a solvent that can reduce the surface energy of ePTFE. Avoid spilling DEET on any rainwear.
- Gore-Tex Pro Shell, the latest incarnation of Gore-Tex (and commonly found in Arc'teryx jackets), routinely receives raves for breathability in customer reviews at REI.com.
- Gore-Tex is widely available in a variety of designs from a large number of manufacturers; eVent remains a relative newcomer with a modest number of styles.
Varieties of Gore-Tex
The Gore-Tex product line periodically changes. Original Gore-Tex (with unguarded ePTFE) has long been out of circulation. A version called Gore-Tex XCR is now used only in footwear. The latest Gore-Tex apparel product mix uses ePTFE-based laminates all styles, all of which carry Gore's "Guaranteed to Keep You Dry" pledge:
Gore-Tex Pro Shell: Top of the Gore-Tex line. Best for rugged, athletic, demanding use. Available in 3- and 2-layer versions; 3-layer styles are usually a touch lighter and have a slight edge in breathability.
Gore-Tex Paclite Shell: W.L. Gore's contender in the 2.5-layer product space, targeted at weight-conscious adventurers. The ePTFE membrane is shielded on the inside by a very thin layer that Gore-Tex describes as "an oil-hating substance and carbon."
Gore-Tex Performance Shell: Used in garments intended for lower-intensity activities, from casual recreation to travel. Available in 2- and 3-layer versions. Excellent waterproof performance; good but not exceptional breathability.
At-a-Glance Summary: Gore-Tex vs. eVent
|Gore-Tex laminate||eVent laminate|
|Membrane material||ePTFE plus anti-contaminant PU layer||Coated ePTFE filaments|
|Technical designation||Hydrophilic monolithic||Hydrophobic microporous|
|Which means it is:||Water-attracting and solid||Water-resisting w/tiny holes|
|Subjective observations||Gore-Tex laminate||eVent laminate|
|Durability||Excellent||Believed good to very good|
|Weight||Varies by garment||Varies by garment|
Full disclosure: REI manufacturers and markets a line of outerwear that uses eVent fabrics. (REI also offers a variety of waterproof/breathable fabric applications, laminates and coatings, all marketed under the umbrella brand name of REI Elements.) As such, any notations about eVent that appear favorable could be interpreted as biased self-promotion. We can only affirm that the intent behind every Expert Advice article at REI.com is to offer fact-based information—with some clearly identified subjective guidance based on our own collective experiences—that can make readers better-educated consumers.
Many laminates use membranes made from a superthin film of polyurethane. It is the same hydrophilic monolithic material used in Gore-Tex to form the protective wall connected to the ePTFE membrane in its laminate. On its own, a PU film performs the same waterproof/breathable function as it does when combined with ePTFE—moving moisture in an adsorption-diffusion-desorption "monkey-bar" process (described earlier).
So if a PU film can do all that, what function is ePTFE performing in the Gore-Tex laminate? ePTFE, due to it the microscopic texture of its filaments, bonds to PU in a way that allows the polyurethane film to be uncommonly thin—thinner (so far) than any stand-alone PU films used as laminates. Admittedly, the differential in thinness is measured in microns, but even such tiny variances can make a discernable difference in technical performance and the feel of a garment.
Newer PU laminates are narrowing the performance gap. Marmot's MemBrain Strata (introduced in 2009), for example, has ambitions of establishing itself as a game-changer in this space, promoting its "20/20" (waterproofness/breathability) capabilities. (What does "20/20" mean? Performance numbers are discussed in this article's Breathability section.)
Not all polyurethane is created equal. Polyurethane can be formulated to create a wide, wide range of products, from spongy foam to a hard faux-wood furniture finish. Thus whatever proprietary steps are taken to formulate polyurethane for use as a WP/BR membrane determines its performance attributes. The wizards who stir the most advanced polyurethane cauldrons are concentrated in Asia (primarily Japan), and the race to create better-performing PU films never ends.
Note: In any REI apparel, any laminate identified as REI Elements waterproof/breathable technology is a polyurethane film.
While not yet widely used, polyester-based membranes are a gradually emerging WP/BR category. The best-known example is Sympatex, which combines polyester (hydrophobic) and polyether (hydrophilic) components to create a pore-free hydrophilic film that transports water vapor by the adsorption-diffusion-desorption process used by polyurethane films.
Sympatex boasts that its film is exceptionally thin, a mere 5 microns, and thus capable of moving moisture vapor quickly. Its maker also claims it provides above-average stretchiness, which makes it comparable to PU films. (Laminates using an ePTFE membrane offer almost no stretch.)
In terms of performance, the general industry view of polyester laminates is that they slightly lag the best PU laminates in terms of breathability. Yet they offer a key sustainability advantage: Once worn out they can be recycled as long as they are bonded to a polyester textile that can also be recycled.
PU Films vs. ePTFE
Advantages of polyurethane films:
- Usually results in lighter, smaller-packing garments.
- Can accommodate stretch in a garment's design; ePTFE laminates cannot. The inherent stretchiness of PU films may make them better equipped to handle hard impacts (such as a fall on rock or ice).
- Lower cost, yet the newest versions are approaching levels of breathability comparable to 2-layer and even high-end, 3-layer ePTFE laminates.
Advantages of ePTFE:
- The membrane itself (and fabrics that surround it) are better equipped for rugged use.
- Better breathability than other laminates or coatings (though the breathability gap between PU films and ePTFE is narrowing).
- Lower risk of condensation on garment's interior (particularly eVent's ePTFE, which excludes the PU layer used in Gore-Tex laminates).
"Coated" rainwear uses a layer of polyurethane to cover the interior of garment, mechanically applied like paint brushed on a wall or mayonnaise spread on bread. Their chief appeal: Decent WP/BR performance for a low price.
Coatings can be used to fully seal a fabric and make it waterproof and nonbreathable. In the waterproof/breathable category, however, polyurethane coatings are formulated in 2 ways:
Microporous coatings: Include a network of infinitesimally small channels—too small for water to penetrate, yet large enough for vapor to escape. Most coated WP/BR rainwear uses monolithic-hydrophilic approach. How is a paint-like coating made porous? A foaming agent may be added so gas bubbles form and expand within the coating, creating permanent interconnected holes within the coating as it dries and becomes solid. Another method: Minute solid particles are mixed into the coating solution, causing tiny cracks and fissures to form next to the particles as the coating dries. This creates super-small passageways for water vapor to escape.
Monolithic coating: A solid, hydrophilic (water-attracting) layer that transports moisture via the adsorption-diffusion-desorption process described earlier in this article.
Unless the manufacturer identifies what type of coating is used, the 2 are visually indistinguishable. Which performs better? Likewise—differences are mostly indistinguishable. (Some garments, such the hugely popular Marmot PreCip jacket, combine the 2 coating technologies.)
Coated rainwear is the least sophisticated and least expensive entry in the WP/BR category. Its low cost makes it a popular choice among:
- Outdoor explorers who move at a casual or recreational pace.
- People who make only periodic visits into the backcountry when rainy weather threatens.
- Anyone trying to minimize weight or stretch a budget.
Coatings usually do not appeal to demanding outdoor athletes who routinely pursue high-energy, high-abrasion activities. But for travelers or casual/infrequent outdoor travelers, lower-cost coated WP/BR rainwear makes sense.
Coated WP/BR rainwear advantages:
- In rainwear's 2.5-layer category, coated garments are lighter and pack smaller than 2- or 3-layer laminated designs because, like 2.5-layer laminates, they include no lining, only a barely-there protective layer of raised lines, dots or resins to keep the WP/BR barrier off the wearer's skin and give it a dry touch.
- Less expensive than rainwear that uses laminations, sometimes by triple-digit amounts.
Coated WP/BR rainwear disadvantages (compared to laminated WP/BR rainwear):
- Less breathable.
- Often a little heavier than laminated rainwear. When spread onto a fabric, polyurethane must fill in the hills and valleys of that fabric, allowing it to become comparatively thick in spots (though only by a number of microns). As a result, a coated garment may also feel somewhat stiffer.
- Slightly lower textile tear strength.
- When washed in mass-market detergents, microporous coatings tend to collect some surfactants (cleaning additives or brighteners) that can decrease the surface tension of the coating, making it possible for water to pass through the fabric. Two solutions: Use at least 2 rinse cycles when laundering the garment (to purge all detergent residue); use a specially formulated, clean-rinsing laundry product for technical fabrics.
The elusive grail of achieving bare-skin breathability in rainwear has challenged designers and frustrated wearers for decades. A key obstacle for waterproof, windproof garments is that little or no air can pass through them—technically, they are not air permeable. (Try blowing air through a sleeve or body panel of a WP/BR jacket.)
When active, perspiring human bodies stir up a moist microclimate inside a garment that cries out for dispersion and evaporative cooling. At very high rates of exertion, moisture from sweat can begin to collect inside a garment, raising the potential for overheating when active or chills (due to evaporative cooling) when resting.
Not surprisingly, shoppers invariably ask the REI sales team to guide them to the "best" jacket or most breathable options in waterproof/breathable rainwear. Is there a way to know this?
A variety of tests exist for measuring a WP/BR fabric's water vapor transfer rate (WVTR; a clinical term for what most consumers call breathability as they perspire). Most of these tests carry head-scratching names (upright cup test; inverted cup test; sweating hot plate test). Regrettably, none has emerged as the consensus reference standard among manufacturers by which all rainwear breathability is evaluated.
This is an important factor for shoppers to recognize when they feel confounded by the complex-looking lab results they see promoted by some rainwear manufacturers: No universally accepted standard for fabric breathability exists.
Where does this leave the consumer? Pretty much at square one, according to Dr. Liz McCullough, a long-time fabric tester for the textile industry and director of the Kansas State University Institute for Environmental Research. It is difficult, she believes, for lab tests to mimic changing environments in the field, and manufacturers could be inclined to selectively promote test results that are based on a single set of conditions that are most favorable for a particular fabric.
"There will never really be a day when consumers can look at a Web site or a tag and say, 'OK, the moisture rate on this jacket is 604 and the rate on this one is 3,004, so this 3,004 one is better,' " McCullough says. "It could just be that a manufacturer is just using a method that gives its fabric higher numbers. So I don't think you're ever going to be able to help consumers other than to tell them that the labels cannot be used in an effective way unless you know exactly the protocol used within each test method and were able to do a comparison."
In 2003 McCulloch sampled fabrics using 5 different test methods and described the variances in results in a technical paper she coauthored for the academic journal Measurement Science and Technology (A Comparison of Standard Methods for Measuring Water Vapor Permeability of Fabrics, Vol. 14, No. 8, 2003, pp. 1402-1408). She found minimal correlation among the tests.
Her best advice when you spot breathability statistics on the hangtag of a rainwear item? "Not to pay any attention to them whatsoever," she says. "It's like apples and oranges, and it's too hard to explain to consumers what they mean."
Dr. Phil Gibson has also conducted independent fabric testing for outerwear manufacturers (including REI) for decades. Gibson is a government-employed civilian scientist who since 1989 has been assigned to the U.S. Army Natick Soldier Systems Center, a research lab in Natick, Mass. He serves on the center's Molecular Sciences and Engineering Team, which is primarily focused on the development of new materials, particularly those involved with chemical protective clothing. An outdoor adventurer himself, Gibson has both a professional and recreational interest in fabric technology. His lab accepts commercial fabric samples for breathability testing.
He has employed multiple breathability tests on outerwear fabrics and is not bothered by the absence of a universally accepted standard. "I don't know if it matters that much," Gibson says. "I've noticed over the past 15 years that all the materials seen to have gotten a whole lot better. It's very rare now to find a really poor material being tested or showing up in a jacket.
"That used to the case a lot more frequently in the past," he says. "Materials that were just not breathable were being sold as breathable. I really don't see that any more at all."
Commonly Cited Test Figures
Even though no universal breathability standard exists, outerwear manufacturers persist in trying to sway consumers by publishing impressive-sounding (though hard to fathom) lab results in their promotional materials. The following explanation may elicit a too-much-information response from many readers, but we believe some interpretation of these numbers is needed.
Over time, the use of the following measurements has grown increasingly commonplace:
- Water resistance: The amount of water pressure (in millimeters) a fabric sample can withstand before leakage occurs. For example, garments using Marmot's MemBrain Strata laminate claim 20,000mm.
- Breathability: The amount of water vapor (in grams) that can pass through a square meter of fabric during a 24-hour period. Using engineer's shorthand, Marmot's MemBrain Strata laminate reports 20,000g/m⊃2;/day.
- Wind resistance: The maximum wind speed a fabric can block.
Not all rainwear manufacturers publish these numbers, but many do. As Dr. McCullough points out, manufacturers may promote a specific test result simply because those numbers present their products in a favorable light.
And many of these products perform exceptionally well. The limitation of test results, however, comes when a consumer attempts to put one brand's numbers up against another brand's. Both may boast 15,000mm and 15,000g/m⊃2;/day performance, but it is possible they arrived at those figures using unrelated test methods.
So when comparing measurements between 2 brands of rainwear, keep in mind that you have no guarantee that the numbers you are reading are true apples-to-apples comparisons with other rainwear choices. (Technically speaking, not all test results can be correlated.) In addition, lab results do not take into account many variables that impact individual perception of comfort—personal metabolisms, the type of clothing worn underneath rainwear, exertion level, temperature, humidity, wind exposure and so on.
Accordingly, any numbers that you see promoted are at best a general estimate of how a particular garment might perform for you in the field. These measurements could be most useful when comparing garments within a single brand, when it's more likely identical tests were applied to each garment.
Water Resistance (i.e., Waterproofness)
- Water-pressure measurements are shown in millimeters (mm) or pounds per square inch (psi); 704mm = 1 psi.
- Technicians determine water resistance in fabric by employing a water column or hydrostatic pressure test (JIS 1092 method A; AATCC test method 127). Fabric samples have been laundered 10 times to simulate field use.
- What level of water resistance qualifies as waterproof? No widely accepted waterproof industry standard for fabric exists. Based on its own tests, the REI Quality Assurance Lab considers rainwear fabric to achieve a minimum standard of "rainproof" performance if it permits no leakage at 3 psi (2,112mm).
- 3 psi far exceeds the force of typical rain. Some comparative figures:
- Wind-driven rain: approximately 2 psi (1,408mm).
- Hurricane-force rain: 10 psi.
- Fire hose: 100-300 psi.
- Pressure washer: 1,000-10,000 psi.
- Note: No fabric is truly "waterproof." Even sheet metal is routinely cut with high-pressure water jets.
- How much water pressure is exerted by 3 psi (2,112mm)?
- Imagine a square column with 1-inch sides placed atop a piece of fabric.
- Fill the column with water.
- To reach 3psi, the column would be nearly 7 feet tall (the equivalent of 2,112mm).
Again, as with breathability testing, no waterproof standard for fabric exists, not even in the military. "The army seems to have several definitions of 'waterproof,' depending on whether it is clothing, tents, bags or packs," says Gibson, "Even for clothing, there are several definitions and numbers used by different military groups, and they don't all agree—though I do see the 25 psi [16,700mm] often used as a specification."
What about kneeling in a marsh, sitting on a wet rock or carrying a backpack's shoulder straps atop a rain jacket during a downpour? Must a fabric carry an exceptionally high waterproof rating in order to keep a wearer dry in such circumstances? Opinions vary on this topic.
One rainwear manufacturer calculates that a 180-pound person exerts about 16 psi when kneeling and 8 psi sitting. Accordingly, it uses fabrics that offer a minimum rating of 20 psi.
REI's Nagode contends (and Gibson concurs) that such claims have never been supported by hydrostatic resistance testing. In the marsh/wet-rock/backpack scenarios mentioned above, the pressure created by the actions described simply displace water away from the fabric. "The water would need containment to generate pressure," says Nagode. Everyone agrees, meanwhile, that 3 psi is more than adequate resistance to repel rain.
Summarizing water resistance:
- REI considers any rainwear fabric with a rating of 3 psi (2,112mm) or higher to be rainproof, even if exposed to wind-driven rain.
- Many rainwear fabrics are marketed with manufacturer waterproof ratings of 5,000mm, 10,000mm, 20,000mm and higher, though REI recognizes no significant benefit to higher ratings.
Five main laboratory test methods for breathability exist. Results from 4 tests are extrapolated and converted into g/m⊃2;/day:
- g = grams of water vapor that pass through . . .
- m⊃2; = a square meter of fabric . . .
- day (or 24h) = during a 24-hour period.
The g/m⊃2;/day numbers manufacturers present to consumers could come from any of 4 methods (upright cup; inverted cup; desiccant inverted cup; dynamic moisture permeation cell). While useful to a degree, efforts to compare results these numbers are hampered by the following shortcomings:
- Not all methods are correlated to one another (so the results may be the equivalent of comparing apples to oranges).
- Most of the methods evaluate a single simulated atmospheric condition, not the variety of conditions a person might experience in the field.
- Identical fabrics will perform well on one test, not as well on another.
- Results cited in promotional materials typically come from whatever test method a manufacturer believes provides the most impressive-sounding results.
Another specification, called Ret or RET (resistance to evaporative heat transfer, or heat loss) is measured by the sweating hot plate method. During this test, a porous metal plate is heated while fabric is suspended just above the plate. Water (simulating perspiration) is channeled into the plate.
As the plate "sweats" and water vapor evaporates through it and the fabric, more energy is needed to keep the plate at a constant temperature. This is measured by an arcane formula involving Pascal watts (m⊃2;/Pa W-1). A low number (such as .06 Pascal watts) means the fabric offers little resistance to evaporative heat loss. That translates into high breathability.
A Germany-based testing laboratory, Hohenstein Institute, then supplemented Ret lab results. It dressed people in garments constructed from fabrics of various Ret values and had them exercise on treadmills. Hohenstein combined lab numbers and user comments to unilaterally create a Ret-based comfort rating index:
|0-6||Very good (extremely breathable); comfortable at higher activity rate|
|6-13||Good (very breathable); comfortable at moderate activity rate|
|13-20||Satisfactory (breathable); uncomfortable at high activity rate|
|20-30||Unsatisfactory (slightly breathable); moderate comfort at low activity rate|
|30+||Unsatisfactory (not breathable); uncomfortable and short tolerance time|
An Example of Comparisons
Below we offer a chart that blends test results promoted by 2 unnamed manufacturers. We do so NOT to provide a definitive comparative resource, but only offer a general impression of what the numbers represent and to demonstrate how such figures can vary from source to source.
One of the manufacturers emphasizes that their results represent "very generalized" overviews of the technologies mentioned, and that "all [WP/BR] technologies will have a range of performance due to variations in the face fabrics." Considering the many variables that can affect individual comfort in changing wilderness conditions, we at REI translate all of that to mean: "These numbers are an educated guess; your experience may vary."
|Fabric technologies||Ret||B2 method*|
|Gore-Tex Pro Shell 2- and 3-layer; some PU laminates||4-6||25,000+|
|Gore-Tex PacLite/Performance 2-layer||6-8||15,000+|
|Gore-Tex Performance 3-layer; PU coatings||7-10||10,000-15,000|
|Most soft shells that include film laminates||8-13||6,000-10,000|
* These are 2 versions of a test method known as JIS (Japanese Industrial Standard) L 1099, or the desiccant inverted cup method, where a fabric sample is exposed to an inverted cup of water; the B2 approach adds a layer of ePTFE to keep water off the fabric sample and is neutral on the MVTR results obtained.
Another Example of Comparisons
Gibson invented the dynamic moisture permeation cell (DMPC) method (or ASTM F2298, the designation assigned to the test by the American Society for Testing and Materials). It involves flowing 2 gas streams at specified temperatures and mean relative humidities on 2 sides of a fabric sample. (What is mean relative humidity? Example: If the humidity on one side of the fabric is 25 percent and 75 percent on the other, the mean relative humidity is 50 percent.) This method can effectively mimic the difference in concentrations between the inside and outside of a rain jacket when worn in the field. The amount of water vapor that passes through the sample is then measured. Fabrics that exhibit the least resistance to water vapor diffusion offer the best breathability.
In a 2008 study, Gibson used the DMPC method to test a variety of WP/BR fabrics. The results are shown in the following plot:
What this chart shows:
- eVent demonstrated a degree of breathability (or, water vapor permeability) nearly identical to that of an unguarded ePTFE membrane. (As mentioned earlier, the original version of Gore-Tex was unguarded ePTFE; later versions added a polyurethane film to protect the ePTFE from contamination.)
- The ePTFE-PU membrane combo used in Gore-Tex functions better in higher humidity settings, when the PU film has already collected moisture. Even in those situations, eVent outperformed other Gore-Tex laminates (and a 2.5-layer Marmot PreCip, which uses a blend of PU coatings) in this test by roughly 30% or more. In drier environments, eVent's advantage rose to 100% or more. (Note: Gore Windstoppper is not a waterproof fabric.)
Note: Though not included on this chart, the REI Elements family of laminates and coatings (which excludes garments constructed with eVent) offers performance characteristics comparable to the majority of 2- and 3-layer technologies sampled in this test.
A concluding reminder on breathability: Many factors beyond WP/BR fabric technology influence individual perception of breathability, including exertion levels, personal metabolisms, weather, clothing worn underneath rainwear, use of vents, even the type of lining a garment uses (woven linings are slightly less breathable than mesh linings).
- No universally accepted standard for fabric breathability exists.
- Five different test methods can be used to arrive at breathability figures, yet most do not correlate to one another.
- Manufacturers may promote lab results with similar-looking numbers acquired from different tests, meaning an unsuspecting consumer may unknowingly be comparing apple and oranges.
- The best use for these numbers is comparing rainwear products within a single brand, since a manufacturer is likely to use the same test for its entire product line.
- Among the WP/BR technologies evaluated in all tests, eVent posts consistently high performance results.
- Regardless of which WP/BR technology used, its moisture vapor transfer rate (MTVR) will be faster if the contrast in concentration (or gradient) between the 2 sides of the fabric is high. A sweating human makes the inside of raingear warm and humid. In cool weather conditions (typical of most rain events), a WP/BR should be able to do its job effectively. In warm and steamy conditions, where the concentration variance may be zero, no raingear is likely to feel comfortable.
Wind Resistance/Air Permeability
A fabric's wind resistance is usually displayed in miles per hour (mph) or cubic feet per minute (cfm). The most common test in the U.S. for measuring wind resistance in fabrics is the Frazier Air Permeability Test.
The Frazier test measures the amount of air (in cubic feet) that can pass through 1 square foot of a fabric sample in 1 minute at a pressure differential equal to a wind speed of 30 mph. Such numbers are only sporadically promoted for rainwear and emphasized more on garments designed specifically for high-wind conditions. REI modifies the process when testing REI apparel for wind resistance in order to calculate a miles-per-hour figure. (REI rates a fabric as windproof if it blocks winds clocked at 60 mph.)
Most waterproof/breathable rainwear fabric is also promoted "windproof." Wind becomes a concern to people who are moving at a high velocity (skiing or cycling) or caught in a storm that involves strong winds. Wind can deprive our bodies of heat and moisture, leaving us feeling chilled. Wind can also cool us when we are vigorously exercising.
With rainwear, usually our greater concern is air circulation. Air movement enables our bodies to avoid overheating when active while wearing rainwear. This is why venting a rain jacket (using the main zipper, core vents or underarm zippers) is a key tool for regulating our comfort level when active. (Note: eVent rain jackets often exclude underarm zippers due to elevated breathability of their laminate.)
While windproof, most rainwear offer no air permeability. Soft-shell fabrics that include no laminates, meanwhile, offer good air permeability (and thus superior breathability). The downside to unlaminated soft shells: They cannot repel heavy precipitation. Even so, some high-energy wilderness travelers accept that trade off and choose laminate-free soft shells as their primary outerwear piece in order to maximize breathability.
Summarizing wind resistance/air permeability:
- Most rainwear is considered "windproof," though for fabrics no industry-wide windproof standard exists. When testing REI apparel, the REI Quality Assurance Lab team uses 60 mph as its standard.
- Air circulation helps cool a vigorously exercising person who is wearing rainwear.
- Because most rainwear offers no air permeability, the use of vents (including a jacket's main zipper) is an important tool for regulating air circulation.
- Laminate-free soft shells offer generous air permeability but minimal protection from a sustained downpour; relying on a soft shell as one's principal outerwear is a decision best left to experienced wilderness travelers tolerant of rain.
REI markets its own line of outerwear products that feature WP/BR REI Elements rated fabrics. What are these fabrics? In brief:
REI Elements is an umbrella designation that encompasses most WP/BR technologies used in REI-branded apparel. Some garments use laminates (polyurethane films, not ePTFE), some use coatings. The REI Elements group does not include eVent laminates.
All REI Elements fabrics are rated rainproof to 3 psi (2,112mm) after 10 washings, and all are rated windproof to 60 mph. Due to the lack of industry agreement over which water vapor transport test methods best represent a WP/BR technology's field performance, REI does not publish breathability test results for garments using REI Elements fabrics.
Durable Water Repellent (DWR)
The first line of defense for rainwear is not a laminate or coating but the durable water repellent (DWR) applied to the fabric's outer surface.
All waterproof/breathable garments are treated with a DWR finish (as are most water-resistant soft shells). DWRs do not inhibit breathability because they do not coat the textile surface; instead they bond to the textile's fibers and do not fill in the interstitial spaces between those fibers.
The purpose of DWR finish: Allow a garment's face fabric to shed water, prevent saturation and keep water from sitting atop a WP/BR membrane. Garments remain light when they avoid becoming waterlogged.
Randy Verniers, a technology specialist at Marmot, explains that DWRs work by increasing the "contact angle" or "surface tension" created when water contacts a textile. An optimized DWR forms a chemical chain of microscopic, tightly packed vertical "spikes" on the outermost fringe of a garment's exterior. This dense, spiky buffer leaves no room for water to spread out, forcing it to form in round droplets. As such it beads up and swiftly slides off the fabric, having no opportunity to flatten out and seep into the textile.
Fluorocarbons (sometimes called fluoropolymers) can create the steepest angle and are the most common DWRs. Silicone and hydrocarbons are also used. Nonchemical DWRs are being studied, though none offers the performance standards achieved by chemical DWRs.
DWRs are at their best when new, but their performance can diminish with use. Their molecular chain, says Verniers, is masked by dirt and oils and can also be affected by abrasion. Such things reduce the surface tension and allow water droplets to flatten, spread out and penetrate the textile.
Regular laundering and a brief spin a clothes dryer (about 10 to 15 minutes at medium heat) can revive a DWR. After prolonged or rugged use, though, rainwear will likely need to have its DWR reapplied. Spray-on and wash-in reapplication products from companies such as Granger's, Nikwax, ReviveX or Sport-Wash accomplish this goal. Verniers prefers spray-on products, since wash-in products may impact a garment's breathability.
Additional details on maintaining and reviving a DWR is explained in a separate article, Rainwear: DWR Care.
- Read and follow care instructions included with your garment, usually printed on a tag and stitched into a seam. Manufacturer instructions supersede any tips offered here.
- Rainwear manufacturers usually encourage people to wash their garments regularly. A clean rain jacket outperforms a dirty or smoky garment.
- Despite the expense, specialized cleaning products created specifically for WP/BR rainwear are often recommended by manufacturers. The key reason: They rinse away cleanly. Mass-market detergents include additives (optical brighteners, foaming agents and more) that leave behind surfactant residues that can impact performance. Specialized products are available from Granger's, Nikwax, ReviveX or Sport-Wash.
- If a grocery-store detergent is your only choice, select a variety labeled "free" or "clear." Typically they include fewer additives.
- Whatever cleaning product is used, always rinse thoroughly; 2 rinses cycles are usually advised to remove all soap reside.
- Hand-washing or front-loading washers are preferred; top-loaders can potentially snag a garment on its agitator.
- Close any zippers, empty pockets and fasten any rip-and-stick (aka hook-and-loop) closures to prevent possible abrasion while garments tumble. Turn garments inside-out.
- Prewash stain removers such as Shout and Spray 'n' Wash are usually acceptable. Check instructions for your particular product.
- Things to avoid (unless otherwise specified):
- Fabric softener and dryer sheets. They contain waxes, oils and fragrances that diminish WP/BR performance in fabrics.
- Chlorine bleach.
- Dry cleaning.
- Open flames or intense heat (synthetic WP/BR materials can melt).
- Rain jacket or rain pants may not last a lifetime. Some reasons why:
- Hard use can cause snags, rips and tears on the face fabric; this makes a garment vulnerable to leakage.
- Even if the face fabric does not rip, a hard fall on rock or ice can cause a shearing effect on laminate or coating, which can permit leakage.
- Basic DWR revival and reapplication advice was mentioned earlier in this article. For a more detailed explanation of durable water repellents, please refer to our companion article, Rainwear: DWR Care.
Some Subjective Observations
Other factors beyond rainwear's WP/BR technology should influence a purchasing decision: Weight; packability; appropriateness of the face fabric (Burly for bushwhacking? Wispy for high-speed ultralight travel?) for your primary activity. These and other considerations are discussed in our companion article, How to Choose Rainwear.
Other factors beyond rainwear's WP/BR technology will impact wearer comfort: Lots of good, breathable rainwear choices exist, and some (particularly 3-layer laminates) tend to consistently outperform others in breathability. But even their performance expectations can be overwhelmed by overly aggressive use. A key objective when wearing rainwear during exertion is to avoid moisture build-up inside the garment. The best defense: Be alert and actively manage your comfort level.
- Personal metabolism: If you heat up easily, modify the pace of your activity.
- Exertion level: Same principle; slow down or speed up as needed.
- Weather conditions: If it's warm and steamy out, no rainwear feels comfortable; if it's cool and dry, keep a light insulating layer handy for rest stops; in other words, be alert and nimble; adapt as conditions dictate.
- Clothing worn underneath: Shed a layer if you're uncomfortably warm.
- Use of vents: If you rainwear has underarm zips, core vents, use them (in addition to your main front zip) to speed sweat's evaporation.
Maintenance: Clean your rainwear regularly to keep it performing at its best. If you notice wet blotches on the face of a rain jacket, revive its DWR.
The following individuals provided valuable technical guidance in the preparation of this article: Dr. Phil Gibson, Molecular Sciences and Engineering Team, U.S. Army Natick Soldier Research, Natick, Mass.; Randy Verniers, technical specialist, Marmot; Dr. Liz McCullough, director, Kansas State University Institute for Environmental Research; Dr. Alfred Lo, engineer, eVent development team; Steve Nagode, research and development engineer, REI Quality Assurance Lab.