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, Entrant, Dermizax, MemBrain and many generics), yet people commonly refer to the entire WP/BR category as "Gore-Tex." The knowledgeable Gearhead, of course, understands not all waterproof/breathable rainwear is Gore-Tex—just like not all facial tissue is Kleenex. 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/rain (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.
- Coatingsare 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.
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.
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" rain-wear 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 non-breathable. In the waterproof/breathable category, however, polyurethane coatings are formulated in 2 ways:
Coated WP/BR rainwear advantages:
- In rain wear'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 rain wear that uses lamination's, 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. To learn more, see our companion articleTechnical Fabric Care.
The 3 Key Measures
- Water resistance:The amount of water pressure (in millimeters) a fabric sample can withstand before leakage occurs.
- Breathability:The amount of water vapor (in grams) that can pass through a square meter of fabric during a 24-hour period.
- Wind resistance:The maximum wind speed a fabric can block.
Water Resistance (i.e., Waterproofness)
- The Industry 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.
- No universally accepted standard for fabric breathability exists.
- But at the moment we measure it as
- g = grams of water vapor that pass through . . .
- m² = a square meter of fabric . . .
- day (or 24h) = during a 24-hour period.
- 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
- 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.