How to Rate Waterproof Fabric

Written by allison ockinga
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How to Rate Waterproof Fabric
Appropriate wet-weather gear is essential for rainy days on the trail. (randonnée pluvieuse image by Danielle Bonardelle from Fotolia.com)

An outing can quickly turn hellish when inclement weather catches hikers unprepared. In a downpour, the right wet-weather gear makes all the difference between a happy camper and a bad sport. Savvy shoppers should know how to navigate the complex performance textile industry to stay dry when it counts.

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Waterproof/Breathability

According to T.D. Wood, who offers expert advice on choosing rain gear on REI.com, "No widely accepted waterproof industry standard for fabric exists." However, he writes, two factors determine waterproof ratings: water resistance and breathability.

Most manufacturers measure water resistance in terms of how many millimetres of water a garment can resist prior to leakage. A 15,000mm (15K) rating means the test fabric can handle 15,000mm of water in one day before the wearer gets wet, explains Justin Mool in "Waterproof Breathable Explained" on backcountry.com.

Breathability is measured by the Water Vapor Transfer Rate formula: how many grams of vapour (sweat) can pass through one square meter of fabric in 24 hours. Wood reports that, though many tests exist for determining breathability, again, "none has emerged as the consensus reference standard among manufacturers by which all rainwear breathability is evaluated."

How it Works

Waterproof breathable fabric relies on layers. The interior utilises either laminates or coatings. Wood explains laminates as a WP/BR membrane bonded to the garment’s inside face, like “wallpaper applied to a wall.” Laminates (such as Gore-Tex) are generally lighter and more breathable, so perspiration doesn’t get trapped between the wearer and the garment. Coatings are liquid solutions spread onto the inside face like “a super thin coat of paint,” and are generally less breathable and subsequently lower priced. Regardless of what’s inside, exteriors are always “treated with a durable water repellent finish,” which makes water bead up and roll off fabric.

Ratings Translated

In his article “Waterproof Ratings Demystified”, Rob de Luca of backcountrybeacon.com provides a rough correlation between ratings and performance. Garments like soft shells, rated 0 to 1K, resist rain but are not rainproof. At 1K to 5K, a garment is rainproof but leaks under pressure against a wet surface. A garment is “generally waterproof” from 5K to 15K, but will leak if submerged. At 15K, the garment is waterproofed to withstand shallow submersion, like fishing waders. A rating above 35K indicates a solid, non-porous material, such as rubber galoshes.

Company Bias

Mool and de Luca both acknowledge that most fabric manufacturers determine waterproof ratings in their own labs. Mool writes that labs generally have comparable testing criteria, so “take their waterproof ratings at face value.” However, as Wood assures us, there is no WP/BR standard. Therefore, Mool insists, a manufacturer’s ratings should be taken with “a massive grain of salt.”

Considerations

An essential point to keep in mind is that WP/BR ratings apply only to the fabric, not the whole garment. To totally prevent leakage, look for a fully seam-sealed garment. Finally, Mool reminds us that a garment’s waterproofness can degrade with use. Contaminates like oils, sweat and dirt work into the fabric and weaken its ability to keep water out.

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