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Waterproof Ratings: What do the Numbers Really Mean?
Why does your snow jacket soak through when it rains? Why does your rain jacket soak through if you lean against a rock? Why does one jacket seem to handle everything and another turn into a wet towel with any precipitation while both claim to be waterproof? Hopefully the information below can clear up a bit of this ever-confusing issue.

While almost nothing is standardized in the outdoor industry, two waterproof ratings are commonly used:

Pounds per Square Inch (psi) represents the amount of water force a fabric can handle before it starts to leak. (For example, rain produces about 7-psi.)

Millimeters show the volume of water a fabric can handle over a 24-hour period before it stops being waterproof. (The rainiest places in the US get around 1000-mm of rain a year.)

Since psi and millimeter ratings reflect different fabric properties, they cannot be accurately compared to each other. While you can state that a 25-psi jacket is roughly the real-world performance equivalent of a 10K (10,000-mm) rated jacket, a smart person could tear your argument apart and be completely right. For our purposes, however, we’ll assume the statement to be more-or-less true and that a higher psi rating means a jacket would fall into the 20-30K range if it was rated with the millimeter system.

The military considers a fabric to be waterproof only when it’s rated to 25-psi or better, and you don’t have to look too hard to find a jacket with a 20,000-mm (20K) waterproof rating. So, why is there so much overkill? Well, it’s because you’re not always standing still. While rain may only cause 7-psi, falling into a puddle or crashing in wet snow creates much more pressure. Anyone who has ever gone out on a really slushy day of skiing knows how quickly a “waterproof” jacket can soak through, especially if you crash a couple times.

So, what does all this mean in the real world? This very basic guide to the millimeter system (which most outdoor companies use) should give you some indication of the minimum requirements for your day in the hills.

5K = Keeps you dry in light rain or dry, cold snow.
10K = Keeps you dry in moderate rain or on an average snowy day.
20K = Keeps you dry in heavy rain or while skiing in wet, snowy conditions.
30K = Keeps you dry if a river falls from the sky, all at once, and lands directly on your hood while you’re skiing through horizontal hail and firemen blast you with a hose.

It’s all pretty simple, right? Well, there’s more to it than that. An important thing to remember is that a waterproof rating does not apply directly to a jacket or a pair of pants but to the fabric that the garment is made from. Zippers, wrist closures, hood attachments, seams, and vents all compromise waterproofing. For example, a jacket with regular zippers will be less waterproof than a jacket with a flap of fabric covering them. A jacket with rubberized, water-resistant zippers will be even better, and a pullover anorak with no zippers will be the most waterproof of all. Along the same lines, a jacket with a fixed hood will be more waterproof than one with a removable hood, because the zipper allows moisture to enter if it gets exposed. Getting the idea? Good.

Seams also play a huge role in a jacket’s real-world performance. A seam is essentially a gap in the fabric where moisture can enter. To combat this unfortunate fact, companies have devised ways of sealing the seams. Taped seams have a piece of waterproof tape that covers the gap and keeps it from leaking. Welded seams are bonded together without stitches using one of a variety of techniques (glues, melting, sonic bonding, pixie dust, wishes to Santa, etc.) to create a seal without the extra fabric or bulk. They both accomplish the same thing in terms of waterproof performance.

There are, however, differing degrees of seam taping (the term we’ll use for taped or welded seams). A critically-taped (a.k.a. strategically taped) shell only uses seam sealing in areas like the shoulders and hood that are more exposed to the elements. This works fine in light rain or while skiing on a cold, bluebird day with no precipitation, but the unsealed seams will start to leak if they come in contact with more moisture. Fully taped garments are exactly what they sound like – every single seam on the entire thing is sealed. This is the only type of garment to consider if you expect to be out in heavy rain or snow.

Now that you have all this information, how do you apply it? Well, when you’re comparing jackets, start by looking at the fabric waterproof ratings since poor fabrics perform poorly no matter how they’re sewn together. Once you have your list narrowed down to fabrics that can take the weather you expect, look for seam construction. After you’ve reached this stage of comparison you can move onto finding the jacket with the features and design you want. Then all you have to do is get out there and get after it.

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Author
Adam Riser lives in Salt Lake City, Utah, where he tries hopelessly to balance his passions for rock, ice, and alpine climbing, backcountry skiing, and downhill mountain biking. He tries to get out of the country on an alpine trip every summer, makes his yearly pilgrimage to Whistler for biking, and can live on Mac-N-Cheese for months on end.
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Cred: 6
Comment by dpbrick
2011-12-01
I hate to say it, but some of this information is just plain wrong. First of all: "Pounds per Square Inch (psi) represents the amount of water force a fabric can handle before it starts to leak". Water leakage through a membrane, such as fabric, is dependent not upon force, but rather, upon pressure. And psi is a unit of pressure, which is defined as force per unit area.

Next: The number of millimeters in a waterproof rating has nothing to do with average rainfall in a region or "the volume of water a fabric can handle over a 24-hour period before it stops being waterproof". From a rigid viewpoint, millimeters are a unit of length or height. But the height (in this case, of water) can be translated to represent measures of pressure or volume. In this case, millimeters of water are used not as a unit of volume, but of pressure, thus relating directly to psi.

The method of measuring the degree of "waterproofness" of fabrics is essentially the following: A solid vertical cylinder (such as a pipe) is placed on top of the fabric and the edges of the fabric are solidly sealed to its circumference. Then water is added to the cylinder until leakage through the fabric is detected. The standard unit of measure for the height of the column is millimeters. So the height of the water in the cylinder just before leakage is detected is reported as the waterproof rating for that fabric. The height of the water translates to pressure, P, as P = rgh where r = density (of water in this case) g = acceleration of gravity and h = height of the water column.

You can perform the calculation yourself or leave it up to your favorite handy-dandy web unit converter to convert 25 psi to mmH2O. You'll have to choose a temperature (usually 4 C or 20 C) since the density varies with temperature. You'll quickly discover that 25 psi is equivalent to about 17610 mm of water.

Thus a 20000 mm waterproof rating really isn't "so much overkill"; and a 10k mm rating wouldn't really be considered waterproof by the military, according to the article, since it would be equivalent to only about a 14 psi rating.

So pretty much the first half of the article is total poppycock. There is a fairly good discussion in the second half, though, of why the waterproof rating of fabric doesn't translate directly to the waterproofness of the garment made from the fabric. So, yeah, the article needs work.

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