Let's start with the basics. The higher the number, whether it's listed as 10k or 10,000 mm, the more waterproof a garment is. For example a jacket rated at 10,000 mm is twice as waterproof as one rated for 5,000 mm. Pretty simple so far, right? Now another simple fact is that 10k and 10,000 mm mean the same thing. 10k is just much easier to print on a label, and for that matter to write in a blog post so from now on I will refer to the rating in that form. Still with me? Good.
In order for a jacket, coat, or gloves to be considered waterproof in the outdoor industry, or more specifically, the ski industry it must be rated at 5k or higher. Anything below 5k will be in the class of water resistant, aka not waterproof. So far it all seems pretty straightforward and I bet you probably have already figured most of that out on your own. The real mystery lies in the "5k," what the heck does that mean anyways.
The waterproof rating of 5k (or remember it also means 5,000 mm) really refers to a lab test. In order to come up with these ratings clothing manufacturers test their fabric based on how much water one square inch of fabric can hold without leaking in a 24 hour period. So for example a 5k rating means it held out 5,000 mm of water in one 24 hour period without leaking. A 20k rating means it held out 20,000 mm of water. The test is done by placing a column of water (note chemistry terms are not my strong suite ..it could be a test tube, cylinder, beaker, etc...) on the fabric allowing gravity to pull the water down onto it. If nothing seeps through then the fabric is waterproof to whatever level they tested it at.
Now that you understand what the rating means there are just a few more items we must discuss. The next is the difference between DWR and Membrane (Think Gore-Tex) protections. Both are waterproof and both can be rated very high in that department. So what is the difference you may be asking? Well, there is a pretty substantial difference. The DWR stands for "durable water repellent coating." This is a treatment for the exterior fabric where a chemical treatment is baked onto the fabric creating a waterproof barrier. DWR works great and there is no question about that. Where Membrane fabrics excel is longevity. Over years of use the DWR can break down and become less effective. Fabrics can be retreated with a new DWR layer but without it, the waterproof layer will break down years later.
The Membrane system is a layer sandwiched in the fabric to provide the waterproof factor. The illustration below from Gore-Tex makes this easier to understand.
The membrane layer keeps the water out and is protected by the external and internal layers so realistically it will never break down. This will generally give a ski jacket or pants a much longer life while maintaining the waterproof efficiency.
Now that we are really digging into what waterproof fabrics are all about we might as well keep going and touch on breathability. While the waterproof rating is a huge factor in keeping you dry while skiing and/or snowboarding it is not everything. You need to also be aware of breathability. Think of a rubber rain coat, there will be no water getting through that baby but would you want to run a marathon in it? Probably not as it would be like a swamp inside from your perspiration. Skiing and snowboarding may not be as strenuous as running a marathon but you generally will work up a sweat. The breathability of jackets, pants, and gloves will ensure that the inside will stay dry from your perspiration as well.
"It makes no sense now" you might be saying. "How can water stay out, yet sweat can escape too? Its madness." It just so happens that the water molecule created by perspiration is significantly smaller than the water molecule from rain/snow. Therefore the engineers can create a fabric with pores small enough to block the incoming rain and snow molecules, yet large enough to allow your sweat out. Pretty amazing stuff I know.
So now you understand what the ratings mean how do you choose the correct level. Well lets start with this, you know you need at least 5k so do not even bother looking at anything below that. If you are an occasional skier who waits for sunny blue skies then 5-10k should be fine. If you are a bit more serious of a skier and venture out every weekend no matter what the weather is then look at 10k-15k. If you live and breathe the sport then that is where you will see the benefit of 15k-25k. Again make sure to check out breathability to avoid that rubber coat effect.
And there you go, you are now a waterproof expert armed to make an informed purchase for your next ski jacket or ski pants. Check out some really cool options at our online ski shop to see what these great items look like.
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