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Waterproof Clothing Ratings Explained

We've been on the hunt for new waterproof layers recently, what with all the rain we've had recently showing us how old some of our kit is getting. And with winter on the way and brands filling their online stores with this season's gear, we figured we'd pass on some of the things you need to look out for when buying waterproof clothing.

Waterproof clothing in a nutshell

Weirdly, waterproof fabrics don't actually repel water themselves, but are instead normally treated with a solution called DWR (Durable Water Repellent) so it doesn’t soak up water, or wax in the case of some wax cotton jackets (looking at you, Barbour and Fjallraven). This is the stuff that makes water bead off your jacket. If you start to notice water isn't beading up any more and is starting to be absorbed into your jacket this shows the DWR coating needs reproofing. (Nikwax tends to be our preference).

Just as important but not always immediately considered is breathability, which allows your own body's moisture and vapours to be released, allowing the drying process to happen quicker.

In short, a rubber raincoat is going to be completely waterproof, and may be the ideal garment for standing in the pouring rain aimlessly, but if you tried to ski or hike in it, you’d be drenched pretty quickly from your own sweat, defeating the point in a waterproof and turning you in to a walking wetsuit.

When you are looking for the right waterproof, it is important to balance protection from wet things on the outside with the ability to let your sweat escape from the inside. Simple, right?

How do waterproofing ratings work?

Manufacturers typically describe the waterproof and breathability ratings of fabrics using two numbers, typically separated by a backslash. The first number is waterproof, the second is breathability. Pay attention to both! These ratings are the result of lab tests, and the higher the rating, the more waterproof, and the more breathable the item is.

There are a number of different testing protocols in use, but in the UK this involves what's called a Hydrostatic Head test, where the fabric is pulled tight under a 1 inch diameter sealed tube of water. This is then reviewed over 24 hours to determine how many millimetres of water the fabric can withstand before water gets through. The UK's standard requires at least 1,500mm for something to be called waterproof. Some manufacturers have also developed their own testing methods that involve adding pressure to the process to simulate the effects of wind, but always make sure you pay attention to the base mm of resistance.

What waterproof clothing rating do I need?

Jackets with a rating of 1,500mm to 2,000mm are suitable for everyday use, whether commuting, moving outdoors or walking the dog. These jackets are also suitable for anyone walking or hiking who isn't likely to get caught out in heavy, sustained rain. These may be described as 'showerproof' in stores or online. Anything less than 5,000 however will leave you wet and cold in a storm. 5,000 mm is also the minimum waterproof rating for ski and snowboard jackets, and is also considered suitable for 'dry snow'.

Anything between 5,000 mm and 10,000 mm is perfect for spending long days outdoors through cold and wet weather conditions, such as winter hikes or trips to the Lake District! It can withstand rains and winds, making it ideal for hunting and fishing. Waterproof ratings beyond 10,000 mm are best suitable for avid skiers, snowboarders and mountaineers. 

How do breathability ratings work?

Just like waterproof ratings, breathability ratings are determined by both manufacturers and independent labs, but in this case the testing methodologies are fairly diverse and almost harder to compare.

However generally within a given brand or family of fabrics it’s safe to say more grams is more breathable (if the brand grades breathability on an RET scale – Resistance to Evaporative Heat Transfer – a lower value is better).

As we said earlier, you can get away with a lower breathability if you aren’t going to work up much body heat or maybe you won’t be on a hill that long. 5,000 to 8,000 grams will probably be fine. Likewise, if you’re just wearing the jacket around town, you don’t need to focus as much on breathability. Higher breathability is better for mountaineers, climbers, cyclists and anything else that requires more exertion than one foot in front of the other on the flat.

If you do a lot of “high energy” stuff, hiking, or running where you often break a sweat, look for anything in the 10,000 to 15,000 gram range or above. Backcountry hikers, runners, and mountain bikers should look for garments with breathability in the 20,000 plus range for the best results.

Having a layer of warm, moist air between your body and your shell isn't always a bad thing as long as your base layers don’t become saturated with moisture however. In cold and dry weather, a very breathable shell can actually lead to visible clouds of vapor exiting your body, which leads to heat loss so make sure you pick the right kit for the right conditions, for improved performance.


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