Rain wear like a hardshell rain jacket is mostly waterproof and breathable: sweat will go out and rain can’t go in and so we stay dry. But what is waterproof actually and how is it measured? Read the article and enjoy the video!
This article and the accompanying video is the second in a small series about outdoor rainwear. The first article and video answers the question: How can a fabric be waterproof and breathable in de the same time. The link to that video is below this article. Now: what is waterproof exactly?
Waterproof is of course in our outdoorish life a material that water cannot penetrate. And for a long time, this was visible with the naked eye: water on one side, no water on the other. But with breathable membranes and coatings this changed and a method was needed to measure how waterproof a material is. The method that became the standard is the hydrostatic head or water column test.
Hydrostatic head or water column test
You have probably seen the result of the hydrostatic head or water column test in reviews, advertisements or the specs when you were looking for a new rain jacket. You would find a number like 5K and 5000 mm or 10K or 10.000 mm. The numbers mean that the fabric can resist a column of water on top of it of 5000 e.g. 10.000 mm, yes 10 meters of water! Of course this testing is done officially in laboratories and there is a lot of mumbo jumbo about the execution of this test.
30 Meter water column
Most explain that the actual test is done with samples of a fabric that are pulled over several tubes of a certain diameter and that those tubes are filled with a certain amount of water for a certain amount of time. For the lower end of the water column test this is quite doable; a tube of 3 meters is not that high but 30 meters… In real life the hydrostatic head or water column test is performed with a machine that builds up pressure on the fabric. Up to a pressure of 1 Bar – this equels 10.000 mm – a Sutertest device is often used and for the hydrostatic head test a … hydrostatic head tester. The latter is bigger and can build up higher pressures; Lab safety seems to be an issue here to.
Now, back to the question: what is considered waterproof? Legally a fabric is waterproof if it can withstand a water pressure of 0,13 Bar or 1300 mm. That is not a lot. The answer to what is considered waterproof varies a lot from brand to brand and even amongst ‘the people who know’. In general a rating of 3000 mm is the lower end of the waterproof scale and 28.000 at the Mount Everest highest point of the scale.
I do believe that higher is better if you spend a lot of time in mountains in bad weather conditions. But… it is not only weather that demands waterproofness. The shoulder straps or the hip belt of a backpack apply pressure too. Even if you kneel down in wet grass the pressure on the fabric on your knees is about ¾ of your bodyweight and that is a lot of pressure too that the waterproof breathable fabric must withstand.
Waterproof versus water resistant
Just to complete this article on what is waterproof: What is the difference between waterproof and water resistant? A fabric is water resistant if it can counter a pressure of 0,08 Bar or 800 mm so the difference is 0,05 Bar/500 mm. Now you know what waterproof is!
I wrote a couple of more in-depth articles about waterproof and breathable fabrics. See the list below.
- The first is about ‘How does Waterproof and Breathable Rainwear work?’.
- I also made a list of waterproof and breathable fabrics that are produced and that finds its way into our rainwear.
- The third article is about ‘What is considered to be breathable and how is it measured?’
- If you are looking for a new rain jacket or hardshell? I reviewed a couple and this is were they are: Hardshell Rain Jackets Reviews.