Worst case scenario: it’s the second day of a two-week camping trip. Your backpack is stuffed with the necessary equipment and the only clothing you have with you is what you’ve got on your back. Extra clothes get left behind to save weight and space. But then, the weather takes a turn for the worse and doesn’t look like it’s improving any time soon. The worst thing that can happen to you now is your clothes get wet. You have nothing to change into and you don’t know when you might get an opportunity to let things dry.
You’ll have heard the annoying cliché that “there’s no such thing as bad weather, only inappropriate clothing”? Well, there’s a lot of truth in it. In a situation like this you need a good waterproof layer to protect you from the rain and the cold and of course the hotspots from rubbing rucksack straps and clothing.
But how can you tell the difference between good and bad waterproof clothing? What will really keep you dry?
It all comes down to a standard
The standard is the way in which the water permeability of a fabric or a membrane is measured. Or simply put, the amount of water that can get through a fabric or a membrane within a certain amount of time.
This involves exposing the outside of the material to water. The water pressure starts at zero, and the hydrostatic then increases by 100 millimetres or 600 millimetres per minute, depending on the standard.
The time it takes for three drops of water to make their way through the fabric is measured. The height of the water column above the material before it leaked is measured in millimetres of water and forms the standard. Just remember the the higher the number, the better.
As always, when standards are involved, things get complicated between countries. Here goes. For clothing, the European standard DIN EN 343:2010-05 (protective clothing against rain) comes into play.
Several factors are taken into account here, such as:
- the water column
- the water vapor transmission resistance (RET value).
Whether you get wet from the outside or inside rarely makes a difference. But more about that later. Here are a few exemplary hydrostatic heads and what the values mean:
- 1,000 mm: pressure created by a person lying down, weighing 80 kg and 1.80 m tall
- 1,500 mm: value above which textiles are generally considered waterproof in Germany
- 4,000 mm: according to the Swiss Federal Laboratories for Materials Testing and Research (EMPA) in Switzerland, waterproof.
- 5,000 mm: pressure created by a seated person weighing 80 kg
- 10,000 mm: waterproofness for good outdoor clothing
- 14,000 mm: pressure caused by a kneeling person with 80 kg
What are the methods of measuring the hydrostatic head?
There are various methods for determining the hydrostatic, which can often confuse the end user.
Basically, the hydrostatic head is determined by a water pressure test – usually the Suter test. Here, the material is subjected to increasing water pressure. This is done until the material allows water to pass through. When the third drop has penetrated to the inside, the test ends. The time elapsed until then determines the hydrostatic head.
However, the amount by which the water pressure is increased in the test procedure differs depending on which standard is used, and the laboratory conditions are also not identical. This is the first reason why the hydrostatic head should rather be understood as a guideline value, to which other factors such as taped seams, the number of layers used or the general processing of a material should be added.
Then there is a second crucial difference to consider with hydrostatic heads specified by manufacturers. This mainly concerns tents. The hydrostatic head is determined differently in the USA than in Europe. There, the material is subjected to various rapid aging processes to simulate five years of use of the material.
The hydrostatic head is then measured on this basis, which naturally leads to significantly lower values than for European hydrostatic heads. Here the practical tests could often prove that an American hydrostatic of of e.g. 3000mm is denser than a European one.
Basically, you can multiply an American hydrostatic head by a factor of 2-3 to get approximately the European standard.
So how much of a hydrostatic head do I need?
As always, this depends on what you are planning to do and where? The question is, how much water will the fabric have to deal with? A decisive factor is whether or not pressure is to be applied to the material as well as water. Will you just be standing in the rain with your rain jacket or will there be strong wind hitting the jacket, as well? Will the straps of a heavy backpack also be applying pressure on the jacket and forcing water through the fabric?
As already mentioned, high pressure acts on the clothing when sitting and kneeling. It is similar when lying, sitting and standing in a tent. For this reason, you should make sure that the hydrostatic head is as high as possible.
Here are a few examples:
- Tent floor and tarpaulin: at least 5,000 mm
- Rain or ski trousers that allow you to kneel or sit on wet ground: 15,000 mm or more
- Hardshell jacket designed to carry a heavy backpack: from 20,000 mm
A significantly lower hydrostatic head is perfectly adequate as long as weatherproof garments and textiles are not exposed to any particular pressure.
- Tent tarpaulins, outer tents and tarps: at least 3,000 mm
- Rain trousers that are primarily designed to protect against splashing water: 10,000 mm
- Running and cycling jackets that are not designed to be worn with a backpack: 10,000 mm
In other words, the boundary between waterproof and water-repellent is a grey area and depends on several factors:
- Type of water exposure
- Duration of the effect
- Pressure of the water on the fabric (e.g. due to wind, backpack or the wearer)
What values are there for jackets?
But here’s the good news. All hardshell jackets are in fact waterproof. At least, all of the ones in our shop. Most manufacturers, such as Mammut and Jack Wolfskin start at a hydrostatic head of 10,000 mm, which means they are way above what the standard defines as waterproof.
Most hardshell jackets can deal with 20,000 mm and some even get close to 30,000 mm.
But you should always keep in mind that it depends on what you are planning to do with the jacket (or trousers). And remember that these figures apply to new jackets. Used and worn fabrics and membranes quickly lose the properties they had when new.
Waterproofing your jacket on a regular basis is vital. Water will bead and roll straight off of a well-waterproofed jacket giving the wind no chance to force it through.
It’s also worth noting
The hydrostatic head only refers to the fabric itself. There are a number of other factors that influence the protective function of a jacket as a whole. If a front zip is of a low quality, for example, it may let water into the jacket, particularly in a strong headwind.
Poorly-designed hoods are another factor that should not be ignored. Taped or sealed seams are also very important because they can protect weak points.
The breathability of your jacket is extremely important of course. What use is a completely sealed jacket, that cannot transfer your sweat to the outside?
Getting wet from the inside is no better than getting wet from the outside. Although, sweat is somewhat warmer than rain to start with. The decisive factor here is a membrane like GORE-TEX, which offers some breathability.