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BLOG | TECH | 04 October 2016

The Secrets Behind Membranes

Everything about the biggest advance in the history of mountaineering clothing: membranes such as Gore-Tex, which are both waterproof and breathable at the same time.

Facing up to a blizzard; a membrane is essential. Photo: Haglöfs
Garments for mountaineering activities that are breathable and waterproof probably mark the biggest advance in mountaineering clothing in the last century.

This waterproofing capacity and breathability is all down to the garment’s major component, the internal membrane.

We will try to explain what types of membranes and finished fabrics we can find in the market and look at some examples.

There are some interesting basic concepts about membranes and it helps to understand what waterproofing or breathability are, what resistance to water means, or what are the different treatments to repel water.

We advise you to read this first part carefully. It will help you resolve many of your doubts when choosing your garments. You will understand how they work and know what to choose depending on your needs.

The thing is, simple concepts such as waterproofing aren’t always exactly as we expect them to be.

Waterproof or Breathable Garments?

In 1976, Bob Gore marketed the first garments that were not only considered waterproof, but were also breathable, wicking sweat away from your skin. This was astonishing for the time.

It was something that was incomprehensible at the time, and it seemed contradictory and absurd. Many didn’t believe it and even today it isn't clearly understood by many people. How can a textile garment expel humidity from the inside, and at the same time not allow it to come in?

The story began in 1971, when Bob Gore discovered in his laboratory, in his parents’ basement, the expansion possibilities of polifluoroetene (PTFE, commonly known as Teflon, accidentally discovered in 1938 by Roy Plunkett, while working in New Jersey for DuPont).

This discovery of ePTFE (expanded PTFE) led him to invent a product that would not only revolutionize how millions of mountaineers would carry out outdoor activities, but also much more, from guitar strings to the world of medicine.

But, how is it done? Well broadly speaking, if you stretch a piece of Teflon, it starts to create small holes (pores), which is similar to what happens when we stretch the membrane that surrounds certain cuts of meat, or chewing gum.

Photograph of a Gore-Tex membrane under a microscope
When PTFE is expanded in order to create ePTFE, Gore-tex creates a micro-porous material made up of 70 percent air. These micro pores are what allow the internal humidity to escape.

To get an idea of how microscopic these pores are, in each square centimetre there are 1,400 million.

How the ePTFE waterproof and breathable Gore-tex membrane works

In reality, it isn’t so difficult to understand why, in a porous membrane, humidity can escape. Looking at the photograph of the microscope, there is no doubt.

However this doesn’t fit in with the other part of the equation: How is it that humidity can get out and at the same time can’t get in? The answer lies in the molecular dissociation that is produced in water molecules once turned form liquid to gas form.

In other words: rain water that falls onto the jacket in liquid form, can’t penetrate to the inside of the jacket because the molecules are bigger than the size of the pore.

On the other hand, as sweat evaporates, due to body heat, it can get through these pores because the molecules have become disassociated.

A bad day in Aiguille des Cosmiques. A membrane jacket is essential. Photo: Carlos PĂ©rez
What does “waterproof” actually mean?

Strangely enough, if, by waterproof, we understand that something that prevents water passing through, then there are very few things that are really waterproof, not even plastic. Of course under this pretext it would be impossible for a garment to be both waterproof and breathable.

How can we explain this? Instead of talking about waterproofing, we should talk about water resistance. A garment is considered waterproof when it’s resistance to water is high enough for it to hold up to the kind of weather conditions found in the mountains.

And how do we know what is considered waterproof, and what isn’t?

At this point we must say that there isn’t a unified standard in the outdoor industry that can inform us, but there are some very strong indicators.

The Measurement of Water Resistance

The tests are conducted under the following norm ISO 811:1981 (Textile fabrics – Determination of resistance to water penetration – Hydro static pressure test). This specifies how to conduct a water resistance test of fabric in a laboratory.

To measure this resistance in simple terms, they employ an apparatus that measures the “water column”. A fabric is placed horizontally and pulled taut, without anything touching it from above or below. Then a square tube 1×1 inches is placed on top, and it is filled up with water. This increases the pressure, until the water passes through the fabric.

The measurement in millimetres of the height of the water indicates the grade of waterproofing.

That’s why jacket labels often indicate the “water column” which is measured in mm. The higher the figure, the better the waterproofing.

Why does a tent a have a 3000mm water column and a jacket can reach 20,000mm?

Lets turn this question on it’s head. How can we know the water resistance of a jacket from the measurement in mm of the water column, when we consult the specifications on the label? Here is the answer.

  • 0-5.000mm: Zero is no resistance (a technical t-shirt, a shirt etc.), 5.000mm is a low resistance, light rain dry snow etc.
  • 6.000mm-10.000mm: Light to moderate rain, moderate snow.
  • 11.000-15.000mm: Moderate or heavy rain, heavy snow.
  • 16.000-20.000mm: Heavy or very heavy rain, intense blizzards.
  • More than de 20.000mm: Very high protection.


These figures, in general, are accepted by the outdoor industry. However, in other areas we can hear that something is waterproof with a water column measurement of just 2500mm. That’s 10% compared to some high range jackets. What is the explanation?

There are various reasons. The first one has a lot to do with pressure. We have seen that the test measures the height of the water contained in a tube. This means that the pressure of the water increases as we add water, until it passes through the fabric.

Of course, any pressure the jacket receives from the outside results in the water penetrating much sooner.

A clearer example are the straps on a heavy backpack. Not only do they cause pressure on the fabric, they are also in a place that is much more exposed to the rain. It isn’t easy for rain not to penetrate in that zone, especially when subjected to continuous rubbing against rocks, or the different elements of the garment itself. This makes it essential to significantly increase the “laboratory” water resistance, for safe use in the outdoors.

Another important aspect is the exposure to weather. Although the majority of the water that falls onto a jacket just runs off, after several hours of exposition the water will penetrate the garment. In fact, and as mountaineers know, there are very few garments that w

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