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 Pérez
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 will withstand penetration, albeit in specific points, in a full day of heavy rain in the mountains.

A long day of sleet and rain in the mountains can saturate membranes
Therefore in real conditions, water resistance is marked by the relationship between the water column, the external pressure and the amount of exposure time.

Lastly, wear and tear, and stains destroy the properties of the membrane. For this reason the Gore-Tex membrane has an ultra thin layer of Polyurethane to protect it, and others like eVent, have an anti-oleic treatment in their fibres, to prevent loss of performance. In this way, even after extreme wear and tear, they continue to maintain a high proportion of their water-resistant properties.

The laboratory tests mark a reference, but in real use in the field, different internal factors that are not taken into account begin to form part of the equation. This obliges each manufacturer to conscientiously study the necessary characteristics that meet up to their quality standards.

And the tent?

In answer to the original question… why is it OK for a tent to have a water column of 3000 mm but not for a jacket?

An expedition tent, prepared for the worst storm, can have a water column in its double roof of between 1200mm and 3000mm only, and of 10,000mm in the floor.

If you think about it it’s logical. Remembering what we said before: a taut double roof will hold up to while it isn’t brushed, so the water column in the laboratory is very similar to the field conditions (2500mm is already considered waterproof). The floor must withstand the pressure of having someone inside putting pressure on it, and with a water column of 3000mm, the water would penetrate. Up to a point it has the same problem as the jacket.

Perhaps now we can understand better why, and as so many of us have experienced, when we are inside a “normal” tent, if our backpack touches the walls while it’s raining, water will penetrate the fabric.

If waterproofing indicates the resistance to humidity penetrating the fabric, and the higher the resistance the better the fabric is, breathability is the complete opposite. It indicates the capacity that the humidity has to pass through the fabric, where a lower resistance means a better garment.

And here is where we find the paradox of these fabrics… and the great difficulty in their designs. There are very few textile or protection materials that incorporate the technology, the I+D and the years of investigation that a high quality mountain jacket has.

This question has a simpler answer to that of waterproofing which, in reality doesn’t really exist in mountain garments. We have seen the key features at the start of this article.

The breathability of a jacket indicates its capacity to evacuate internal humidity (mostly caused by sweat) and keep us dry.

That’s why there’s no point comparing them for example with a thick waterproof plastic used to stay dry when it rains. Plastic is incredibly waterproof, but unless the conditions were freezing and we didn’t move, we would literally end up soaking wet with our own sweat. This would result in losing just as much body heat as if we were not wearing a waterproof garment, with the added complication of dehydration.

So breathability indicates the capacity to evacuate water vapour in the form of sweat from the inside of our jacket?

When our body overheats, it exudes sweat onto the skin. Body heat causes this to evaporate, and in doing so reduces body heat, lowering the temperature of the skin and of the organism. It’s the body’s way of cooling down.

In other words: sweat evaporates because of body heat. As water vapour, it is able to “fit” through the micro pores of the membrane and evaporate to the outside.

Wind and rain stop us after intense exertion. Sweat would make us cold if it wasn’t for breathability.
As you can imagine however, the process isn’t as fast as with a fabric that isn’t waterproof. Our sweat evaporates bit by bit.

This is why, when a jacket is waterproof and breathable it doesn’t mean we will always be dry. It means that the fabric will help, humidity will be minimised and will avoid serious problems.

We all know that membranes “get saturated”, but sometimes exaggerated complaints are heard. We must try to keep things in perspective. If we compare this type of garment with a plastic jacket or an old fashioned sailors coat, there is no competition.

Measurement of Breathability

Breathability is measured in a very complicated way. Factors such as external conditions, (relative humidity and atmospheric humidity, the difference between internal and external temperatures) have a big effect.

In Europe there is a reliable method, but it should only be used for comparison. It doesn’t indicate the true breathability in every moment, because this depends to a large extent on the conditions. However being a precise and set norm, it allows us to compare the different types of membrane, being from the same manufacturer or comparing between different manufacturers. This is the European norm ISO 11092.

This is expressed as WVR (Water Vapour Resistance) measured in m²/pa/w. It indicates the resistance the membrane has for allowing humidity to escape. The lower the WVR value, the better the breathability.

Some brands may express breathability in grams. This indicates the quantity of humidity that they can evacuate in a certain amount of time. In this case, the higher the figure, the higher the breathability. However few well-known outdoor brands in recent times use the WVR measurement.

This is possibly because these tests are more suitable for industrial use than for mountain conditions, where the product is used by people which means it produces a difference in the internal temperature (minimum 36º), and the exterior.

Measurements of breathability are useful for comparing different garments

Breathability in real life depends on many factors. Some of them are in our hands, such as the system of dressing in layers.

Up until now we have talked about the membranes, and later on in the article we will discuss how fabrics are integrated into the garments (2 layer, 3 layer and 2.5 layer). But before that, we are going to explain a technology that isn’t used directly on the membrane. It is found on the exterior fabric and is essential for the correct functionality of the membrane. It is water repellent treatment or DWR.

DWR, Durable Water Repellent

DWR stands for Durable Water Repellent, and practically all of the exterior fabrics used for jackets and trousers with a membrane (and sometimes without one), use them in one way or another.

This system avoids water soaking the outer fabric. The thing is that if it did get soaked the garment would still be waterproof (thanks to the membrane). However the jacket would become heavy, and worse still, it would lose a lot of its breathability.

This is because humidity would create a layer that would block a large portion of the pores which allow water vapour to escape. The water wouldn’t get in, but the vapour would have serious problems in escaping.

The bad news is that with use, wear and tear and stains, the DWR treatment on the garment deteriorates. The good news is that this isn’t irreversible.

There numerous products specialized in regenerating DWR fabrics. In fact, correct maintenance can bring a garment back to life.

DWR treatment, the water form drops, not a sheet
Despite what it may seem, DWR treatment is physical. What is does is create, on a microscopic level, a rough surface on the fabric, in a zigzag. As the drop doesn’t fall onto a smooth surface, it stays as a drop. As a result it can’t penetrate, as you can see in the image. Overtime the roughness flattens, although with the correct treatment the fabrics can return to their original condition.

But one can talk about this so much (maintenance, cleaning and recovery of waterproof garments), that we will discuss it in an independent article.

How do we use the membrane? Two layer or three layer

Now we know all about the membrane. However, we have to understand that this is an internal element, hardly 0.01mm thick. To use it, we have to laminate it. In other words, we fuse it an exterior and interior protective textile layer.

Laminated: An exterior layer, the membrane in the middle and an interior layer.
There are various ways to do this. According to the use, we will have the famous names “2 layer”, “3 layer”, and even “2.5 layer”. It’s become part of the name in fact that many mountaineers will refer to their jackets according to the number of layers they have.

However, we have to understand what these names mean, because it isn’t very intuitive…

Gore-Tex 3 layer:

It’s a “sandwich” with a protective outer layer (the material that we see), the membrane, and an internal lining. The most technical and sporty garments are made of three layers, as they minimize weight and volume and give more freedom of movement.

We can see how the layers weld together, making one single material
The 3 layers are integrated, making what appears to be a single layer

Gore-Tex 2 layer:

An external protective layer and the membrane, integrated together. To avoid that the membrane becomes damaged, an internal mesh lining is incorporated on the inside.

2 layers, exterior and membrane. A webbed lining is added on the inside

We said that the name can cause a misunderstanding because the user, in the case of Gore-tex 3 layer will see and have the sensation of only one layer. In the case of Gore-Tex 2 layer, they will see and feel two layers; the joint exterior layer welded with the membrane (the two true layers), and then the mesh lining.

It is heavier and bulkier.

Gore-Tex 2.5 layer is two layers that, instead of putting a webbed lining, has a type of relief drawing on the inside to minimize the rubbing and wear on the membrane. It’s not as long lasting as the other types of Gore-Tex.

There different types of membranes and their use must be adapted according to the activity. It has to be made clear that we are talking about the membrane here, each of which can be laminated in 2 or 3 layers.

This is the basic one, but it is very effective, with very high waterproof and windstopper capabilities. It has a lower breathability ratio, but depending on the use it can be the best option. It is an excellent membrane, and is more than enough for less strenuous activities such as hiking or mountaineering or for use in the city.

A perfect day for the classic Gore-Tex membrane
Gore calls it “daily use technology”. For a reasonable price it has amazing capabilities. One of the big advantages of this membrane is that it can be laminated onto smooth and flexible materials, which means that the garments are very comfortable.

In other words: it is less breathable than the Pro membranes, but for anyone that doesn’t do intense alpine activity, they will have a garment that is more comfortable, and cheaper. It’s unlikely they the membrane hard enough to notice the difference.

As we said, the construction can be of two or three layers. Gore incorporated an interesting innovation last season. The new “C-Knit” lining increases comfort considerably with a silky touch. It is lighter and more flexible and breathability has been increased.

GORE-TEX® Active
This is the latest membrane of the range to be incorporated. It was born as a result of huge increase in the number of people doing intense activities in the mountains in the past years, especially Trial running, Ski touring, or MTB. They are light, and above all, highly breathable. (They have WVR <3. This is such a low resistance that until recently it was only possible on membranes that hadn’t been laminated.) One of it’s qualities is that it is nice to the touch on the inside. This is important because it is normal for the jacket to be directly in contact with the skin due to the type of use.

The North Face Hyperair Jacket, a perfect example of a waterproof breathable jacket for aerobic activities
Last summer The North Face, together with Gore-Tex, gave an important twist to this type of jacket by presenting the Hyperair GTX jacket. It has no internal lining, is ultralight, and boasts a WVR hard to compete with.


There are a small number of membranes in the market, and not all of them are efficient. However, some are very interesting. One of these is independent, and several are marketed by specific brands for their own products:

eVent works especially well, with a very high ratio of breathability. It’s employed on occasions by mid range brands instead of Gore-Tex. Essentially, it’s a micro porous membrane, just like Gore-Tex, but instead of a PU laminate to protect the membrane, the protection is added to each filament, which decreases the WVR.

The North Face has its own membrane called DryVent, which it uses in some of it’s mid range garments (in the rest, it always uses Gore-Tex). It has been completely tested, and has the same qualities as Gore-Tex.

Patagonia, which also employs Gore-Tex in the majority of its range, employs its own membrane in some of its mid range products, called H2No®. As in the previous case, it has very good capabilities for a membrane of this type.

With so many options available in so far as membranes and formats, it’s important to look for a garment that best adjusts to the activity you are going to do. We must look for the optimal balance between waterproofing, breathability, durability, agility, comfort and price.

Visit our online store to explore the full range of products with waterproof and breathable membranes that we have to offer.

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