Guide to Functional Membranes

Hiker in rain with rain jacket and backpack

Outdoor clothing is all about functionality, because the clothing should be breathable and preferably also waterproof. In the following, we give you an understandable and practical overview of the topic of functional membranes and explain what you need to know about membranes, breathability, water column value and Co. and what you should look for when buying.

Hiker with rain jacket over a fjord

Why are membranes important?

All of us will be familiar with the following phenomenon: Anyone who engages in sporting activity, whether it's an evening jog or climbing a high mountain, will inevitably start to sweat at some point. This is exactly where the membranes come into play!

They ensure that you stay comfortable and that you don't sweat too much in your waterproof jacket on a rainy day, for example. In this case, a good membrane should ensure that the sweat is transported from the inside to the outside, while the rain should not penetrate from the outside to the inside.

What is a functional membrane?

  • A functional membrane is like a film with tiny holes, so-called pores.
  • The microporous structure is extremely light
  • Breathability is ensured because sweat molecules can escape through the pores
  • A membrane iswaterproof because the pores are too large for water droplets to pass through
  • Result: rain beads off, sweat can escape
Hiker with rain jacket on a rainy day at the lake

What is the water column?

The water column is a unit for measuring pressure, which in the textile sector indicates the waterproofness of technical fabrics.

To determine the water column, the outside of the material is exposed to water. A tube is placed on the smooth and stretched fabric, which is then successively filled with water. In a measurement, the water pressure starts at zero and is then increased. The higher the pressure the fabric can withstand, the more waterproof it is.

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Orientation values for the water column

There is no uniform standard that defines when a material or garment can be described as waterproof. Depending on the product and the area of use, there are decisive differences as to which water column value should be achieved.

The following water column values can serve as a rough guide:

  • 5,000mm: A tent should at least reach this value, but rather 8,000-10,000mm.
  • From about 10,000mm: Outdoor clothing suitable for most weather conditions should reach at least this value
  • 14,000mm: Pressure created by a kneeling person weighing 80kg
  • From about 15,000mm: Rain or ski pants that can be used for kneeling or sitting on wet surfaces
  • From about 20,000mm: Hardshell jacket designed for carrying a heavy backpack and prolonged rain

How does the breathability work?

When we sweat, the body produces liquid, which then evaporates due to the heat of the skin, thus cooling us down. Breathability is about how much water vapor can pass through the pores of the membrane to the outside. The more, the better.

When you sweat, the temperature and concentration of water vapor rises inside your clothing, and so does the pressure, creating a temperature and moisture gradient (actually a pressure difference) between the inside and outside. As a result, the resulting water vapor migrates through the microporous structure to the outside. The higher the temperature gradient between the inside and outside, the better this process, which is called diffusion, works.

With their breathability, good functional membranes therefore ensure that as much moisture as possible is transported to the outside.

Graphical explanation of breathability in textiles

Why do jackets have multiple layers?

Why should a good functional jacket have several layers? Outdoor jackets in particular are often exposed to high stress in the shoulder area, as the straps of the backpack rest there. The multi-layer system ensures that a protective layer can be applied over the functional membrane. This guarantees that the membrane underneath is subjected to as little stress as possible and does not lose its function through direct friction or dirt.

The functional, but also sensitive, membrane is thus laminated onto an outer protective layer. There are different forms of laminate, the 2-layer, the 2.5-layer and the 3-layer variant: the more layers the jacket has, the more robust it is.

Multi-layer clothing lasts longer and guarantees excellent performance even after many uses, as the functional layer with the membrane is better protected. It therefore makes perfect sense to purchase a piece that is a bit more expensive, but also lasts much longer than a single-layer alternative.

What is GORE-TEX?

For more than 40 years, GORE-TEX product technologies have provided the highest level of protection and optimum comfort for outdoor enthusiasts. Therefore, numerous manufacturers rely on the functionality and experience of GORE-TEX and use the membrane in many of their products.

The GORE-TEX membrane is not only waterproof and breathable, but additionally windproof.

The membrane has more than 1.4 billion pores per square centimeter. These pores are 20,000 times smaller than a drop of water, but 700 times larger than a water vapor molecule. This allows sweat to escape in the form of water vapor, but rain cannot penetrate.

Graphic explanation of how GORE-TEX works


The membrane is a decisive factor for the functionality of a garment: With membranes it depends on what gets through (namely as much sweat as possible) and what does not get through (namely as little water as possible and possibly wind).

Exactly which garment or product is right for you depends a lot on what you want to do with it and what purpose it will serve. We hope that this article has shed some light on the subject and will make your purchase decision easier. If you have further questions on the subject, just stop by one of our Sport Bittl stores, our knowledgeable staff will be happy to advise you!

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