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What does breathable mean?


 Gore-Tex Membran Erklärung
Breathability: an important topic when it comes to functional clothing

In the outdoor sector, it often feels like you hear this word all the time, and it always seems like the egg of Columbus when it comes to the functionality of outdoor apparel. But what is actually behind the term breathability? Does the clothing actually breathe? And what does it breathe? The outside air, or perhaps our sweat? Why is it so important?

We’ve collated the most important information on the subject of breathability for you.

breath·a·ble [ˈ/ˈbriːðəbl/] : (of a material) allowing air to pass through; Example: the fabric is breathable.
Part of speech: adjective
Use: specialist
Frequency: 1 of 5

Looking in the dictionary doesn’t provide much more than this – and this is usually my first move when I’m asked what a word means. So I did this when the awesome topic “What does breathable mean?” popped up in my to-do list for the Alpinetrek Base Camp blog. Of course, it’s not like I’d never heard the word before. On the contrary, you see it everywhere: from socks to hardshell jackets, it seems these days like no outdoor clothes are released without at least having breathable properties.

It’s like this or similar for anyone who is looking for functional fabrics every now and again: You know the word and what it means, too. “It’s when your jacket doesn’t stick to your body when you’re playing sports, right?” Yes, that’s right! Nevertheless, breathability is a confusing description for this phenomenon – I mean, since when are inanimate substances able to breath? So what’s behind it?

Mann zieht sich ein blaues Shirt über und schwitzt
When looking at breathability, the question is what happens to sweat

If you do sport, you sweat.

A rule that is as simple as it is universal: an active person generates energy! The body immediately converts approximately 20% of this energy into mechanical performance, while practically all the rest of it is emitted in the form of heat. No question about it, energy efficiency is something else – and so the body urgently needs to come up with something to handle this build up of heat.

The solution is two to three million sweat glands housed in the human body’s largest organ, the skin. They are particularly well represented on the surfaces of your hands and feet, in your armpits, on your neck, and on your head and forehead. For an average level of physical exertion, these glands produce 200 to 700 ml of sweat each day. Under extreme levels of exertion or in very hot temperatures, though, up to 1.5 litres per hour of the salty secretion can accrue.

When looking at breathability, the question now is quite simply what happens to this sweat. If it stays on the skin, it can start to get really uncomfortable: sweat can cause heat to accumulate between your body and your clothes, or it can cool down unpleasantly – and the body with it. And this quite apart from the fact that it just doesn’t feel good when your clothes get damp and stick to your skin.

Mann läuft über roten Schotter
Wasserundurchlässigkeit beim Sport
Breathability is about water vapour permeability

Particularly for sports, for really sweat-inducting activities that is, clothing should therefore be able to conduct and transport moisture outside, away from the skin. So the breathability of a piece of clothing really means its water vapour permeability. This can of course be measured precisely, in grams (g) of water vapour per square metre (m²) of surface area over a period of 24 hours: so, for a breathability of 5000 g/m²/24h, 5000g of water vapour can escape from a square metre of the fabric over the course of one day.

Breathability, water vapour permeability or moisture management?

To make things even more confusing, I’ll now throw a third term into the ring: moisture management, or wicking, as some outdoor adventurers also like to call it. When it comes to this subject you have to differentiate between two kinds of water vapour permeability:

  1. Materials that are waterproof and yet still allow sweat to escape
  2. Non-weatherproof materials that enable the active transportation of moisture

Related to the word for the piece of material in a candle, ‘wicking’ as a form of moisture management primarily refers to this second category of textiles. The idea behind this is that individual strands of the fabric actively draw moisture outside, away from the body. But first, let’s take a look at the first category!

Membrane, closed or open

Rain and hardshell jackets usually come with an advanced membrane. It is designed to enable the jacket to reliably keep out the elements while simultaneously ensuring a high level of breathability. This is achieved through two different principles of function and design: microporous membranes vs closed-cell membranes.

Microporous membranes are, exactly as their name suggests, porous, i.e. they are covered in microscopic little holes. They are just big enough that water vapour molecules can escape through them to the outside. The holes are too small for normal water molecules, though, so the material is still completely waterproof. Usually, these membranes are made of polytetrafluoroethylene and a wafer-thin protective layer of polyurethane is also applied. Well-known examples include the GORE-TEX membranes or ones by Gore’s competitor eVent.

With closed-cell membranes, on the other hand, the moisture first accumulates on the inside of the jacket, until the membrane swells a little and the water vapour molecules can finally be transported via piggyback outside – similar to the principle of osmosis. These membranes may be considered significantly more robust, but they only work after a certain time lag. They are mostly made of polyester, like for example the environmentally friendly Sympatex membrane.

How breathable are cotton, fleece, merino, etc.?

Waterproof and hardshell jackets form the outer layer of the clothing – they are the end user, so to speak. But in order for the sweat to get to the membrane in the first place, the base and mid layers have to transport it, too. This doesn’t depend so much on the amount of moisture the material absorbs but on how quickly it releases it again. Cotton, for example, draws sweat away from the body quickly, but also stores it, preventing moisture from being transported all the way through.

Lightweight synthetic fibres such as polyester, polyacrylics, polypropylene and polyamide work better. In standard atmospheric conditions of 20°C and 65% humidity, polyester, for example, absorbs only about 7% of its own weight in moisture. What’s more, it dries very quickly.

layering man and ram
Merino wool is breathable and can absorb a lot of moisture until you feel wet with perspiration

Merino wool is also popular everywhere – it’s a natural material that not only feels comfortable and soft to wear, but it is also impressively functional: it absorbs moisture and transports it outside away from the skin, while also drying very quickly. Rounded off with its odour-inhibiting properties, merino wool always creates a great microclimate.

So at the end of the day…

… breathability isn’t about air at all, it’s about water. To be precise: it’s about moisture and the best way of getting rid of it. These days, the wide world of functional fabrics offers a practically impossible number of solutions and suggestions, some of which have been tried and tested for years, and others which have only just arrived on the market. Of course, there’s no universal recipe, so get on outside and do your own field research!

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