The onion principle refers to the layering of clothing during outdoor activities. The conventionally known and 'learned' onion peel principle is outdoor in my opinion, however, not optimally adapted to higher loads than walking.

When choosing the right clothes, it's basically about Protection from the elements like rain, snow and wind! Clothes should be selected according to this principle. The second principle is especially important in sports activities: the physical effect of evaporative cooling, When water evaporates on a surface, energy is released. This leads to a cooling effect. This effect is now to use intelligently for yourself: in summer moisture or its evaporation can help to cool the body to the optimum operating temperature. In the winter Of course, this cooling effect is absolutely to be avoided. In short: sweat in summer should and can cool the body, in winter this can have serious consequences. With the onion principle, you can control these effects.

For climbing, mountain hiking or trekking, so in mountain sports in general, but you should not be too much of wonders materials let the manufacturer impress. Of course, these not only offer the right clothes for every imaginable situation, they even awaken needs and create situations that lead to higher sales figures but do not correspond to the actual field of application.

Conventional onion principle

The traditional onion shell principle is based on the idea that several layers of functional clothing are worn one above the other. Adapted to the climate and the weather, different layer thicknesses and lengths are combined, from inside to outside: sweat-transporting underwear (Base layer), about one insulation layerif necessary, and then a breathable and / or waterproof layer (hardshell or Softshell - depending on the weather either with waterproof membrane or only windproof material).

In principle, this layered look is a good basis for walking, but it has to be significantly concretized and carefully adapted for sweaty outdoor applications such as hiking, mountaineering or running and trail running. In this article, we address other important issues that should be considered when choosing functional clothing.

Often (consumption!) too much clothes and too warm clothes taken. Kilos quickly come together that are never worn or worn wrong anyway. Who does not know them: the perfectly equipped, the summiteers wrapped in chic black softshell parts who torture themselves with their heads red and sweaty through the midday sun. I confess, I tend to do it myself. One packs in his biggest fears and sacrifices these fears even on the way a certain buoyancy and naturalness.

Here is a brief overview of the layers and the underlying tasks of the individual layers:

1st location: Baselayer

The first layer in onion principle is worn directly on the skin, so called underwear or Baselayer. This must absorb the sweat and pass it on to the next layer. The material itself should not hold moisture, but only continue to transport.

The biggest mistake made here is to use cotton. Cotton absorbs the moisture, but does not give it off and is so wet and clammy. In addition, the cotton fibers collapse as soon as they are wet. These fibers have a smaller surface area and lie close to the skin. Water is a good conductor of heat, so the wet, collapsed cotton fibers provide for cooling. 

The so popular parts out merino are also not really optimal, because they also hold the moisture. The wool fibers collapse when wet but not as strong as cotton, so a moist Merinoteil is perceived as more pleasant than a wet cotton part. 

A functional T-shirt made of polyester or another functional fiber is still the best choice, as it absorbs hardly any moisture but directs it further. Such functional laundry must also be as tight as possible, so that the sweat removal is guaranteed.

2nd situation: isolation

The isolation layer comes on the Baselayer. This layer provides for heat retention and should be adapted to the weather according to their thickness and heat output. In the cold, the midlayer can also consist of several layers. Here is also true, as in the Baselayer: a false insulation layer sucks with water full. A cotton hoody may look good, but is soaked after a sweaty ascent and no longer isolated. The second layer should be airy and breathable, and should not be windproof or watertight, as these properties make it difficult to steam through. This is, as discussed below, the reason why down or synthetic fibers are better "on top".

3rd location: weather protection

The outer layer of the onion protects against the effects of the weather, keeps out wind and / or water and should be robust enough for the intended use. The weather protection and the associated membrane technologies are a science in itself, so I explain here only the absolute basics:

• Softshells are windproof and water repellent - but not waterproof. But they are - usually - much more breathable than rain jackets. Of course, there are also different processing qualities here. Some softshells on the market have made use of a windproof membrane that even protects against moisture from the outside for a certain period of time. However, such a membrane naturally lowers breathability. In part, the vapor permeability through the membrane is so reduced that, in terms of breathability, they have no advantage over a rain jacket. Example: the MVTR of a cheap rain jacket is 6000g / m2 / 24h, the softshell is here at a value of 7000th Although this difference is measurable, whether he is also noticeable, I doubt it.
• Rain jackets are also windproof, and also waterproof. This is usually achieved by a microporous (or even a hydrophilic / hydrophobic and thus non-porous) membrane, which can have different qualities and properties. Rule of thumb: the more expensive the jacket, the better the built-in membrane, the more water vapor can escape and the more dense the jacket is against rain.
• Values such as "MVTR" or "RET" and "water column" are given by the manufacturers to their own products. Of course, it makes a difference whether the manufacturers test "honest", so test the complete product with outer material, seams, seam tapes and zippers, etc., or just a piece of membrane. There are such and such. Independent tests have not yet prevailed.
• Information of the RET (indicates the resistance, the material opposes the water vapor) are more serious than information to MVTR, At the MVTR each manufacturer cooks his own soup, the measurement is not scientifically reproducible across manufacturers. The RET value is much more comprehensible in terms of the reproducibility of the measurement results. But even here, the manufacturers have not agreed on a common, independent test. Everyone tests for their own benefit.

The advantage of co-ordinated layers is: good vapor transmission and the flexibility of the system to quickly adapt the apparel system to the given and changing conditions. With several layers of clothing you can quickly respond to changing weather conditions by donning and undressing.

Optimized onion principle

principle: who overheats, loses - that means conversely: freezing is more effective than sweating.

A word in advance to Breathability: in order for the miracle membranes to function at all, a difference in air humidity between inside and outside is required. When it is warm and moist in the clothing, the breathability works well, the water vapor passes through the membrane and is released into the ambient air. For physicists, the keyword is here Vapor pressure gradient, The higher the humidity outside, the worse the whole thing works. The often cited difference in temperature only plays a role insofar as warm air can hold more moisture than cold air - and thus the humidity at higher temperatures tends to be greater. But you should expect no miracles from the most expensive membranes, you are still wet with sweaty activity. By clever combination of the clothes but this can be reduced a little.

As comprehensive as possible tour is the prerequisite to go to the mountains at all. So you have to know with some certainty what the weather will be like. Is it getting hot? How much does it cool in the evening or at night? Will there be rainfall and what kind will it be? Rain or snow? What applies to other equipment applies even more to the selection of clothing: as perfect as possible Preparation and selection decides - certainly also about well-being and woe.

Onion principle in summer or in heat

feet: waterproof and breathable mountain boots can not breathe very well in summer weather! Better avoid dodgy shoes. These also dry much faster if they have gotten wet.

legs: Short, well-ventilated pants are usually quite sufficient, for longer breaks or in the evening you can fall back on the long pants in the backpack. The body loses little heat over the legs. Beware of zip-off pants: they have to fit exactly, so that nothing rubs! Another option: combine jogging tights and short pants. Underwear should, as described above, consist of functional material - or you just leave it completely away, if you have in the short pants, such as in running pants, a net-like inner pants.

upper body: In hot or even humid weather, the rapid removal of sweat away from the skin is in my opinion not only unpleasant, but can also lead to overheating. Overheating leads to significant performance losses to heat stroke. So in the summer I prefer to wear light, loose outerwear and no tight fitting Underwear or functional shirts. Old-fashioned mountain shirts from our forefathers or airy T-shirts are quite appropriate in summer. A lightweight, windproof jacket protects against overcooling during breaks.

headA well-ventilated sunscreen protects the skin from sunburn and heat stroke. Long-wave sunbeams, which can lead to heat stroke, are also well-held by a thin hat or cap.

hands: Do not forget sunscreen on the back of your hand! Tip for climbing or via ferrata: gummed gardener gloves from the hardware store on your fingers: lots of grip and protection for little money.

Onion principle in winter or cold

feet: Schurwollsocken have with me well proven, Always take a pair of waterproof packaged spare parts with you! At low temperatures, a waterproof and breathable membrane in shoes makes perfect sense. However, when the shoes get wet inside, they seldom dry until the next day. Two plastic bags as VBL / Vapor Barrier Liner can help here.

legs: Here are two options: to keep warm a long underpants. They are available from relatively thin to fleeces. Of course you have to pay attention to functional material and not to put on the old cotton. Then either a softshell or even hardshell pants for weather protection is pulled. If it's really cold, you can combine both.

upper body: a long, tight-fitting functional undershirt or a functional shirt that can be unbuttoned, possibly with a thin layer of fleece insulation, covered by a breathable wind shirt or a thin softshell. A hardshell Wear only in really wet conditions - no hardshell is as breathable as a good one Softshell. The most important: the thick down jacket really only during breaks and if really necessary - about it pull.

head: Over the head, the body actually loses most of the heat, so protect well! Put on your hat or even balaclava, maybe the hood of the soft shell or hard shell to keep the wind out.

hands: Combine! Keep replacement gloves as well as the spare socks in a waterproof pack in a backpack. Rubberized gardener gloves from the hardware store are also available in the winter version. Possibly thin silk under gloves (in the motorcycle shop there are such liner gloves often much cheaper than in the outdoor store), then on the waterproof gloves. Depending on the application and cold use better mittens, which keep the fingers warmer. Again, cheap disposable gloves can serve as VBL.

I have summarized in a further article some tips and tricks, as well as in the cold stays warm.

General principles for choosing clothes

• If you do not freeze when you go out, you are dressed too warm!
• Too many layers over each other brings nothing. The body uses a lot of energy to heat the many individual layers of air. It is better to pack the right clothes and cleverly combine the individual layers. Especially the insulation layer / the midlayer should be appropriate to the expected temperatures.
• Even if the weather seems to be good, always bring adequate and adequate insulation (as well as change things) and pack an (emergency) bivy bag - ours 10 essentials articles There are valuable tips.

Tip: more practical tips and detailed articles can be found in our Know-How Collection.