To succeed, and be comfortable, in the outdoors you need to have a good clothing layering system – a base layer, insulation layer and shell layer. This article will explain what that means and how to create one. I’ll also pass on a practical ‘top tip’ for adding additional layers when conditions are not on your side.
You can learn more about all of this on our mountain skills courses.
Table of contents
- Clothing layering system – the theory
- Clothing layering system – the practice
- Top tip for adding additional layers
Estimated reading time: 13 minutes
Clothing layering system – the theory
If you read any good text on clothing layers, it will mention base layers, insulation layers and shell layers. From a theoretical standpoint, these are the three layers in which we dress. But what do these terms mean? It’s important to understand the theory and the concept before looking at the practical aspects.
There are a couple of scientific facts worth understanding. These facts have influenced animal evolution and, in turn, our approach to the clothing layering system.
Humans have a ‘constant’ body temperature which we must maintain for our survival. This is in the range of 36.5-37.5 degrees Centigrade. Any more than a few degrees either side of this, our bodies start to shut down gradually, to the point of death. Maintaining this constant body temperature is vital for life.
Hyperthermia and Hypothermia
Getting too hot (hyperthermia) can kill us. A body temperature of more than 40 degrees Centigrade is life threatening and requires immediate professional medical treatment. Also, allowing our body temperature to drop too low (hypothermia) can also kill us. Anything below 32 degrees Centigrade is a medical emergency and around 25 degrees Centigrade death occurs.
Hypothermia in UK
In the UK and most of north-western Europe, we live in a temperate climate. As far as our outdoor activities are concerned, especially in the UK mountains, getting too cold (hypothermia) is of prime daily concern. It is much rarer that getting too hot is our main concern.
Conduction of Heat
Conduction is one of the ways in which our bodies give off, and therefore lose, heat. If we can minimise heat conduction we can minimise heat loss. Air is a poor conductor of heat – it basically conducts no heat. Therefore if we can surround ourselves with a layer of trapped air, we will massively reduce heat loss.
Cooling Effect of Wind
If the air moves over the surface of our body then it will cause us to lose heat more quickly than if the air were still. This explains why we feel hot on a still day but get cold quickly in the wind.
Cooling Effect of Water
Water is a better conductor of heat, and in this particular context this is not a good thing. If immersed in water, the human body will lose heat 25 times more quickly compared to cooling at the ambient air temperature. The rate of cooling of the body in wet clothing is around 5 times the rate compared to cooling at ambient air temperature. Therefore if we can keep our body dry we can also conserve heat.
This is the layer of garment in contact with your skin. It has two main functions. First, it traps a thin layer of air close to your body in much the same way as body hairs or fur do. This thin layer of air warms up and helps to maintain our body temperature. Secondly, a good base layer will ‘wick’ moisture away from the skin and transport the moisture on to the next clothing layer. This keeps our skin drier and therefore reduces heat loss. The moisture in question will be a combination of sweat and/or rain that has found its way in from outside.
The next layer in the clothing layering system is the insulation layer. The purpose of this layer is to trap air to keep our bodies at the right temperature, bearing in mind the ambient air temperature. In winter we need to trap much more air to insulate ourselves than we do in summer. Of course, in very warm temperatures we may not want any insulation at all.
Although the insulation layer is just one layer in conceptual terms, it could be made up of one or many physical garment layers.
The shell layer is the layer in contact with the external environment. Its purpose is to protect our body, and the rest of our clothing layering system, from the harmful effects of the weather around us. This means cutting out the wind, which will chill us more quickly. It also means preventing rain from wetting our clothes, which will also cause us to lose heat more quickly.
This is the concept of the ‘shell’, like the shell of a tortoise. It is an outer protective layer. However, what we need to be protected from (wind, rain or both) will vary according to the prevailing conditions.
Clothing layering system – the practice
An effective base layer must trap a thin layer of air and ‘wick’ moisture away from the body. Good base layers are usually made from man-made fibres (both hi-tech and lo-tech designs work) or from wool.
It is worth discussing cotton in this context, along with the mantra ‘cotton kills’ You only need to think about cotton towels to understand this. Cotton is highly absorbent and will hold a large amount of water. But it dries very, very slowly. A cotton base layer would therefore absorb a lot of sweat and rain but hold it all in the garment. This will cause you to lose heat 5 times more quickly than you would wearing dry clothing.
Anything that traps air will work well as a garment in the insulation layer. What’s more, multiple garments worn in layers trap more air in the space between each layer. This has given rise to the conventional wisdom of ‘wearing lots of thin layers rather than one thick layer’.
However, you should take this with a small pinch of salt. Certainly, it’s true that having a number of thinner layers gives you flexibility. You can add or remove them as desired to control your body temperature. But don’t go overboard! Over the decades, wearing 5-6 layers of thin woollen jumpers gave way to wearing a couple of fleeces and maybe an insulated jacket. These clothing layering systems will still work effectively, there is no doubt. But modern hi-tech garments generally mean that only a couple of garments are needed in the insulation layer (depending on the external conditions, of course).
Natural versus Man-Made
Down is worth mentioning. This natural product is unrivalled in its ability to trap air and so it is the best known insulating material. However, it loses its ability to trap air when it becomes wet. For this reason a down jacket is an appropriate choice for the Alps or Himalayas (where is cold and dry), but will usually be a poor choice in Scotland, Wales and the Lake District (where it is almost always wet). Whilst acknowledging the insulation properties of down, many people have concerns over animal welfare and reject down as a choice. If you do choose to buy down, most good companies use only ethically-sourced down in their products.
There are also many options for garments using man-made insulation. Man-made insulation (Primaloft is a leading product) is not as warm as down, weight for weight. However, it is cheaper and also does not lose as much of its insulating properties when wet.
The shell layer is the one in contact with the outer atmosphere and therefore at any one time you can only be wearing one shell. When choosing a garment to buy for your clothing layering system, remember that the shell needs to keep out both rain and wind. A fabric that keeps out the rain (‘waterproofs’) will also keep out the wind. However, there are fabrics that will keep out the wind extremely well (‘windproofs’) but which only keep out rain to varying limited extents.
For the UK, it is vital to have a ‘waterproof’ to cover all eventualities. But, for reasons mentioned below, it’s also nice to have a ‘windproof’ for times when you are concerned only about the wind.
‘Waterproof’ or Hard Shell
The type of shell that we generally call ‘waterproofs’ (jacket and trousers) is also called a ‘hard shell’. The classic, typical product is a Gore-Tex jacket. These can be quite heavy, noisy to wear, and despite all discussion of ‘breathability’, they inevitably impede the flow of air compared to wearing no jacket. Therefore, wearing them results in more sweat being retained inside the jacket and our base layer getting wet.
If it is raining hard outside then getting sweaty is the lesser of two evils compared to getting soaked by the cold wind and rain. That’s why a ‘hard shell’ is a vital life-saver in your clothing layering system that you should not be without.
There are many alternative products to Gore-Tex ones, usually based on a physical membrane within the jacket or a coating on the outside of the jacket.
‘Windproof’ or Soft Shell
The other general type of shell garment available is often called ‘soft-shell’ or in days gone by, ‘windproofs’. These fabrics are generally lighter, more flexible, and more comfortable to wear. They allow water vapour to pass through more easily, so are more ‘breathable’ than many hard-shells and as a result we get less sweaty inside.
They are very effective at stopping the wind, and this plays a major part in keeping us warm. However, they vary in their ability to keep out the rain. The lightest soft-shells will barely keep out the rain from a short shower. The thicker and heavier ones will slow down the penetration of the rain but not keep it out properly.
Soft-shells therefore have pros and cons compared to hard-shells. But on a dry, cold, windy day wearing a soft-shell will provide appropriate protection from the environment but yet be much more comfortable than a hard-shell.
The outdoor clothing market is constantly evolving. One of the ‘holy grails’ is a fabric with the waterproofing capabilities of a ‘hard-shell’ but the comfort, flexibility and breathability of a ‘soft-shell’. Different manufacturers are constantly making strides in this direction so our options for what to buy is always increasing and can be confusing.
Layers and garments do not match up 1:1
In the section on ‘The Theory’ I said that the clothing layering system has three layers – base-layer, insulation and shell layer. But this does not mean that we will always wear three layers, nor that this theory translates into three garments.
Wearing one or two layers
First, we may not always wear all three layers (although we almost certainly are carrying them ready to add during the day):
- On a very hot, still day it may be appropriate to be wearing only a base layer.
- In very mild but wet conditions when we are working hard, an insulation layer may cause us to overheat. Wearing a base layer and a hard-shell might work best. We just accept that we will be wet from sweat until we add an insulation layer later in the day.
- On a cold, dry day with no wind, a good choice might be to wear a base layer and an insulation layer but no shell layer (though it would be carried for use if required).
More than one garment in insulation layer
Next, it should already be apparent that layers and garments do not match 1:1. If we are wearing all three layers because of the prevailing conditions, this could actually be, say, 5 garments (on the upper body). These might be a base layer top; then a thin insulation layer (often called ‘mid-layer’ tops); on top of that maybe a fleece; then a Primaloft jacket; and finally a Gore-Tex ‘hard-shell’. So, three layers has on this occasion translated into 5 garments.
However, as fabric and garment design advance, things become very exciting but also complicated. For this reason, it’s really important that before buying a garment you have understood what part you need it to play in your clothing layering system.
There have been garments that combine a waterproof shell outer with an insulation layer for a long while now. Depending on your needs, these might be an appropriate choice (though they rarely are for UK mountain walking and mountaineering). But now there are garments which combine being a base layer and a soft-shell in a single item of clothing; or a base-layer, thin insulation and soft-shell in one. The list goes on and the choices open to us mean we can fine tune our clothing layering system to be exactly as we like it.
The jacket below combines wicking properties, insulation properties and a reasonably windproof shell into a single garment. It doesn’t perform all these functions to the complete extent needed so it is still used as part of an overall system.
Top tip for adding additional layers
Very often, but especially in winter, we start off walking with minimal layers (“Be bold, start cold”) but perhaps need to wear our waterproof jacket if it’s raining, snowing or very windy.
However, when we either reach the foot of our climb, or find ourselves further up the mountain, then we need to put on an extra layer underneath our waterproof jacket. For the uninitiated, what then follows is a great flap-fest as the jacket comes off, nearly gets blown away in the wind, and then once the extra warm layer is on, a further struggle to get the jacket back on despite the wind’s attempts to prevent this.
Next time you find yourself in this situation, try this tip. Don’t remove your waterproof jacket, but unzip it nearly all the way, then carefully take your arms out and let the jacket hang from your waist. This is a little like removing the top part of a wetsuit. Then quickly put on the new warm layer, and simply roll-up the jacket once more, slipping your arms in and doing up the front zip. It’s much quicker, easier and you are less likely to lose something!