An introduction to layering

If you’ve read my various blog posts about hiking, and reviews of kit, you’ll no doubt have heard me talk a lot about layering systems and so on. Likewise, whenever you read any advice about staying warm and having outdoor-appropriate kit, people talk about ‘make sure to wear layers’. At it’s simplest, that makes sense; layers keep you warm by trapping air, and have the advantage that you can adjust for temperature by simply removing a layer.

But there is so much more to it than that. Base layers, mid-layers, shells. Fleece, down, merino, coreloft. What do you need and how do you choose?

Well, here’s your primer: an introduction to layering that will understand the theory and ensure you have the gear you need.

First of all, what are hiking clothes actually trying to achieve? I’d say there are five key criteria, some of which are to a certain extent mutually exclusive or at least very hard to achieve in conjunction:

• Insulating: maintaining body temperature by trapping warm air within the garment.
• Wicking: draws sweat away from the body to the surface of the garment where it can quickly dry, preventing dampness and therefore cold.
• Breathable: similar to wicking, but applied mainly to waterproof and wind proof fabrics and in this case it is about allowing evaporated sweat to pass through the fabric rather than condensing on the inside of it and becoming damp.
• Waterproof: prevents water from entering (like this Montane jacket).
• Windproof: prevents air from passing through the fabric from the outside, so reducing wind chill.

Clearly not all of these aims are of equal importance on different garments. Having a waterproof base layer is utterly pointless, as is having a wicking shell. So what are the different layers and what is important for each one?

Base layers

Base layers are worn directly against the skin, so as a rule you only wear one true base layer on each half of your body, although in some conditions there may be a grey area between a second heavy base layer and a lightweight midlayer.

For a base layer wickability is arguably the most important criteria. More than anything else, your base layer will get sweat-soaked and this can become uncomfortable and even dangerous once you stop moving and begin to cool down. A fabric which moves sweat away from your body and dries fast will both enhance the effectiveness of sweating, so keeping you cool when exerting yourself, but also stay drier and therefore keep you warm when not exerting yourself. Of the other criteria, breathability is important although rarely a concern, insulation is somewhat relevant but not the prime issue, and being windproof or waterproof are essentially irrelevant.

So, your base layer should be merino wool or an artificial technical fabric designed with wicking in mind. Cotton is the very opposite of wicking – it will soak up litres of sweat and then sit next to your skin, sopping wet, taking forever to dry. Jeans are made of cotton by the way. 

Mid layers

Mid layers cover an absolutely massive range of items from the flimsiest, lightest fleece pullover to these huge down jackets designed for belaying at -20 degrees C. I can’t cover them all off but I can explain the logic behind it.

The main criteria for mid layers is insulation – that, after all, is the purpose of them. What they are made of and how heavy they are just depends on the insulation you need. The secondary criteria, however, change as you move away from your skin. Mid layers worn closer to the skin are likely to favour being wicking, and therefore fleece is a particularly good material. As you move away from your body, being wicking may become less important and perhaps just be considered in fleece panels under the arms (as found on the Arc’Teryx Atom LT) and eventually not considered at all on heavier items. Down and artificial insulation like coreloft therefore starts to be more useful. Instead, as they are more likely to be outer garments, having windproof and waterproof qualities becomes more of a factor, and these outer midlayers may be made water-resistant through treatment with DWP or a similar product.

In choosing midlayers, then, you want to think about the conditions you are likely to face and how warm you want to be, and ideally have a range that balance being light and wicking to wear close to the skin and when generating a lot of heat, or heavier, warmer and more windproof for extremely cold days or when stopping for a time.

Shell layer

Shells are typically aimed at being either waterproof or windproof or both. Being wicking is irrelevant and being insulating is usually not a priority – indeed having an insulated shell just means you can’t wear it in warmer weather which may be a nuisance depending on the conditions you are hiking in. Being breathable is much more of a concern with these jackets though, and there will always be some trade off since few if any truly totally waterproof jackets are also really breathable.

The shell could be mainly windproof, or it could be waterproof in which case it is likely to be windproof as well. Either way, the weight and degree of wind/water-proof-ness will depend on the conditions you expect to encounter.

So that’s that. Of course, there’s plenty of scope for variation. What you call a layer is less important than what it does, and understanding what it does, and how that compares to what you need it to do, is vital in making wise purchasing choices.