When our movement increases we need to reduce that insulation to prevent sweat building up and when we stop moving we need to increase the insulation to retain the heat we already have.
No single layer of clothing can achieve all these things but by wearing multiple layers and adjusting them as we change our activity, we can regulate the insulation and thereby our heat and moisture retention.
This approach is described in detail by many sources and unsurprisingly is referred to as the layer system.
For our purposes I’m going to break it down into just three separate layers. A base layer to transport moisture away from the skin. A mid layer where most of the insulation is provided and a shell layer which protects these layers from environmental conditions such as rain and snow but most importantly wind which would strip away the warm air trapped in the insulation and replace it with cold air.
It is important to realise that these layers can be made up by more than one physical layer of clothing and in very cold conditions this may well be the case.
The main difference between the layer system as normally described and use in cold environments is that normally great emphasis is placed on the waterproof properties of the shell layer, often at the cost of breathability. However, if conditions are cold enough then that priority is reversed. Breathability becomes much more important than waterproofing because the main source of moisture is now the body itself.
Some fabrics retain more moisture than others. In general, synthetic materials retain less than natural ones, so from a purely technical point of view, synthetics are an obvious and sensible way to go.
I would add an important caveat to that though. If you have no practical laundry facilities, as is often the case on expedition, synthetic materials soon start to develop an aroma that is far from pleasant.
Another consideration is what is happening to the moisture retained in natural fibres. Cotton absorbs a lot of water which can feel comfortable for a while but when it becomes damp it can quickly chill the body if it is in contact with the skin. Wool absorbs quite a bit of moisture too but will tend to store it in the centre of the fibres and away from the body until it completely wets out. I have read many articles that claim that wool is warm when wet. That is simply not true... It will maintain most of it’s insulating properties better than cotton when damp though as will good synthetics.
Wool does have a few disadvantages. First of all not everyone can wear it next to the skin. Merino wool is less itchy than some other types but even this can feel itchy on very sensitive skin.
If it does become wet, it gets heavy and takes a long while to dry out. I would recommend airing your kit at every practical opportunity as this will reduce the build up of moisture over prolonged usage.
Purpose made woollen clothing for the outdoors is quite a niche market and unfortunately that makes it quite expensive. Personally I measure value by fitness for purpose and longevity which I find wool sizes up very well on but it is often possible to find good woollen jumpers in sales or even charity shops for very reasonable prices.
Invest in fine merino for your base layer which is in contact with your skin by all means but coarser, cheaper wool is perfectly acceptable in your mid layer.
Finally, I have to say that contrary to the outdoor folklore that “Cotton is a Killer”, It does have it’s place in a good clothing system for extreme cold and that is as a shell layer.
Tightly woven cotton such as canvas or Ventile can block wind very effectively and is far more vapour permeable than waterproof fabrics, even the supposedly breathable ones. The important thing is to prevent it getting wet and that is fairly straight forward in the sub zero environment.