I can’t stress enough the need for good moisture wicking long underwear. It is your foundation base layer. This layer will transport the moisture from your skin to outer layers and keep your skin dry and warm. There are three long underwear fabrics that are leaders in the field: Polyester, polypropylene, and wool. These exist in many fabric trade names, and are sometimes blended with each other. They all work well, and the high quality ones are often regarded as “wonder” fabrics, because they really do work very well just as advertised. However all do have advantages and disadvantages, and you will definitely develop a personal preference.
The material I use is polyester. If you are not familiar with modern polyester fabrics, fear not – its not like the polyester leisure suit from the 70’s! Modern polyester threads are spun and woven into amazing hydrophobic (water repelling) and water transmitting thin fabrics that are stretchy and very soft against your skin. The 3 dimensional thread structure, and the 3D structure of the fabric provide the properties of moisture wicking and insulation. The modern weaves also are very good at resisting bacterial growth, and so resist getting “ripe” and nasty smelling like the synthetics of old.
Polypropylene was the first of the modern synthetic wicking long underwear. It gained the nickname “polypro”, but that nickname is often incorrectly used in media to refer to both polyester and polypropylene, but be aware they are different fabrics. Just like polyester, polypro is extremely hydrophobic and transmits moisture very well. The first generations of polypro fabrics unfortunately were superb mediums for growing bacteria gardens, and within hours of wearing it, it began to stink real bad. The concept of wearing the same top for three days on a trek was horrible to even contemplate. It quickly turned into “toxic waste” in a tent, and it was tough for your buddies to be even near you! The good news is that the fabrics have been reengineered into different 3D structures and that problem has been solved. I have not used the newer material in tops and bottoms so I can’t comment on their performance. If you try polypropylene, please let us know what you thought.
Wool of course is not new. Its been around a long time and for good reason – its one of the original wonder fabrics. Wool however behaves quite differently from the synthetic fabrics. Wool holds more moisture in the fiber and the fabric, although it wicks out excess. The moisture it holds however is warmed against your skin and users say it is quite comfortable. This little bit of warmed moisture insulates against sudden changes in activity where the synthetics don’t. The result is that you can get a chill with the synthetics when you suddenly stop moving and working hard, as the material catches up to wick the excess moisture away. Users of wool underwear report that they don’t experience that chill effect as much, so the fabric seems to have more breadth to insulate when it is loaded with excess moisture.
Wools of old have the reputation of being very itchy and prickly on the skin, and some people are allergic to it, including yours truly. The modern wool long underwear is made from merino wool, which is advertised as being soft and not itchy. My experience has been that it’s a vast improvement, but its still too itchy for me to have against my skin (except my feet and hands). But many people love it and swear by it, and I have no doubts in their claims of superb function. Wool is also the best fabric out there at resisting bacterial growth, and stays fresh smelling for long periods.
There are brands which blend one or more of the above fabrics plus others, into one. Some garment manufacturers will have information on their website to explain the composition of their fabrics in their garments. Others will use the fabric’s brand name, and then you have to search out that information.
Thickness and Weight:
The thickness, or fabric weight of your long underwear system depends on how cold it is, what your outer layers are, and your own personal physiology. Some people are heat and sweat factories, and some people seem to have more even heat production and moisture production across many activity levels.
Commercial brands will often market weights as “lightweight”, “medium” and “expedition” or “heavy duty”. Of course the commercial brands are all different, so there is no standard. For camping, hauling sled, and back country skiing I personally use an “expedition” weight polyester bottom under my outer wool pants up until 00C. Above freezing, I go to a medium weight if I have it on the trip. My wool pants are very scratchy wool, so I need at least a medium to protect my leg skin from the wool fibers. For a top, I use a medium weight polyester for all temperatures.
Your long underwear system will no doubt evolve with the rest of your clothing system. Each brand and weight functions somewhat differently, so you will have to experiment to find what works best.
Your sock “system” is critically important to your comfort and safety. Cold feet are no fun, and can be a trip and activity limiter. A consideration of your sock system must also be in context with your boots. But because boots are such a large topic on their own, we will deal with boots separately.
Your choice of sock “system” will evolve with different footwear and your experience. I have evolved one which works interchangeably with my felt pac snowboots and my back country ski boots, so when changing boots in the field, I don’t need to change socks.
Against my skin I use a very thin polypropylene (polypro) liner sock. On top of that, I wear a second liner sock. I chose brands which have a shiny surface/ finish. The two socks slide on each other and have solved the heel blister problem I had in my ski boots. If you have heel blister problems, consider this double polypro sliding system for your inner socks.
If there is not a blister problem, there are excellent merino wool very thin liner socks, which I can also wear against my skin, even with my wool allergy. However they don’t slide against other socks as well as the polypro liners. If you have no blister problems however, I highly recommend merino wool liner socks for their performance and especially their antibacterial properties. Your feet will be continuously in boots and socks in winter, so foot hygiene is important.
You don’t necessarily need specialty liner socks. If not a liner, then I still recommend a thinner sock against your skin is better because it will hold less moisture.
Outside your inner sock, for outer socks almost anything goes for thickness and materials. You can’t go wrong with pure wool, or wool blended with various hydrophobic synthetics. There are so many brands and blends out there, that you will just have to experiment.
It is important to size socks properly so that they don’t bunch up when layering. A principle which works for me is thinner and stretchier socks on the inside, with thicker, bigger girth socks on the outside. Outside of my polypro liner socks, I use a lightweight merino wool hiking sock, and over that a very thick mostly wool “ice sock”.
On multi-day trips, it is important to change into fresh clean socks from time to time. With the layering system, you can change your lighter inner socks more often, and get more days out of your outer socks before changing, thus minimizing the bulk and weight of your clothing pack.
Vapour barrier socks are another fascinating topic. Some swear by them, and some don’t like them. These will be dealt with in the boots section, since they are designed for keeping all moisture in, thus keeping boots dry.