Thermal underwear – what do you need to know?

thermal underwear thermal baselayers

SleddingAs a child growing up on the east coast of Canada, thermal underwear were a fact of life. The long johns were badly fitting, sagged at the bum and left waffle weave impressions on your skin after only a few minutes. They were also made of cotton, so once they got wet they got even heavier, sagged more and lost their thermal properties.

Doing a number of sports outside in the winter I experimented with numerous ways of keeping warm. Layers of pantyhose under jodhpurs with chaps over top for riding horses. Jogging pants under splash pants or snow pants for cross-country skiing or snowshoeing. Pantyhose under snow pants for sledding. All of these kept me warm enough to participate, but none managed to be warm, dry and comfortable all at the same time.

What has changed now?

Fortunately for my outdoor undertakings now, the world of thermal underwear has advanced significantly, and technology has now come! Thermal underwear, or base layers as they are now commonly called have benefited from increased understanding of exercise physiology as well as research and development of technical fabrics. When these are married to improved efficiency in fabric and clothing manufacturing processes, the prices come down to an affordable level. As the understanding of the benefits of exercise and sport is increasing globally, it is increasing the availability of these advanced products and materials, which can now be found everywhere and for all budgets.

Sport Specific Thermals

I’m of two minds about sport-specific thermals or base layers. I have two sets of lovely, but very expensive scuba diving thermals, made specifically to fit underneath a dry suit. They are plush, definitely fit both me and the suit, rated for specific temperature ranges and don’t lose any thermal properties when they get wet – bonus if your suit springs a leak. They spent most of the year in my drysuit bag until I realized how stupid it was to have these lovely, warm, expensive thermals sitting around doing nothing and wore them on my bicycle one winter day. Admittedly, sport-specific thermals or base layers will be designed with the demand of the sport in mind, so cycling thermals will be reinforced at the common stress points – thighs and bum, whereas my diving thermals are designed to avoid pinching your skin and causing pressure points if a dry suit squeeze were to occur. So the extra sport-related bum and thigh reinforcement which extends the working life of the thermal base layer designed for cycling isn’t there in the diving base layer, but is or better to get use out of them anyway or maximize their useful life in the one sport?

Fabrics and Technologytechnical fabric

The same technical fibers used in sports underwear are employed in thermal base layers, more about these technical fibers and their care can be found here in my discussion of women’s running underwear, and also in my indepth discussion of common technical fibers used in athletic clothing. However, thermal base layers have used technology to improve even further on the technical fibers – things like:

  • seamless knitted garments to reduce chaffing and pinching
  • innovative knitting and weaving patterns to increase temperature control
  • compound materials and innovative knitting and weaving techniques
    • use of ceramic particles in the yarn to help keep you cool by transmitting excess heat rapidly away from the skin – Odlo use this on their Ceramicool range
    • intelligent design to warm you up when you are cold, and cool you down when you sweat – X-Bionic have their 3D Bionic Sphere system combined with merino wool on their Apani Merino range

How Fit affects Function

Your base layer functions through contact with your skin both to provide warmth through trapping a warm air layer next to your skin when you are too cold and to wick your sweat away from your skin so you don’t get too hot. To facilitate this, it is important to choose a snug-fitting base layer, but not one that is so tight that it restricts your movement or is uncomfortable. If your base layer is too loose on top of losing both thermal and wicking performance, you could also be leaving yourself vulnerable to chaffing or skin sores due to the excess fabric movement over your skin.

The stretchy fabric of your base layer should accommodate your body shape with ease if it has been well-designed, therefore you should focus on any sport-specific requirements or body length issues that can’t be resolved. For instance, I have fairly short, muscular legs and bum, so I often find base layers are too long in the leg (and some seamless knitted varieties can’t be hemmed), and not high enough cut in the back so that when cycling I get an unpleasant cold draft working its way up my back. Other areas to consider in particular when trying on base layers are arm length and torso length.

And finally – with or without underwear?

Thermal base layers get a lot of use in my house, you’ll find me lounging in them, using them as pajamas, working out (obviously), wearing them on the sailing dingy or SUP in the winter instead of a wet suit (and cursing loudly when I do end up in the cold water) and always wearing them at the cold CrossFit gym first thing in the morning. 99% of the time I will wear underwear too; however it is a personal preference, so once you’ve chosen your thermals to give it a try, it will have a minimal effect on your laundry cycle if you are exercising in them; although always adjust for the situation – on a multi-day hike you will probably appreciate a clean pair of undies inside your thermal layers, and spare undies pack much more efficiently then spare thermals!

What do you think? How do you choose and use your thermals? I look forward to hearing from you!

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4 Responses

  1. Kerry says:

    Hi Lisa,
    Great article!! My wife and I are runners in colder climates in the US. We like to complain about running in the cold but it’s really not bad except for a handful of days during the season. On those really cold days we definitely struggle with what to wear and always seem to be settling on our “new” favorites. Your info above is great.

    I’ve forwarded this site off to my wife so that she can get another perspective on it. We’ll both be sure to visit again and keep up to date with more of your expertise on the subject.
    Thanks for helping!

    • Lisa says:

      Hi Kerry,

      Thanks for your comments, I’m sure you and your wife have a number of suggestions and key products that we could all make use of this winter to try and keep the perfect temperature, I’d love to hear some of them!

  2. Danny says:

    Thanks for this great article, Lisa! I’m from California where it’s nice and sunny but I’m currently living in Denmark where it’s nice and frigid in the winter haha. This article is super helpful for knowing which thermal underwear to use! I’m a novice in the subject because of where I come from.

    The only thing I knew about thermal underwear was merino wool… good way to keep warm. It’s good to know that when I’m exercising in the winter, there are ways for the sweat to be wicked too.

    I’m interested to check out the cycling thermals you mentioned after reading this. Thanks again!

    • Lisa says:

      Hi Danny, thanks for your kind comments! Personally I am a big fan of Merino wool thermals, but I only tend to use them for multi day hikes because in the winter (when you want to use them) air drying things in my UK flat (apartment) means the whole place feels a bit damp and I’d rather use less technical ones daily and use the dryer.

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