What is thermal underwear, and when do you need it?

 girl in thermal underwear with trekking poles walks along the trail among the snow-capped high peaks of the mountains
What is thermal underwear? We answer all your questions about this winter essential, including what to look for and how to wear it to stay warm and dry all winter long (Image credit: Zhanna Danilova)

Gearing up for a winter outdoors requires a whole slew of insulated clothing that you might never have needed before, all the way down to your skin where you’ll want to wear thermal underwear beneath those thick, outer layers like a down jacket. But wait, does that just mean a fleecy pair of boxers or a G-string made using goose down? What is thermal underwear and do your nether regions really need that much protection from the cold? In this article, we answer all your common questions about thermal underwear, including what to look for and how to wear it to stay warm and dry all winter long.

A hiker's legs climbing a steep, snowy slope

Also known as long underwear or long johns, thermal underwear are thin, stretchy pants that look a lot like a pair of leggings (Image credit: Zhanna Danilova)

What is thermal underwear?

Also known as long underwear or long johns, thermal underwear are thin, stretchy pants that look a lot like a pair of leggings and are usually worn underneath clothing such as your hiking pants or ski pants during your outdoor activities to provide an additional layer of insulation. When you’re camping in cold weather, thermal underwear can also make good pajamas to wear inside your sleeping bag.

In the hiking layer system, thermal underwear forms part of your base layer, along with a long sleeved top made of the same material, and the two terms are often used interchangeably to describe both the top and bottoms, a bit confusingly. For this article, however, we’re speaking about the bottoms when we say thermal underwear.

Runner's legs in the mountains

Good thermal underwear is usually made using merino wool (Image credit: Zhanna Danilova)

What makes thermal underwear thermal?

If thermal underwear looks and acts a lot like a pair of leggings, can you just wear an old pair of cotton leggings under your hiking trousers to stay warm on a winter hike? Assuming you’re not going to sweat or otherwise get wet, this would certainly provide some more warmth, but what makes true thermal underwear thermal is typically the fabric that it’s made from, and sometimes the construction. 

When we’re talking about thermal underwear for active pursuits like hiking and skiing, then good thermal underwear is usually made using merino wool, such as the Icebreaker Merino 250 Vertex Thermal Leggings. The benefit of merino wool is that it traps air to provide insulation, and wicks away moisture to keep you dry if you get sweaty, and dries quickly.

The main downside of merino wool, however, is that it’s expensive, so lots of great and more affordable thermal underwear is being made using synthetic fibers, such as polyester, which shares similar moisture-wicking properties to keep you warm and dry.  Some, such as the Smartwool Intraknit Merino 200 bottoms, are made using a merino wool/synthetic blend. If you’re very sensitive to wool, you’ll appreciate the non-itchiness of synthetic alternatives, too. You might also see some thermal underwear made using bamboo rayon, which will trap air and wick sweat, but most bamboo clothing is slow to dry, so for high-energy frigid adventures, it may not be your best choice.

Active thermal underwear may also be constructed using two thin layers of fabric, to better help trap heat. The most important thing to know is that if you’re shopping for thermal underwear for outdoor adventures, you’ll want to avoid cotton since that won’t dry quickly or keep you warm if it gets damp – save these for days when you’re working from home and want to keep your central heating bill down.

A woman hiking in the snow pauses for a drink from her hydration vest

Active thermal underwear may also be constructed using two thin layers of fabric, to better help trap heat (Image credit: Newman Studio)

Is thermal underwear necessary? 

If you’re adventuring in cold weather, it’s a good idea to own thermal underwear though you won’t always wear it. It all depends on how cold the weather is, and what you’re going to be doing. Assuming we’re only talking about the bottoms, your legs probably don’t feel as cold as the rest of you in winter, so if you’re really exerting yourself on a hike or uphill skiing, you won’t need long johns unless it’s well below freezing, especially if you’re wearing insulated pants. However, for very cold temperatures, downhill skiing and winter camping, they can be life-saving. Wear them with your regular underwear underneath, and warm layers like a down jacket and windproof hiking trousers over the top and you’ll stay toasty all day.

A man and a woman snowshoeing uphill in winter

If you're snowshoeing in fair weather and have insulated ski pants, thermal underwear might be overkill (Image credit: Westend61)

Should thermals fit tight or loose?

Any base layer should be snug-fitting, and that’s not simply so you can get your clothes on over the top. The air-trapping insulating properties of your thermals work best if the material is held close to your skin. If slim-fitting clothes annoy you, look for thermal underwear with more than 10% stretchy fabric to make it easier to move.

How long can you wear thermal underwear?

Hopefully you wash your regular underwear and each use, but the good news is that you won’t have to wash your thermals quite so frequently. If your thermal underwear is made using merino wool, it will have some naturally antibacterial properties while synthetic garments meant for active use are often treated to be antimicrobial. All this means is that they inhibit the growth of microorganisms and don’t get as smelly, especially if you go for merino wool – for hopefully obvious reasons, you can enhance this by wearing regular underwear underneath your thermals.

So to get back to your question, how long can you wear thermal underwear? Lots of outlets say you can wear them for 3 - 5 days, based on manufacturer recommendations, but obviously it all depends on what you’re doing in them, how much you sweat and whether or not you air them out when you get home or leave them scrunched up at the bottom of your backpack to fester. Basically, hang them up to air them out after a hike and use the good old fashioned sniff test and know that you’ll be able to wear them for at least a handful of adventures without washing them.

A hiker in thermal underwear rummages through his backpack in the snow

They won’t offer a ton of protection from the wind or any shield against wet weather, but in dry climates like Utah and Colorado, you could wear them for a winter hike without outer layers if you see fit (Image credit: Zhanna Danilova)

Can thermals be worn alone?

If thermals are body-hugging, sweat-wicking and insulating, you might well be wondering if you can wear them without clothes on top, and the answer is yes. Particularly for sweaty adventures like winter running and cross country skiing, they can provide better protection against the cold than a simple pair of leggings without making you overheat. They won’t offer a ton of protection from the wind or any shield against wet weather, but in dry climates like Utah and Colorado, you could wear them for a winter hike without outer layers if you see fit, and then there’s no annoying swishing of weatherproof membranes like Gore-Tex while you walk. Some look a lot like underwear, but lots of them are fairly stylish nowadays, so you can wear them around town too if you want.

Julia Clarke is a staff writer for Advnture.com and the author of the book Restorative Yoga for Beginners. She loves to explore mountains on foot, bike, skis and belay and then recover on the the yoga mat. Julia graduated with a degree in journalism in 2004 and spent eight years working as a radio presenter in Kansas City, Vermont, Boston and New York City before discovering the joys of the Rocky Mountains. She then detoured west to Colorado and enjoyed 11 years teaching yoga in Vail before returning to her hometown of Glasgow, Scotland in 2020 to focus on family and writing.