The 9 Best Thermal Underwear of 2021

The best baselayers for the ski slopes and lounging on the couch

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Best Thermal Underwear

Tripsavvy / Chloe Jeong

Adding a layer of thermal underwear to your winter outfits can make all the difference. This extra warmth—usually found as insulated leggings or a thin, insulated shirt—is meant to be worn underneath an outer layer. Oftentimes, thermal underwear is thought to be something worn while participating in winter sports like skiing and snowboarding. But it can make any day spent outdoors a little more pleasant.

Thermal layers like this are most commonly made of wool, silk, or synthetic materials since they’re all great at wicking away moisture, keeping heat trapped inside, and drying quickly. The latter is what makes it great for winter athletes, but find a pair of quality thermals to wear on any given winter day and you’ll find yourself a convert.

Ahead, we researched the best of the best when it comes to thermal underwear (both shirts and leggings).

Cyber Monday Deal

Our Cyber Monday pick for women is the Arc'teryx Rho LT Bottom, which is $67 on Backcountry.com, down from $89. For men, find great savings on the Salomon Essential Wool Tight, priced at $75 down from $100.

The Rundown

Best Overall: Smartwool Intraknit Merino 250 Thermal Bottoms at Amazon

"Thermal underwear that works well to keep you at a comfortable temperature no matter what you’re doing."

Best Budget: TSLA Thermal Underwear Set at Amazon

"There’s an incredible stretch in this set."

Best Leggings (Men): Salomon Essential Wool Tight at Backcountry

"These wool tights from Salomon are great for outdoor activities."

Best Leggings (Women): Arc'teryx Rho LT Bottom at Backcountry

"These lightweight leggings are deceivingly warm. "

Best Set: Thermajane Ultra Soft Thermal Underwear Long Johns Set at Amazon

"Thermajane’s thermal long john set is fleece-lined and softer than soft."

Best Wool: Icebreaker 175 Everyday Leggings at Amazon

"Both the top and bottom are stretchy so they can easily be worn underneath any type of winter gear."

Best Lightweight: Ortovox 185 Rock'N'Wool Long-Sleeve Top at Backcountry

"The brand’s high-quality wool blend has better strength and durability than 100 percent Merino wool yarns."

Best for Layering (Men’s): Smartwool Merino 250 Base Layer at REI

"Smartwool is known for its rock-solid basics and these leggings are no exception."

Best for Layering (Women’s): 32 Degrees Base Layer Thermal Leggings at Amazon

"This baselayer from 32 Degrees feels like wearing nothing (in the best way possible)."

Best Overall: Smartwool Intraknit Merino 250 Thermal Bottoms

Smartwool Intraknit Merino 250 Thermal Bottoms

Courtesy of REI

What We Like
  • Innovative construction

  • Ribbed knee

  • Anti-odor

What We Don't Like
  • Expensive

These days, the merino wool construction may seem run-of-the-mill, but Smartwool gives it a tech-focused boost thanks to the brand’s Intraknit design. These leggings are woven with terry loops combined with mesh, which helps simultaneously ventilate and insulate. If you need a pair of thermal underwear that works well to keep you at a comfortable temperature no matter what you’re doing, invest in this pair from Smartwool. The knees of these leggings are also ribbed, which allows for more durability and flexibility. There is also a men’s style in this option.

Sizes: XS, S, M, L, XL | Fabric: Merino wool, polyamide, elastane | Care: Hand wash

Best Budget: TSLA Thermal Underwear Set

TSLA Thermal Underwear Set

Courtesy of Amazon

What We Like
  • Inexpensive

  • Great color variety

What We Don't Like
  • Not super durable

Microfiber won’t keep you the warmest in the outdoors, but finding a set that can get you by in multiple situations at a budget-friendly price is a win and a good item to have in your cold-weather layering quiver. This long-sleeve shirt and legging set is moisture-wicking and fleece-lined, plus it comes in 12 color options. There’s an incredible stretch in this set, which makes it a great option for those who work outdoors year-round in need of an extra layer. This set is also available in a women’s style.

Sizes: XS, S, M, L, XL, XXL, XXXL | Fabric: Microfiber, spandex | Care: Wash on a cold gentle cycle and tumble dry on low

Best Leggings (Men): Salomon Essential Wool Tight

Salomon Essential Wool Tight

Courtesy of Salomon

What We Like
  • Breathable

  • Odor-resistant

What We Don't Like
  • Thin construction

  • Shouldn’t be worn alone

These wool tights from Salomon are great for outdoor activities, thanks to the merino wool’s natural ability to keep odor away. The fit of these tights is certainly slim and best worn underneath a layer or two. The material delivers great breathability, which is key to anyone getting active outside.

Sizes: S, M, L, XL, XXL | Fabric: Nylon, wool, elastane | Care: Hand wash

Best Leggings (Women): Arc'teryx Rho LT Bottom

Arc'teryx Rho LT Bottom

Courtesy of Backcountry

What We Like
  • Compression

  • Ability to wear solo or under a layer

What We Don't Like
  • No extended sizing

The Rho's from Arc'teryx are lightweight leggings that are deceivingly warm. Worn under another layer, they’re a great option for skiing and snowshoeing in the winter and climbing or hiking in the warmer months. There’s a built-in brushed lining that makes these all the more comfortable and soft. These leggings are thick enough to wear without a top layer, as well, for chillier days.

Sizes: XS, S, M, L, XL | Fabric: Polyester, elastane | Care: Hand wash or machine wash on cold and tumble dry on low

Best Set: Thermajane Ultra Soft Thermal Underwear Long Johns Set

Thermajane Women's Ultra Soft Thermal Underwear Long Johns Set

Courtesy of Amazon

What We Like
  • Versatile

  • Quick drying

  • Easy to move in

What We Don't Like
  • Snags easily

Thermajane’s thermal long john set is fleece-lined and softer than soft. Consisting of a long-sleeve shirt and a pair of leggings, this odor-resisting set has a special four-way stretch that allows you to move comfortably in any direction (great for skiing or any sport that can send you bending in weird positions). Available in 12 colors, this thermal underwear set is just as nice for lounging around the house as it is keeping you warm under all of your outdoor gear. This thermal set is also available in a men’s style.

Sizes: XXS, XS, S, M, L, XXL, 3XL | Fabric: Polyester, spandex | Care: Hand wash

Best Wool: Icebreaker 175 Everyday Leggings

Icebreaker Women's 175 Everyday Leggings

Courtesy of Amazon

What We Like
  • Fitted design

  • Flat seams

  • Temperature control

What We Don't Like
  • Limited color options

Form-fitting and tight on the ankle, these wool leggings are the ultimate base layer for winter sports enthusiasts. They can certainly be worn casually, but those who put them to the test on a hike, on the slopes, or with some alpine climbing will experience them at their best. The seams are minimal, which helps keep chafing away, and there’s a functioning fly for convenience in the men’s style.

Sizes: XS, S, M, L, XL | Fabric: Merino wool | Care: Machine wash on a cold gentle cycle and tumble dry on low

Best Lightweight: Ortovox 185 Rock'N'Wool Long-Sleeve Top

Ortovox 185 Rock'N'Wool Long-Sleeve Top

Courtesy of Backcountry

What We Like
  • Multiple color options

  • Odor resistant

  • Durable

What We Don't Like
  • Expensive

Orotvox’s Rock’N’Wool top was built for outdoor performance, from its odor-resisting merino wool to its top-notch temperature regulation. If a pop of color is what you’re after, you found it. Opposed to most thermal underwear and its ever-neutral palette, this lightweight, long-sleeve option comes in three bold colors and patterns. This thermal top is also available in a women’s style.

Sizes: S, M, L, XL | Fabric: Wool | Care: Hand wash

Best for Layering (Men’s): Smartwool Merino 250 Base Layer

Smartwool Men's Merino 250 Base Layer Bottom

Courtesy of Smartwool

What We Like
  • Flat-seam design

  • Traditional fly

What We Don't Like
  • Can’t be worn solo

Smartwool is known for its rock-solid basics and these leggings are no exception. The midweight Merino 250 Base Layer was designed with a flat-lock seam, meaning there’s less opportunity for chafing. The leggings come with all of the benefits of merino wool (odor resistance, temperature regulation, soft-touch) and a convenient, traditional fly.

Sizes: S, M, L, XL, XXL | Fabric: Merino wool | Care: Hand wash

Tested by TripSavvy

I purchased Smartwool's Merino 250 Base Layer a few years back for the same purpose most do: I wanted a midweight layer for skiing from a brand I knew would deliver a solid product. The 250 has delivered on that and more. It's now infiltrated my layering system for basically all of my cooler weather outdoor pursuits. I recently wore it backpacking California's Ansel Adams Wilderness, then hiking Duck Pass outside of Mammoth Lakes, then fly-fishing on Crowley Lake just outside of Mammoth, and even the local brewery.

My favorite aspect of this layer? Merino wool is truly a wizard at masking stink. I've worn the 250 on runs multiple days in a row, logging upwards of 50 miles in-between washes. My second favorite aspect? It's style. (Note: I own the hooded version.) Never have I been complimented so much on the looks of a layer meant to go under outer layers. Style shouldn't be prioritized over performance when it comes to activewear. But why not have both? — Nathan Allen, Outdoor Gear Editor

Best for Layering (Women’s): 32 Degrees Base Layer Thermal Leggings

What We Like
  • Anti-odor

  • Four-way stretch

What We Don't Like
  • Thin construction

This baselayer from 32 Degrees feels like wearing nothing (in the best way possible). Both sides of the polyester fabric are brushed and very soft to the touch. With four-way stretch and an elastic waistband, this layering piece is invisible—again, in the best way possible—perfect for slipping on underneath a pair of hiking pants, snow pants, or denim. Keep in mind that the thinness of the fabric, while warm, can be a bit see-through if you’re wearing them solo.

Sizes: XS, S, M, L, XL, XXL | Fabric: Polyester, spandex | Care: Wash cold on the gentle cycle, tumble dry on low

Final Verdict

If you don’t mind making an investment, look into the Smartwool Intraknit Merino 250 Thermal Bottoms (view at Amazon). Available in both men’s and women’s styles, this legging does a great job insulating and ventilating. If you are looking for an inexpensive pair to wear around the house or in less strenuous scenarios, opt for the Thermajane Ultra Soft Thermal Underwear Long Johns Set (view at Amazon), which provides both a thermal t-shirt and leggings at a great price.

What to Look For in Thermal Underwear

Fabric

There are three main materials used to make thermal underwear: silk, wool, and synthetic fabric. All of these are fantastic insulators, meaning they keep your body heat close to, well, your body. Silk is the most fragile of these fabrics and requires hand washing. Wool is a bit sturdier and synthetics even more so. Synthetic fabrics and wool tend to be best for winter athletes, while silk is great for casual wear and lounging.

Sizing

Thermal underwear is meant to fit close to the body, as it’s often worn as a base layer underneath other clothing. A closer-fitting base layer means less chafing and less bulk.

Frequently Asked Questions
  • How do I wash my thermal underwear?

    Always follow the manufacturer’s instructions, but a good rule of thumb is to hand wash silk, and wash wool and synthetic fibers in a cold gentle cycle and tumble dry on low to avoid any shrinking.

  • Are all thermal underwear odor-resistant?

    Yes and no. Merino wool is naturally odor-resistant, but synthetic fabrics can pick up slight smells. Make sure to wash your base layers often with gentle detergent.

Why Trust TripSavvy

Erika Owen is a travel writer and a cold-weather enthusiast. She loves bundling up to take on the elements and put hours of research into these picks.

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