The 12 Best Thermal Underwear of 2021

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The Rundown

Best Overall: Smartwool Merino 250 Base Layer at Smartwool

"The warmest base-layer option that Smartwool offers."

Runner-Up, Best Overall: Duofold Midweight Thermal Pant at Amazon

"Consists of a cotton inner layer for softness along with merino wool in the outer layer for natural insulation."

Best Budget: 32 Degrees Lightweight Thermal Baselayer Legging Pant at Amazon

"A cold-weather favorite with an affordable price tag that's ideal for everyday activities."

Best for Women: L.L.Bean Midweight Base Layer Pants at L.L. Bean

"Featuring a 190-gram polyester fabric, these thermal pants will keep you warm and wick moisture away."

Best for Men: Patagonia Capilene Zip-Neck Long Underwear Top at Backcountry

"Its long, center-front zipper allows for easy venting when you get sweaty."

Best Plus-Size: In'Voland Plus-Size Long Johns Set at Amazon

"Designed for every body with material that breathes exceptionally well and stretches nicely."

Best Leggings: ColdPruf Women's Platinum Dual Layer Bottom at Amazon

"Designed without tags and feature flat seams so they can easily be worn under any pair of pants."

Best Set: Thermajohn Men's Thermal Underwear Long Johns Set at Amazon

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

Best Wool: Brynje Wool Thermo Long Sleeve Shirt Base Layer at Brynje

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

Best Style: Under Armour HeatGear Leggings at Amazon

"The lightweight HeatGear fabric is just a tiny bit shiny, so these leggings are more stylish than most"

Thermal underwear is essential for staying warm in freezing temperatures. But it can be hard to know where to start when trying to find the perfect piece to deal with the cold. There are so many options out there—and it’s easy to get overwhelmed by just the fabric options alone. Some people swear by wool (especially Merino wool, which is known for its lightweight thermal insulation); others can’t imagine thermal underwear in anything but silk. There are also decent affordable options available, and these tend to be durable and easier to care for. Of course, some people will need these critical base layers more than others, so if you’re living in a cold climate—versus just taking a trip to one—it might be worth swinging for one of the more expensive options to stay warm all winter long. Whether you're looking for tops or bottoms, we've scoured the web to help make your search easier.

Scroll through for the best thermal underwear available online.

Our Top Picks

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Best Overall: Smartwool Merino 250 Base Layer

Hands down, the best long underwear for extreme temperatures is Smartwool’s Merino 250 top. Not only is this midweight, merino wool base-layer the warmest base-layer option that Smartwool offers, but it’s also highly breathable and ultra-comfy, with none of the itchiness of other thermal underwear fabrics. Shoulder panels help to reduce chafing and optimize comfort, while flatlock stitching eliminates abrasion and enhances fit by reducing bulky seams. Plus, you’ll stay protected from the sun’s harmful UV rays, thanks to the top’s UPF 50+ layer of fabric. All in all, when it comes to sheer quality and durability, nobody does thermal underwear better than Smartwool. A men's version is available here.

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Runner-Up, Best Overall: Duofold Midweight Thermal Pant

Duofold’s Midweight Double-Layer Thermal Pants are some of the company's warmest styles, and they’re the perfect choice for those who are looking for a budget-minded pick for a cold vacation—or just need to stock up on a few pairs to get through winter. Its two layers consist of a 100 percent cotton inner layer for softness, along with merino wool in the outer layer for natural insulation. The anti-microbial fabric keeps smells out, while moisture-wicking tech keeps you warm—and sweat-free. The flatlock seams prevent chafing, no matter what activity you’re up to, and the tagless label doesn’t irritate or itch skin. You won’t want to wear these solo in the cold, as they’re not incredibly windproof, but under a thicker outer layer, they do great. Keep in mind that for long legs, these might run a tiny bit on the short side. A similar version for women is available here.

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Best Budget: 32 Degrees Lightweight Thermal Baselayer Legging Pant

Budget-conscious shoppers will delight in the 32 Degrees Thermal Leggings, which are a cold-weather favorite with an affordable price tag to boot. Ideal for everyday wintry activities, these snug-fitting leggings also offer maximum stretch, allowing for full freedom of movement. Available in three neutral shades (charcoal heather, stormy night, and black) so you can pair them with any outfit, these leggings are made of a lightweight yet warm blend of polyester and spandex that wicks away moisture. They’re also lightly brushed on both sides, which gives them a soft, cozy feel that’s smooth to the touch.

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Best for Women: L.L.Bean Midweight Base Layer Pants

L.L. Bean Women's Midweight Base Layer Pants — Courtesy of L.L. Bean

L.L.Bean does base layers incredibly well, and the Midweight Base Layer Pants for women are no exception. These pants are easily one of the most reliable choices for thermal underwear, especially if you’ll be in extra-frigid temperatures. Featuring a 190-gram polyester fabric, these thermal pants will keep you super warm on the inside, regardless of how cold it gets outside. The pants wick moisture away from your body, making them extremely breathable and odor-resistant. As a bonus, L.L.Bean also uses recycled content when making these pants to minimize the impact on the environment. These pants are available in regular and petite sizes ranging from extra small to extra large.

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Best for Men: Patagonia Capilene Zip-Neck Long Underwear Top

Patagonia Capilene Midweight Zip-Neck Long Underwear Top — Backcountry

Patagonia is the crème de la crème of activewear, so it’s no surprise that the Capilene Midweight Zip-Neck Top for men is a winner. To begin with, there’s the specialized, quick-drying Polartec Power Grid fabric that boasts warmth, breathability, and optimal moisture-wicking performance. There’s also the top's genius structure—a long, center-front zipper that allows for easy venting when you get a tad too sweaty, as well as full underarm gussets and raglan shoulder seams that provide shoulder mobility. Plus, the fabric’s smooth face is also ideal for layering. This Patagonia top has it all.

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Best Plus-Size: In'Voland Plus-Size Long Johns Set

Body-inclusivity for the win. The In’Voland Women’s Plus-Size Long Johns are designed for every body, with material that breathes exceptionally well and stretches nicely, ensuring a comfortable fit. The top and bottom of this set are lined with soft fleece for extra warmth, and together, they work equally well as snuggly pajamas or as layers underneath bulky winter clothing. This set comes in a wide variety of colors that run the gamut from wine red to purple to champagne blue, and the round neck and long sleeves make for a flattering fit.

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Best Leggings: ColdPruf Women's Platinum Dual Layer Bottom

For extra warmth for your lower body (without adding a bulky under-layer to your outfit), ColdPruf's Platinum Dual Layer leggings are a classic choice. These fashionable leggings are designed without tags and feature flat seams so they can easily be worn under any pair of pants. Made out of a polyester and wool blend, these leggings feature an array of cold-weather comfort features including insulating, moisture-wicking, and anti-odor/anti-microbial to keep you warm while outside.

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Best Set: Thermajohn Men's Thermal Underwear Long Johns Set

Made of ultra-premium poly-spandex fabric, the Thermajohn Long Johns Set is like a second skin for the whole body, to be worn under your winter outerwear. Offering great breathability and anti-microbial properties, these thermal base layers efficiently trap heat while simultaneously wicking away moisture from the skin and dry fast for comfort during intense physical activity. Both the top and bottom are stretchy but body-hugging, so they can easily be worn underneath any type of winter gear for added insulation. And, for more comfort, the elastic waistband stretches and moves with your body. Thermajohn's Long Johns come in five colors: white, navy, grey, charcoal, and black.

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Best Wool: Brynje Wool Thermo Long Sleeve Shirt Base Layer

Brynje Women’s Wool Thermo Long Sleeve Shirt Base Layer with Inlay — Courtesy of Brynje

Handcrafted with 80 percent merino wool and 20 percent polyester yarn, the Women’s Wool Thermal Shirt from Brynje is incredibly soft on the skin, providing a significant layer of warmth in freezing temperatures. In fact, the classic ⅜-inch open weave design provides four to six times the insulation value of any solid knit by weight, making this the perfect layer to wear if you’re going skiing or winter camping. The brand’s high-quality wool blend has better strength and durability than 100 percent Merino wool yarns, and the fabric’s odor management properties are excellent. Not to mention, the specialized mesh design allows moisture to transfer in vapor form—so you stay nice and dry, no matter how hard your body works.

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Best Style: Under Armour HeatGear Leggings

Why go basic black when you could don a pair of sweet, sleek thermal leggings in shades of electric purple or midnight navy? Under Armour’s Women’s HeatGear No-Slip Leggings come in four vibrant colors, and the lightweight HeatGear fabric is just a tiny bit shiny, so these leggings are more stylish than most. The stretchy, supportive fabric also delivers optimal coverage without feeling too bulky, and it effectively wicks away and dries sweat. Plus, you don’t have to worry about getting stinky on long hikes since the anti-odor technology successfully prevents the growth of odor-causing microbes. You’ll simply love the fit, too—the double-sided silicone print helps keep your waistband from sagging or slipping as you move around, which can be especially freeing if you’re running or doing other high-intensity activities. See a men's version here.

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Best Alternative: Terramar Women’s Thermasilk Pointelle Pant

A natural alternative to wool thermal underwear, Terramar’s Thermasilk Pointelle leggings are crafted from 100 percent filament silk. Lightweight and stretchy, these underwear bottoms come with the EC2 Qwik-Dri Electrostatic process, which has the ability to lift the moisture molecules away from your body and break them down for swift evaporation. No matter what moderately cold-weather activities you’re undertaking, these Terramar leggings will keep you snug and warm—conveniently, they’re machine-washable, too. Just keep in mind that silk-based thermal underwear may not be as thick or insulating as a wool blend, so these leggings probably aren’t the best choice for subzero temperatures.

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Best Wool Blend: Icebreaker Women’s BodyFitZone 200 Zone Leggings

Whether you’re hitting the ski slopes, traipsing around on snow-covered trails, or just trying to get home from work without turning into a human icicle, the Icebreaker Women’s BodyFitZone Leggings are ideal. They’re perfect for year-round use, thanks to the lightweight merino wool fabric with strategically placed mesh panels for active ventilation. Flatlock seams prevent chafing and the LYCRA content aids in stretch and mobility. And, the Icebreaker-branded brushed elastic waistband offers ultimate comfort.

Final Verdict

Smartwool’s Merino 250 Top (view at Smartwool) is at the top of the heap when it comes to performance, fit, and protection. You just can’t beat the fabric—100 percent merino wool made with an interlock knit—and construction, which includes shoulder panels, a flatlock seam, and back body seams, all of which help to minimize chafing and provide next-to-skin comfort, while still allowing for ample breathability.

What to Look For in Thermal Underwear

Material: Selecting the right material for your thermal underwear is critical—different types of fabrics work in different ways to regulate your body temperature. Merino wool and synthetic fabrics like polyester and spandex (or a poly-spandex blend) are, by far, the most common fabrics used for thermal base layers. Both of these types of fabric are great at wicking away moisture, preventing sweat buildup, drying fast, and keeping you warm in extreme temperatures.

In terms of their (slight) differences, merino wool tends to take longer to dry than most synthetic fabrics, though it’s slightly warmer than polyester, nylon, or spandex. Wool is also more susceptible to shrinkage than synthetic fabrics, so you have to be careful to hand-wash it.

Fit: Looser isn’t better in this case. Keep in mind that snug-fitting thermal underwear will keep you warmer than loose-fitting layers, which allow for more air circulation and therefore have less insulating properties. Also, to be able to wick sweat from your body efficiently, your thermals should have a true next-to-skin fit.

Weight: Depending on what activity you’ll be doing outdoors and what the climate is like, there are different weights to consider when you’re looking for the right thermals. As a general rule of thumb, lighter weights work better for warmer temperatures and high-movement activities. So, if you’re going backpacking, hiking, or running, you’ll want to seek out thin, lightweight thermal layers with moisture-wicking properties. Going cross-country skiing, snowboarding, or winter backpacking? Thick, insulated thermal underwear is what you need.

FAQs

What is the difference between thermal and base layers?

A base or thermal layer is the layer of clothing that touches the skin and regulates your body temperature, keeping you either cool or warm. The terms “thermal underwear” and “base layer” are basically interchangeable—when people use either term, they mean lightweight pieces of clothing to wear underneath outerwear to keep you warm and regulate heat.

How should I wash thermals?

Washing your thermals by hand is always preferable in terms of preserving them for as long as possible. That being said, technically, you can machine-wash your thermals if they’re made of synthetic fabrics (like spandex or polyester); just make sure you do so on a delicate cycle, in warm water, using mild detergent. If, on the other hand, your thermals are made from Merino wool, you should always hand-wash them.

Do I need to wear underwear underneath my thermal layer?

Yes. Thermal underwear is just clothing that’s worn underneath your other clothing; it isn’t underwear.

Why Trust TripSavvy

Justine Harrington loves nothing more than being outdoors—she’s a fan of the slow sports, like hiking and backpacking—but as a long-time Austinite, she’s grown accustomed to 100-degree temperatures as the norm and is a tad fragile when it comes to facing the cold. As such, when preparing for a winter backpacking trip or hike (or, let’s be honest, a mere walk when it’s colder than 60 degrees), she’s positively fanatical about wearing the perfect base layers to stay as toasty and comfortable as possible.

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