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Thermal underwear is an absolute must 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. Some will opt for the lower costs of synthetic materials, which have come a long way in recent years and tend to be easy to care for and durable. Of course, some will need these critical base layers more than others, so if you’re 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.
We’ve scoured the Web to find the best thermal underwear, whether you’re looking for tops or bottoms (hint: buy them separately so you can get the fit you need)—as well as the best of what’s available in wool, fleece and synthetics (don’t worry, there’s a budget pick in there, too). Scroll through for the best thermal underwear to buy this year.
Our Top Picks
Best Overall: SmartWool Merino 250
SmartWool makes one of the best products on the market, and for good reason: The slim fit stays close to the body and flatlock seams help prevent chafing—a bonus when you’re being super-active or have multiple layers going. The wicking properties of the natural material are incredible, with sweat getting pulled off the skin and evaporating in no time. The fit stays incredibly comfortable, thanks to the ultra-soft wool and a slightly duck-tailed back for tucking in (or avoiding plumber’s crack). Plus, the odor-resistant material keeps smells from sticking to the fabric, even after multiple washes (or consecutive days of wear). You can tell that it’s well manufactured, too: The seams are more heavily stitched in areas that see a bit more action. Yes, SmartWool tops come with higher price tags, but with proper care, these thermal layers are built to last. (Find the men’s version here, and it also comes in a lighter-weight version)
Best Budget: Duofold Heavy Weight Double-Layer Thermal Pant
Duofold’s Heavy Weight Double-Layer Thermal Pants are the company’s heaviest style, 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 a cold winter. The spandex-polyester blend, anchored by a drawstring closure, have 360-degree stretch, meaning these hold their shape and stay up, even if you’re shoveling snow like a champ. 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 a tagless label that 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. (Also available for women.)
Best for Men: Carhartt Base Force Wicking Cotton Super Cold Weather Legging
Carhartt is known for making tough clothing for working men that won't wear out, and these leggings are no exception. Surprisingly, they’re made from 100-percent extremely soft cotton, but that doesn’t diminish their incredible thermal properties, which pack a punch on even the coldest days while still wicking away every drop of sweat you produce. A snug, tagless waistband keeps these leggings from falling down while you’re on the move, and these fit close to the skin without being full-on compression leggings (we also like the rib-knit cuffs to further retain warmth). In addition to stain-breaker technology that helps spills wash out easily, the highly durable fabric has been treated with antimicrobial tech, so smells won’t build up after a lot of use. It’s also available as a top to complete the set—and for larger men, the brand offers extended sizing, plus a special cut for taller dudes.
Best Wool: Minus33 Men’s Katmai Expedition Bottom
These expedition-weight leggings, made from 100 percent merino wool, are honestly incredible. They’re machine washable and dryable while still being made of truly heavyweight (400 g/m2), super-soft fabric that keeps adventurers warm in the coldest climates. Whether you’re running, hunting, climbing or snowboarding, these do their job of keeping you warm (but not overheated), and the flatlock seams prevent harsh chafing as you move. Ribbed cuffs help keep the heat in, too. Plus, built-in UPF 50 is great for those still getting a ton of sun exposure from hitting the slopes. Speaking of working up a sweat, merino wool naturally wicks away moisture, so feel free to engage in some high-octane adventure with these leggings, whether you wear them as your base, mid or outer layer. They won’t win too many awards for being ultra-light—weighing in at 14.4 ounces for a large-size pair—but there are too many other positives to pass up. (Also available for women.)
Best Alternative Fiber: FitExtreme Men’s MaxHeat Fleece Compression Long Johns
Once upon a time, your main fabric choices for thermal underwear were limited to just silk and wool. While wool—especially merino—is still a solid choice, pure silk thermal underwear has fallen out of favor in recent years, thanks to advances in fabric tech that yield more durable, easier to clean and warmer tops and leggings. One such replacement is fleece. FitExtreme’s fleece thermal underwear feature silky-soft fibers, along with four-way stretch compression, meaning these will hold up—and retain an unbelievable amount of heat—in the cold, no matter what activity you’re braving the weather for. At the same time, they stay dry, even while doing a mid-level exercise like skiing, and the material stops odors from clinging to the fibers, so you’ll stay fresh even after a few days of wear. We don’t usually recommend buying thermal underwear as a set—it’s better to get separates that each fit you perfectly—but this is one that passes every test. (Available for women also.)
Best Wool Blend: Duckworth Vapor Wool
When it comes to warm but breathable thermal underwear, Duckworth’s Vapor Wool blend gear is a favorite among both avid adventurers and those that just need an extra layer in cold weather. With T-shirts that work great as undershirts under sweaters, long-sleeved shirts and leggings for men and women, this is a great lightweight line to keep you warm—whatever pieces you require. (Guys especially love the three-quarter length of the leggings, which keeps them from getting bunched up in boots.) Each piece is a 38 percent merino wool, 50 percent polyester, and 12 percent modal blend, with the company’s high-quality wool hailing straight from a Montana ranch. The end result is durable, low maintenance pieces that are thin and heat-trapping, while ensuring moisture doesn’t just sit on the skin. The microbial fibers also keep odors from sticking to the fabric, even after dozens of washes. Speaking of washing, these pieces dry extraordinarily fast, too.
Best Onesie: Patagonia Capilene Thermal Weight One-Piece Suit
Patagonia’s Capilene line gets our vote for a few different situations: We love the mid-weight range for running—but if you need an old-fashioned onesie, their take on it goes above and beyond. Made with thin-but-mighty Polartec Power Grid fabric—including an odor-control component—the material’s open-grid pattern means that this one-piece is breathable, warm, and gets any moisture off the skin ASAP. With an eye to the environment, the fabric is between 30 percent and 92 percent recycled content. Another feature we fell in love with? With a single-unit garment, you don’t get any of the cold spots you might get under separates (i.e. no cold stomachs if you have to reach). On the women’s version, there’s a hole in the back of the hood for a ponytail to peek out from, proving this onesie really has it all. (It comes in a men’s version, too—and for extra warmth, there’s the expedition-weight version.)