I grew up in the Northeast and am currently based in the Pacific Northwest—which means I’ve had my fair share of frigid, snowy winters, and practically live in thermal underwear for months. These base layers deliver warmth by insulating while pulling moisture away from the skin, keeping you dry even during intense cold. While I’ve had the opportunity to try countless pairs of thermal underwear over the years, my consistent favorite is Smartwool’s Classic Thermal Merino Layers—they strike that perfect balance between comfort and all-day performance, and they’re built to last. Below, you’ll find these and my other top picks for the best thermal underwear, all evaluated for the chilliest of conditions.
Stay warm all winter long with a set of soft, durable thermal underwear.
Illustration: Forbes / Photo: Retailers
As someone who has tested pair after pair of thermal underwear over the years—in deep snow atop the Cascades, in the dead of the harshest Massachusetts winters and everywhere in between—I know what to look for in a quality set. I evaluated each of the picks below on their type and quality of fabric, plus their fit, breathability, durability and overall comfort. Below, the best thermal underwear that earned my stamp of approval, plus guidance on how to find a comfortable, well-fitting pair.
Smartwool’s Classic Thermal Merino Crew is the quintessential base layer and performed best in all of my testing. The 100% merino construction is buttery soft to the touch and breathes well despite the hefty fabric weight. Above all else, I love its anti-stink properties: As my go-to choice for resort skiing, I’ll wear the top and matching bottoms (available in men’s and women’s) for days between washes, and you’d truly never know (smell?) it. Smartwool also gets the fit just right—it stays close to the body but isn’t too tight. Details like flatlock anti-chafing seams and a backpack-friendly design make it that much easier to wear from the sofa to the summit.
That said, this pick may not be for everyone. For starters, merino’s high quality and many performance properties make it expensive, so a full set of this thermal underwear will often run you over $200. You also have to be careful with use and maintenance; wearing it in especially rugged scenarios and using the wrong detergent to clean it can break down merino fabric and create holes. Last, it’s a bit heavy for spring conditions, and may not be the best choice for any skiers or boarders who run hot. Still, it’s hard to beat this option if you’re after that do-it-all pair of thermal underwear for everything winter throws at you.
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Best Value Thermal Underwear
Quality Base Layers Without The Big Investment
REI
REI Co-op Midweight Long-Sleeve Base Layer
Material: 92% recycled polyester, 8% bluesign-approved spandex | Weight: 220 gsm | Sizes: S to XXXL
REI Co-op comes in clutch when you’re looking for that perfect blend of performance and price, as evidenced by the Midweight Long-Sleeve Base Layer. Made with a combination of polyester and spandex, this top is stretchy, sweat-wicking and perfect for chilly days. And at less than half the price of merino layers, it’s easy to commit to. It’s also got a matching midweight bottom for men and women if you’d like to complete the look.
The fabric weight is still a bit heavy for high-output activities, so I wouldn’t wear it while touring, but it’s great for just about everything else. REI’s generous return policy is also nice to lean on if the style truly doesn’t meet your needs—Co-op members have one full year, and non-members have 90 days to make a return.
Complaints are few at this price, but two stand out: The fabric doesn’t wick nearly as well as merino, nor does it handle odor management after a few days of wear. Again like the Smartwool layer, it’ll likely be a bit too hot for transitional seasons or intense activities, so you can opt for the Co-op’s lightweight layers instead.
Best Thermal Underwear For Extreme Cold
Heavyweight Fabric For Subzero Weather
MOST POPULAR
Icebreaker
Icebreaker 300 MerinoFine Polar Top
Material: 100% merino | Weight: 300 gsm | Sizes: XS to XL (women’s); S to XXL (men’s)
It doesn’t get any warmer than the 300 MerinoFine Polar Top—really. This is Icebreaker’s warmest layer at 300 gsm and the heaviest thermal layer in this guide, designed for the coldest sub-zero conditions. Not only that, but it’s also one of the softest layers I’ve ever worn, thanks to the use of ultra-fine merino that couldn’t cause irritation even if it tried. Icebreaker adds a high neck to keep out chilly drafts, plus handy thumb loops and flatlock seams to prevent chafing. The fit is fairly tight, but a half-zip version makes it easier to layer if you’d prefer to wear something underneath.
The only caveat to all this praise is the fact that this is not versatile enough to be the only base layer in your wardrobe. It’s far too warm for spring conditions, and may even be overkill for most average winter days. Unless you truly run cold or spend extended periods of time in frigid conditions, you may be just as well served with a midweight set.
BlackStrap is known for its balaclavas and face masks, but the Oregon brand has also surprised me with its new Summit base layers that mix style with substance. The combination of polyester and spandex delivers loads of warm comfort when I’m hanging around the house, and it stretches well as I get active. BlackStrap doesn’t list the fabric weight, but it feels somewhere between middleweight and heavyweight, so plan on wearing it with fewer layers or save it for resort days. The regular fit is also something to keep in mind if you prefer tighter base layers, though I’ve never found it to be an issue, and it could be used as a mid-layer, too. It also comes in a hooded version if you’re looking for a bit more protection against the elements.
Downsides? Well, like most synthetics, the polyester-spandex combo doesn’t breathe as well as natural fibers do, and it holds onto some odor even though BlackStrap uses built-in odor protection that the brand says “maintains freshness when the going gets tough.” Ultimately, to find quality thermal underwear from a reputable brand at this price is no easy task, but BlackStrap gets it right. For some other synthetic options that might do the trick, I also like Arc’teryx’s Kyanite (women’s) and Rho SV (men’s) layers for their quality and warmth, though both are more expensive.
After wearing Smartwool’s Classic Thermal Merino Crew Base Layer for years, I tried Stio’s lightweight Basis PeakWool Crew and immediately added it to my rotation. It’s just light enough to prevent overheating on uphill climbs while keeping you warm all the way back down. The combination of merino, tencel and nylon makes it soft and breathable yet strong and stretchy, which is perfect for heavy powder days or hot laps. Stio also uses flatlock seams for smooth layering and thumbholes to keep the sleeves in place. It’s a layer I solely use for backcountry skiing at this point, but you can just as easily wear it around the house or on the town.
The only drawback is its specificity: It’s a lightweight layer, so you’ll need to step up to a heavier fabric if you’re out in cold conditions, or on stop-and-go adventures. But if you’re after an even lighter layer than this one or want to stack a couple of options, consider the Rab Syncrino Base Long-Sleeve (125 gsm), which is good for skiing, biking, running and beyond.
Best Onesie Thermal Underwear
Thermal Underwear That Puts The "Fun" In Functional
Airblaster’s Classic Ninja Suit looks like a goofy gimmick—until you put it on and never want to take it off. The fun-loving brand has been perfecting its famed Ninja Suit onesies for nearly two decades, and the current lineup is better than ever. The Classic Suit’s stretchy blend of polyester and lycra feels like a blanket wrapped around your whole body, and the fact that it doesn’t expose even an inch of bare skin means you’ll never despair over chilly drafts. The fabric weighs in at 230 gsm—in my opinion, it hits that warmth-to-weight sweet spot—and the full-length zipper makes it really easy to put on and take off. I wear my Ninja Suit for days at a time and even sleep in it, but to each their own.
As for cons: Airblaster outfits this suit with a two-way zipper around the waist that makes bathroom breaks a whole lot easier, and while it works for the most part, I find that it’s still an awkward experience. Trying to use the bathroom while wearing a top that’s connected to your pants means moving in whatever way the fabric wants to move you, though even I’ll admit it’s preferable to taking the suit off. Airblaster also makes a merino Ninja Suit that weighs in at 210 gsm, but it’s over $100 more expensive than this synthetic option.
How We Chose The Best Thermal Underwear
To find and recommend the best thermal underwear, I looked into several important factors. Here’s how I went about selecting the pairs featured in this guide:
I started with the fabric itself, because different materials offer different benefits. Merino wool, for example, is naturally temperature-regulating and odor-resistant, while synthetics dry quickly and are often more affordable. The right fabric depends on your activity level, budget and other personal preferences.
Fit is also important when it comes to thermal underwear. You want something that sits close to your skin without feeling restrictive or bunching up when you move. I looked for options that come in multiple sizes and offer enough stretch to move with you throughout the day, no matter what you’re doing.
Durability matters as well, especially if you’re planning to wear your base layers regularly. I looked for reinforced seams, quality stitching and whether fabrics hold up after multiple washes without losing shape or warmth.
Breathability is often overlooked when you’re trying to keep warm, but it’s incredibly important. Even in cold weather, you don’t want to overheat or sweat, which is why the best thermal underwear manages moisture and lets your body breathe while keeping you warm. It’s all about striking the perfect balance.
Overall comfort ties everything together. This includes details like flatlock seams that won’t cause chafing, tagless collars and even thumb holes to keep the layer secure and block out chilly drafts. If your base layers aren’t comfortable, you won’t reach for them when you need them most.
In the end, I personally tested four pairs of thermal underwear featured in this guide (the layers from Smartwool, Stio, Blackstrap and Airblaster). To fill in the gaps, I surveyed snow-addicted friends and got their insights on the REI Co-op and Icebreaker sets, highly recommended layers for those in need of a budget-friendly and ultra-warm pair, respectively.
How To Shop For Thermal Underwear
Finding comfortable thermal underwear can make all the difference when it comes to staying warm in cold weather. Here’s what to consider as you shop:
Fabric
The material your thermal underwear is made from will determine how it performs. Natural materials like merino wool and alpaca are popular choices because they naturally regulate temperature, resist odors and retain comfort even if they get wet. Synthetic fabrics like polyester and nylon dry quickly, tend to be more budget-friendly and provide excellent moisture-wicking properties. And blended fabrics combine different materials to balance warmth, durability and cost.
Ultimately, think about what activities you’ll be doing before selecting a specific fabric. If you’re active and sweat more, synthetics might be your best bet. For everyday wear or lower-intensity activities, natural fabrics offer loads of comfort and are generally longer lasting. I personally switch back and forth between two pairs of thermal underwear (one merino wool, one polyester-spandex blend) based on the day’s activities and conditions.
Fit
Thermal underwear should be snug without feeling restrictive. When you’re trying them on, be sure to move around, bend, squat and stretch your arms out to make sure they don’t pinch or bunch up. Pay close attention to the length of the legs and sleeves—even mild drafts can make you cold—and refer to sizing guides as you shop. Consider whether you prefer a tighter fit or something with a bit more room based on where and when you’ll wear your set.
Durability
Thermal underwear can be expensive, but consider it an investment in long-term comfort. When evaluating durability ask the following questions: Is the fabric type and fit prone to pilling easily? Will it likely lose its elasticity or shape over time? For example, because of merino’s tight weave and durable fibers, options made from this fabric are known for their shape retention and with proper care, are fairly pill-resistant. High-quality pieces might cost more upfront, but they’ll save you money in the long run if they last several seasons, as opposed to just one winter.
Breathability
Even in freezing temps, your body generates heat that needs to escape. Thermal underwear that doesn’t breathe well will ultimately trap moisture, which pulls heat from your body and leaves you feeling cold. Look for fabrics or blends with breathable, moisture-wicking properties, as these materials pull sweat away from your skin and allow it to evaporate, thus keeping you dry and comfortable.
Why Trust Forbes Vetted
The Forbes Vetted gear team spends quite a bit of time braving the cold, which makes thermal underwear a necessity. In addition to thermal underwear, we’ve also covered heated gloves and ski jackets, and we manage an extensive library of winter gear guides.
Forbes Vetted gear editor and author of this guide Cam Vigliotta has years of experience using equipment in the cold. As a native New Englander, he grew up playing in snow drifts before graduating to winter hikes and, eventually, skiing. He now lives in Central Oregon, a stone’s throw from the Cascade Range, and spends just about every winter weekend in the snow, skiing, riding sleds or building the occasional fort that his childhood self would be proud of.
Brinda Ayer, Forbes Vetted editorial director, reviewed this guide. She’s an avid all-weather runner and is no stranger to base layers.
This guide is regularly refreshed to ensure all product information is current. It was last updated in December 2025 by Vigliotta, who started from scratch and reselected every pair of layers in this guide based on different needs.
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