The best thermal underwear is nothing short of essential when the mercury drops. Whether you’re a skier or simply looking to stay warm outside or at night, thermal underwear is your first layer of defense against the cold. Our top pick overall, Smartwool’s Classic Merino Long-Sleeve Top and Bottom, has the right weight and breathability that makes it versatile to wear on its own or as a base layer—essentially, this is a set you’ll get a lot of use out of. Other options work better in different situations, so we tested and evaluated all the specs and came up with 9 picks that we highly recommend depending on your specific needs.
We tested a variety of base layers to find the best thermal underwear for different activity levels ... [+] and budgets. Girls Autumn Clothes
At Forbes Vetted, we’re experts on cold-weather gear, having tested thermal underwear deep in the backcountry in Alaska, winter camping in Utah’s Wasatch, chasing powder in Japan and skiing in subzero temps in Jackson Hole. At the end of this guide, you’ll find more details on how we choose the best men’s thermal underwear as well as additional tips for layering and staying warm, no matter the conditions.
Material: 88% merino wool, 12% nylon | Weight: Midweight | Sizes: S to XXL
Colorado-based Smartwool was one of the earliest outdoor brands to specialize in performance merino wool—which wicks moisture, regulates temperature and resists odors. While Smartwool is certainly not the only brand to work with merino, it’s one of the best—the ultra-soft crewnecks and bottoms have flatlock seams to minimize chafing and feature anatomical fits that wrap around your body for a comfortable, second-skin feel. The prices are also reasonable considering the quality and durability—this is thermal underwear that holds up well after many seasons.
What our tester says: “I’ve tested Smartwool’s 150-grams-per-meter-squared layers on backcountry trips in the winter, brisk fall trail runs and high-alpine fly-fishing trips in the summer, among other adventures. They’re super comfortable, and, most importantly, warm and wicking. This pick can handle sweat, stink and multi-day sojourns extremely well. Even better, the long-sleeve fit is comfortable and snug without being overly form-fitting. In the winter, I recommend it for backcountry skiers and Nordic skiers engaging in high-output adventures, as well as riders who naturally run hot.” —Tested by Forbes contributor Drew Zieff
Material: 100% merino wool | Weight: Midweight | Sizes: S to XXL
When there’s a winter storm warning and temperatures drop, these merino layers are ultra-thick, soft to the touch and warm as can be.
What our tester says: “I usually snowboard in uninsulated shell pants, so I get all of my warmth from my base layers, and I can handle pretty much anything in these toasty thermals. I’ve relied on iterations of these while splitboarding in Alaska’s Denali National Park as well as the Arctic Circle in Sweden. You wouldn’t want to rock them on a warm spring day, but when the mercury plummets, it’s reassuring to have these in your closet. That said, they’re best-suited for skiing and snowboarding in sub-freezing to sub-zero temperatures, dog sledding, snowmobiling and winter camping. Whether you’re hanging out or just surviving in extreme weather conditions, these thermal layers are five-star.” —Tested by Forbes contributor Drew Zieff
Material: 100% Bluesign-approved polyester | Weight: Midweight | Sizes: S to XXXL
Less than half the price of more expensive competitors on this list, the REI’s Midweight Top and Bottoms are cut from a stretchy, synthetic, Bluesign-approved polyester that keeps cost low but comfort high. It’s not as warm as well as those more technical, pricier, merino-based options, but it supplies enough sweat-wicking performance for chilly days. For milder climates, REI also makes an excellent lightweight base layer.
What our tester says: “I generally lean toward natural merino wool base layers, but they’re usually twice the price of this REI bargain. The price tag is low thanks to a synthetic blend of 92% recycled polyester and 8% spandex. The fabric is a welcome 220 grams per meter squared, a bit higher than the 185 grams-per-meter-squared merino layers I normally pick for run-of-the-mill ski conditions. However, that extra heft helped the Midweight provide warmth while snowboarding during a cold snap in Tahoe, and it was so comfortable I took to sleeping in this on chilly nights in the mountains.
The fabric has a brushed back that’s soft and cozy against the skin, and between the looser fit and stretch provided by the spandex, the range of motion is top-notch. I appreciate the use of recycled and Bluesign-approved materials, and the fact that this is crafted in a fair trade certified factory, too. That said, I did have a couple complaints: It doesn’t wick nearly as well as my go-to merino picks, nor does it handle multiple days on the hill well in terms of odor management.” —Tested by Forbes contributor Drew Zieff
Fabric: 100% merino wool | Weight: 300 grams | Sizes: S to XXL
When temperatures plummet past sub-freezing into sub-zero territory, sub in Icebreaker’s Polar Long Sleeve Half Zip. At 300 grams per square meter, the Long Sleeve Roll Neck’s 100% merino fabric is one of the heaviest fabrics included in this article. Not only that, but it’s one of the softest fabrics we’ve ever tested, too. The mock neck cut doesn’t just earn the thermal layer style points but it also helps block cold wind and wet snow from ruining your time on the slopes.
What our tester says: “Knowing this was a 300 gram, pure-merino powerhouse, I wanted to devise a testing scenario that would really put this piece on the chopping block. I decided to head to my local resort in Tahoe extra early on a brutally cold powder day, waiting for over an hour in the shade for the lift to open as temperatures hovered around zero. While I had to bunch up my hands in my gloves to keep them warm, my core temperatures remained comfortable throughout the morning. When the chair opened and the sun came out, I took lap after lap, and throughout an all-out day of riding, the merino wicked well and kept me from overheating.
This Icebreaker piece will be a staple for the coldest days on the hill. It works well as a light mid-layer, too, thanks to the quarter-zip style and relaxed fit—it’s a truly warm, well-made thermal top. My only caveat is that you don’t want this to be your sole base layer: It’s too warm for spring conditions, and it’s even overkill for average days in a fairly temperate climate.” —Tested by Forbes contributor Drew Zieff
Material: 100% Bluesign-approved polypropylene | Weight: 150 grams | Sizes: XS to 3X
Helly Hansen’s bestselling base layer is great for spring or if you run hot. It’s made from Bluesign-approved polypropylene to provide breathable warmth, moisture regulation and comfort during any light outdoor activity like hiking, training or climbing. It’s 150 gram weight means that you can layer more on top to adjust your temperature during changing weather, too. If you want to step things up further, Helly Hansen’s Merino Crew Base Layer is cut from warmer merino with a 280 gram weight to seal in a bit more heat on chilly days.
What the reviews say: “I have a shirt that I have had for over 20 years and it is still in great shape and I wear it to bike and run three to four days per week from November to March. This updated version is not different. Very warm and wicks moisture really well. This is not a thick shirt but is very warm if you are active,” says an Amazon reviewer.
Materials: 84% polyester, 16% elastane | Weight: Lightweight | Sizes: XS to 4XL
High-quality thermal underwear should effectively wick moisture away from the skin—this is essential for staying warm, since damp fabric can lead to a significant loss of body heat. Under Armour’s Heatgear is designed to handle the conditions with smart features not found in all other thermal picks, including mesh underarm and back panels for ventilation and a quick drying fabric that helps absorb sweat from the surface of your body. This crew also has a compression fit to reduce muscle fatigue and improve circulation during your activities.
What our tester says: “If you’re looking for skin-tight compression, Under Armour has you covered with this polyester-and-spandex HeatGear long sleeve. It’s not designed for extreme cold—for that, you want Under Armour ColdGear or the UA Base 4.0 Crew—but it still can come in handy on the hill. I wore this layer on warmer afternoons on the slopes, and found a few benefits, namely the ultralight layer is thin as can be and the mesh panels under the arms amp up airflow. Also, even though I’m not usually a fan of form-fitting base layers, I liked how the compression added some perceived support to my shoulder (an old snowboard injury) and elbow (a new snowboard injury). I especially enjoyed the compression off the hill, while doing core workouts and mobility routines in a drafty mountain condo. My main complaint is obvious given the ultralight weight of the fabric—it’s not remotely warm, and likely provides the least warmth of base layers in this guide. However, for compression fans taking on medium-to-high-output workouts in chilly-to-moderate weather, this is a solid option, especially considering the price.” —Tested by Forbes contributor Drew Zieff
Material: 84% polyester, 16% elastane | Weight: Lightweight | Sizes: S to XXL
Some people prefer the feel of synthetic fabrics, finding them to be smoother or less irritating than even the finest wool. Others have allergies or sensitivities to wool, which can cause skin rashes. Arc'teryx’s thermal Rho LT Crew Neck Base Layer is made with stretch fleece that’s brushed for softness. It’s got a trim fit, chest pocket and stitch construction that reduces seam size and irritation, too. Fleece is not quite as warm as wool, though, so if you need the insulation, Arc'teryx Hybrid Crew Neck is cut from a soft version of merino with stretch comfort as well.
What the reviews say: “I wore the crew neck on a multi-day trip, hiking six to 10 miles with a day-pack in occasionally rainy 30 to 40 degree weather. I started with a thin puffer, but less than a mile in on the first day, I took the puffer off and never put it back on. The crewneck kept me so warm all weekend (with no noticeable stench). It was extremely comfortable to hike in. I cannot recommend it enough,” says an REI buyer.
Material: 50% cotton, 40% merino wool, 10% nylon | Weight: Lightweight | Sizes: S to XXL
If you’re camping or need to stay warm during bitterly cold winter nights, L.L.Bean’s thermal underwear is a classic. The crewneck and pants are made from a blend of cotton and merino wool, so they’re insulating but also breathable, and comfortable with stretch built into the fabric to help prevent it from losing its shape over time. Small details—like rib-knit cuffs and non-chafing flatlock seams—go a long way to make this a cozy set to sleep (and lounge) in.
What the reviews say: “This crewneck is very comfortable, whether you’re wearing it outside or around the house. It provides very good protection and warmth when your walking outside in cold weather. And that's saying a lot when you live near Lake Michigan in Chicago,” says an L.L.Bean reviewer.
Materials: 7% merino wool, 47% recycled polyester, 6% elastane jersey | Weight: 150 grams | Sizes: S to XXXL
When it comes to layering, your base thermal underwear should match the level of activity you'll be doing and the environmental conditions. For instance, if you're going to be in extremely cold temperatures but not very active, a heavier weight might be best. However, for high-intensity activities in cold weather, a lighter, more breathable fabric is preferable. Outdoor Research has options for both situations, so you can build your performance kit. The Alpine Onset Crew (above) is 150 gram weight, which is on the lighter side, while the 240 version of the crew has a heavier texture to better trap in heat. Both are equally great at wicking moisture, standing up to wear and tear and reducing chafing with flat seams, too.
What the reviews say: “I keep one in my pack for dramatic weather changes in mountain weather. Works well for me as a light layer to retain some warmth since I usually wear a lightweight shirt. Putting a Ferrosi light jacket over this has made a great combo to stay warm and block some wind while packing away compact and lightweight,” says a Moosejaw shopper.
Material: 100% polyester | Weight: Midweight | Sizes: S to 4XL
If you’re working outside or traveling in a cold climate, Carhartt’s bestselling thermal crew is a versatile base layer. It engineered with the brand’s “Force FastDry” technology to wick sweat and dry quickly, and has built-in stretch to move with you. This crewneck is not as warm as some other base layers on this list but for milder days, it’s a rugged (and affordable) option that serves you well and has a fitted design so you can wear it under hoodies and jackets.
What our tester says: “I’ve tested this Carhartt base layer on the slopes and handling chores that come with spending time in the mountains (shoveling, scraping ice off the windshield, waxing snowboards—and did I mention shoveling?). While the layer was decent on the slopes, it’s not my favorite pick for skiing and snowboarding. Why? Three reasons: it’s not nearly as warm as other midweight thermal layers I’ve tested; the 100% polyester fabric doesn’t handle stink well and the prominent seams can be uncomfortable, specifically in the shoulders if you’re wearing a backpack or bibs with suspender straps. That said, I find the long-sleeve more alluring for everyday wear. It looks sharp, and the waffled design helps trap heat and wick moisture. For the price, it’s a decent deal, as you can fill out your winter wardrobe with a couple of these everyday thermals.” —Tested by Forbes contributor Drew Zieff
Material: 93% polyester, 7% spandex | Weight: 220 grams | Sizes: S to XXL
It’s tough to find a base layer that’s as comfortable as the Oyuki Hitatech Longsleeve. But that’s not the only reason why this is our top pick for wearing on and off the slopes. Yes, the fabric—a divinely stretchy 220 gsm polyester and spandex blend—is immensely comfortable against the skin. But the fit itself is also on the loose side, adding a sense of ease that simply doesn’t exist in tighter, more form-fitting styles. No need to change out of your gear before you hit happy hour here.
Material: 51% merino wool, 49% recycled polyester | Weight: 190 grams | Sizes: XS to XXL
For backcountry skiers and snowboarders, a built-in balaclava is a game-changer. Patagonia’s Capilene Air Hoody sports a stretchy hood that blocks wind, snow and sun. The fabric—a blend of merino and polyester—also excels at wicking moisture and maintaining warmth.
Material: 100% merino wool | Weight: 260 grams | Sizes: S to XXL
If you’re looking for a one-and-done kit that comes with both top and bottoms, we highly recommend this option from Icebreaker. Both the top and bottoms are made from 100% merino wool and the 260-grams-per-meter-squared fabric is on the heavy side of midweight, making it an ideal everyday pick for unforgiving weather. Perforated sections also boost breathability in areas prone to sweat, including the back and underarms.
Over the years, the team at Forbes Vetted has published a variety of outdoor and fitness apparel articles that feature thoroughly researched products from brands we trust. As with any industry, athletic-wear brands make many claims, and it’s our job to cut through the noise by providing solid recommendations our readers can trust.
When it comes to thermal underwear, we understand how crucial it is to find a set that not only keeps you warm but also offers comfort, durability and suitability for your specific needs.
Our team comprises experienced editors and writers who delve deep into the world of thermal wear. We conduct thorough research, comparing materials, designs and user feedback to ensure we understand what makes a quality product.
Our product recommendations are always unbiased and independent, which ensures our reviews are honest and focused solely on product quality and consumer needs. Additionally, many of the products we recommend have been tested by our team or trusted experts in real-world conditions. This hands-on experience allows us to assess factors like warmth, comfort, fit, and durability firsthand.
We recognize that different people have different needs. Whether you’re looking for thermal underwear for extreme cold, high-intensity activities, or just everyday wear, we aimed to include a variety of options to suit diverse preferences and budgets. Lastly, the world of thermal wear is also always evolving, and we stay on top of the newest materials, technologies and trends. This ensures that our recommendations are not only reliable but also current; this story was last updated in January 2024.
The terms “thermal underwear” and “base layers” are often used interchangeably, but they can have slightly different connotations and purposes, depending on the context. While both thermal underwear and base layers are worn close to the skin to provide warmth in cold conditions, thermal underwear is typically more focused on insulation and is suited for low-activity situations. Base layers, on the other hand, offer a combination of moisture management, temperature regulation and sometimes compression, making them suitable for a wider range of activities and conditions.
The best thermal underwear accomplishes two basic tasks: Firstly, it keeps you warm by insulating your body and trapping warmth. Secondly, it keeps you dry by wicking sweat and moisture away from your body. How well your thermal set accomplishes these tasks depends on two critical elements—fabric type and fabric weight—and you should keep both at the top of mind while shopping.
Synthetic fibers like nylon, polyester and acrylic are common in fitness apparel production. These are man-made, processed fibers that tend to be cheaper to produce than their plant- or animal-based natural alternatives. On the upside, they can be lightweight, quick-drying, stretchy, durable, soft, breathable, easier to clean and care for, and, most importantly for many shoppers, significantly more affordable.
Of course, there are some downsides. Synthetic fibers often don’t have the anti-odor properties that naturally occur in certain natural fabrics, meaning they can get stinkier as you sweat. They’re also, at least in our experience, usually not as warm as high-quality natural fabric base layers. Finally, while certain recycled synthetics are responsibly sourced, some synthetics are petroleum-based, energy intensive and less sustainable than we’d like.
Natural fibers are also common in thermal underwear production, which can include everything from plant-based cotton and bamboo fibers to animal-based yak and merino wool. If you’ve already looked at our picks for the best thermal underwear above, clearly, we have a fondness for merino.
Merino naturally fights against odors, making it our preferred pick for backcountry ski trips and ski travel—if there’s a chance you won’t be showering for a few days, you want merino. It also has an excellent warmth-to-weight ratio, keeping you cozy in the coldest weather, yet still breathes and wicks moisture well, so you can vent if the weather heats up. Furthermore, it’s soft against the skin. However, merino can be less stretchy, less durable and much harder to care for than its synthetic counterparts. Also, some folks have wool allergies, precluding them from this fiber type. Others find the material scratchy, especially before you run it through the wash.
Both natural fibers and synthetic fibers have their pros and cons. One isn’t automatically better than another. Also, there are high-quality natural fibers, and low-quality natural fibers, and the same can be said for synthetic ones. Finally, it’s quite common to see blended hybrid fibers as manufacturers attempt to combine the pros of synthetics with the pros of natural fibers.
Grams per meter squared—also referred to as “gsm”—is the standard unit of measurement for fabric weight. The lightest thermal layers we included in this guide fall closer to 100 gsm, while the heaviest are up to 400 gsm. Thicker tops, ones that can be worn as a base or midlayer, can be found around 300 gsm and up.
For the most part, 185 to 250 is our sweet spot for most winter sports and cold-weather activities. If you’re looking for thermal underwear that can handle many temperatures, shoot for that range, and trend lower if you run hot, or higher if you run cold. If you ski all season long, from the coldest days in January to the sunny days of April, you may want to have two (or more) sets of thermal underwear: A warmer set for cold conditions, and a lighter, more breathable set for warmer weather.
Of course, while warmth varies with fabric weight, don’t forget to continue factoring in fabric type: A synthetic fabric that’s 200 gsm may be warmer than a natural fabric that’s 200 gsm, or vice versa.
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