The best thermal underwear is nothing short of essential when the mercury drops. Whether you’re a skier or snowboarder or simply looking to stay warm outside or at night, thermal underwear is your first layer of defense against winter temps. 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 evaluated all the specs and came up with 9 picks that we highly recommend depending on your specific needs.
The best thermal underwear keeps you warm, comfortable, and active in any condition.
At Forbes Vetted, we’re experts on thermal underwear, having tested countless pairs 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, 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 or activity.
- Best Thermal Underwear Overall: Smartwool Smartwool Classic All-Season Merino Long-Sleeve Top and Thermal Merino Bottoms
- Best Affordable Thermal Underwear: REI Co-Op Midweight Long-Sleeve Base Layer
- Best Thermal Underwear For Extreme Cold: Icebreaker MerinoFine Polar Long Sleeve Half Zip Thermal Top
- Best Lightweight Thermal Underwear: Helly Hansen Lifa Stripe Long-Sleeve Crew Base Layer
- Best Sweat-Wicking Thermal Underwear: Under Armour Heatgear Compression Long-Sleeve T-Shirt
- Best Comfortable Thermal Underwear: Arc’teryx Rho LT Crew Neck Base Layer Top
- Best Thermal Underwear For Sleeping: L.L.Bean Men’s Double-Layer Underwear Crewneck
- Best Thermal Underwear For Layering: Outdoor Research Alpine Onset Crew
- Best Thermal Underwear For Everyday: Carhartt Base Force Midweight Classic Crew
REI
Smartwool Classic All-Season Merino Long-Sleeve Base Layer Top – Men’s
REI
Smartwool Classic Thermal Merino Base Layer Bottoms – Men’s
Material: 88% Merino wool, 12% nylon (top); 100% merino wool (bottoms) | Weight: Midweight | Sizes: S to XXL
Colorado-based Smartwool was one of the earliest outdoor brands to specialize in performance merino wool—it first launched ski socks, and then worked their way up the body, releasing more technical layers in the material. Merino is particularly well suited to thermal underwear since it wicks moisture, regulates temperature and resists odors. While Smartwool is certainly not the only label to work with merino these days, 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 shoulders, chest and hips 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.
MOST POPULAR
REI
REI Co-op Midweight Long-Sleeve Base Layer
Material: 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 crafted from a stretchy, synthetic, Bluesign-approved polyester that keeps cost low but comfort high. It’s not as warm and doesn’t wick as well as those more technical, pricier, merino-based options, but it supplies enough performance for powder days on the hill and chilly adventures on the trail. For spring or milder climates, REI also makes an excellent lightweight base layer that’s even more affordable the outfitter also does offer merino options, and while they’re expensive relative to these synthetic options, they’re some of the more affordable yet high-quality wool options on the market.
Icebreaker
Icebreaker Men’s 300 MerinoFine Polar Long Sleeve Half Zip Thermal Top
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, and, in our opinion, the heaviest you need in your base layer arsenal. Not only that, but it’s one of the softest fabrics we’ve ever tested, too. “I never want to take it off,” reported a tester after wearing it on bitter cold days in the French Alps. 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.
Helly Hansen
Helly Hansen Lifa Stripe Long-Sleeve Crew Base Layer
Material: Bluesign-approved polypropylene | Weight: 150g | 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.
Amazon
Under Armour Mens Armour Heatgear Compression Long-Sleeve T-Shirt
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.
Arc’teryx
Arc’teryx Rho LT Crew Neck Base Layer Top – Men’s
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.
L.L.Bean
L.L.Bean Men’s Double-Layer Underwear Crewneck
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 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.
Outdoor Research
Outdoor Research Men’s Alpine Onset Crew
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.
Carhartt
Carhartt Men’s Size Base Force Midweight Classic Crew
If you’re working outside or traveling in a cold climate, Carhartt’s bestselling thermal crew is a comfortable and versatile base layer made of 80% polyester and 20% wool. It engineered with the brand’s “Force FastDry” technology to wick sweat and dry quickly, and has built-in stretch to move with you. It also fights odors, has smooth flatlock seams and a tagless label for comfort, and has a concealed thumb loop. Since it made from a wool blend, 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.
Why Trust Forbes Vetted
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.
How We Chose The Best Thermal Underwear
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 November 2023.
What To Consider When Buying Thermal Underwear
- Material: The best thermal underwear is often made from materials like merino wool, synthetic fibers (like polyester or polypropylene), or a blend of both. Merino wool is excellent for temperature regulation and odor resistance, while synthetic materials are known for their moisture-wicking properties and quick drying time. Cotton should generally be avoided for cold weather gear as it doesn’t wick moisture and loses its insulating properties when wet.
- Weight and Fabric Density: Thermal underwear comes in different weights or thicknesses, usually categorized as lightweight, midweight, or heavyweight. The choice depends on the level of cold you’ll be experiencing and whether the thermals will be worn alone or as a base layer under other clothing.
- Fit: A good fit is crucial. The best thermal underwear should fit snugly against the skin to trap body heat, without being so tight that it restricts movement. Look for a stretchy material that conforms to the body while allowing full range of motion.
- Moisture Management: Quality thermal underwear is designed to effectively wick moisture (sweat) away from the skin. This is particularly important in cold weather, as sweat can lead to rapid body heat loss. Materials like merino wool or advanced synthetics are adept at absorbing sweat from the skin and transferring it to the outer surface of the garment.
- Breathability: Along with moisture-wicking, breathability is key to prevent overheating and sweating when you’re active. Good thermal underwear should allow air and moisture to pass through the fabric to maintain a comfortable body temperature.
- Seam Construction: Flatlock seams or seamless designs are preferable as they reduce the risk of irritation or chafing, especially in high-movement areas.
- Odor Resistance: Some fabrics, particularly merino wool, have natural odor-resistant properties. For synthetic materials, look for treatments that inhibit the growth of bacteria that cause odor.
- Durability and Care: Quality thermal underwear should withstand regular washing without losing its shape or functionality. Check care labels for washing instructions. Higher-end fabrics often have better durability.
- Additional Features: Some thermals come with extra features like UV protection, anti-bacterial treatments or reinforced panels in high-wear areas.
What’s The Difference Between Thermal Underwear And Base Layers?
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.
Are Synthetic Or Natural Fibers Best?
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.
What’s The Best Weight For Thermal Underwear?
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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