Whether you’re into snow sports or simply want to prepare for winter weather, base layers can help insulate heat while wicking sweat away from your skin. Those base layers can include thermal underwear, also known as long johns, as well as other types of apparel like shirts and accessories.
Below you’ll find everything you need to know about base layers, including tips on how to shop for them, from Ashleigh McClary, a hiking enthusiast and Backcountry "Gearhead" (outdoor expert and customer service specialist), and Ben Clower, an outdoor enthusiast and marketing coordinator at JAX Outdoor Gear in Colorado.
SKIP AHEAD How I picked the best thermal underwear | The best thermal underwear in 2024 | The best thermal accessories | How to shop for thermal underwear | Why trust NBC Select?
How I picked the best thermal underwear
Per expert guidance, I kept the following in mind while gathering the best thermal underwear:
- Types: To make this list as comprehensive as possible, I included base layers of all different kinds, including tops, bottoms and accessories.
- Material: Clower calls Merino wool “the ultimate material” for thermal underwear because of its heat-trapping and temperature-regulating properties. For shoppers who find wool itchy, alternative (synthetic) fabrics like polyester, nylon and spandex offer the best value and good performance — certain blends of these materials, according to McClary, are just as warm as Merino wool. However, if you choose Merino wool, be sure to check the underwear’s temperature rating to be sure that it fits your needs (more on that below). The list below includes both 100% Merino wool and synthetic blend options, so you can find the best choice for your needs.
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The best thermal underwear in 2024
The list below includes expert recommendations and some personal favorites of NBC Select staff.
Editor’s pick: Uniqlo Heattech Tights
- Soft texture
- Good for everyday wear
- Anti-odor
- Nothing to note at this time.
NBC Select commerce editor Jordan Bowman loves these affordable Uniqlo tights for their flexible stretch and wide size range. In fact, he says he can’t survive a winter without them. “These are a necessity when temperatures drop in New York City,” he says. “If I see a gust of wind, that’s when I start reaching for the Heattech tights.” The tights are sleek and fitted for easy layering, and anti-odor, according to the brand. They’re designed to last through everyday wear and tear and hold their shape over time, according to the brand.
Material: 40% polyester, 32% acrylic, 21% rayon, 7% spandex | Fabric weight: lightweight | Sizes: XS-3XL
Best men’s pants: Smartwool Classic Thermal Merino Base Layer Bottoms
- Naturally odor-resistant
- Large size range
- Supportive design
- Nothing to note at this time
These 100% Merino wool bottoms from Smartwool come recommended by Clower. They have a slim fit that hugs the body and a mid-rise waistband designed to sit just at or below the belly button, according to the brand. To help reduce sagging, they also have a curved back seam for added support. To wash, the brand recommends a warm water, gentle cycle and drying on a low tumble. These bottoms are available in both men and women’s versions along with plus-size options up to 4X.
Material: 100% Merino wool | Fabric weight: midweight, 208 GSM | Sizes: XS-4X
Best men’s shirt: Smartwool Classic Thermal Merino Base Layer Crew
- Slim fit
- Lots of color options
- Seamless design
- Nothing to note at this time.
Another recommendation from Clower, this crewneck thermal shirt is the matching top to the Smartwool bottoms on this list. Like the bottoms, this top has a slim fit and is made from the naturally odor-reducing Merino wool, according to the brand. The top has a flatlock seam, which is used in outdoor gear to reduce chafing, and shoulder panels instead of seams for a smoother look. Like the bottoms, this top is available in men and women’s sizing.
Material: 100% Merino wool | Fabric weight: midweight, 250 GSM | Sizes: S-XXL
Best splurge pick: Patagonia Capilene Thermal Hoodie
- Non-itchy
- Lightweight
- Hoodie design
- Not a compressed fit
For people with sensitive skin who find Merino wool too itchy, McClary suggests Patagonia’s “incredibly soft and comfortable” Capilene Thermal base layers as an alternative. The fabric is Polartec Power Grid fleece, a mid- to lightweight base layer material designed to keep you warm without overheating your body, according to the brand. This hoodie has a regular fit, meaning it’s slightly looser than a fitted top, and can be layered with other pieces.
Material: Polartec fleece (92% recycled polyester, 8% spandex) | Fabric weight: midweight | Sizes: S-XXL
Best women’s pant: Patagonia Capilene Thermal Weight Bottom
- Breathable
- Great for intense movement
- Best for outdoor activities
These are McClary’s favorite bottoms from the Patagonia Capilene line. They’re flexible and breathable, making them great for high-intensity activities like snowboarding, skiing, hiking and more. They have a reinforced crotch area to better allow for unrestricted movement, and flat seams to prevent chafing, according to the brand.
Material: Polartec fleece (92% recycled polyester, 8% spandex) | Fabric weight: midweight | Sizes: XS-XL
Best women’s shirt: Icebreaker Women’s Merino 260 Tech Half Zip Thermal Top
- Good for very cold weather
- Anti-friction design
- Half-zip style
- Limited layering with half-zip
This Merino wool half-zip top from Icebreaker is great for layering during extra-cold days, or for more active winter hobbies. It’s soft and lightweight, with design features like a reinforced underarm to allow for greater mobility, and offset shoulder seams to help minimize friction, according to the brand. The top has a slim, body-hugging fit for easy layering, and has a 4.8-star average rating from 788 reviews on Icebreaker.
Material: 100% Merino wool, 260 GSM | Fabric weight: midweight | Sizes: S-XL
Best lightweight women’s pant: Halfdays Sophia Legging
- Breathable design
- Long cuffs
- Sweat-wicking
- Nothing to note at this time.
These base layer leggings are a favorite of NBC Select associate reporter Bianca Alvarez, who wears them for winter activities like snow tubing and skiing. “I love that they keep me warm but not to the point of overheating,” she says. She also likes the mesh calf design, which she says “makes the super soft material very breathable”, and how the long cuffs are comfortable to wear, even with snow boots.
Material: 96% recycled nylon, 4% spandex | Fabric weight: lightweight | Sizes: XS-2X
Best budget pick: 32 Degrees Lightweight Baselayer Scoop Top
- Multipacks available
- Affordable
- Lightweight but warm
- Too thin for extreme cold
Alvarez says that since her mom first bought these tops on a Costco run, her entire family wears them regularly as base layers under their winter clothes. Initially, she thought they were too thin to be effective, but says that after trying the top, she was happily wrong. “These are one of the warmest thermal layers I own,” she says. “I wear it on its own, under vests and of course, under winter jackets.” She also likes that the top is available in packs of two, and is machine washable. “It feels softer and softer after every wash,” she says.
Material: 90% polyester, 10% spandex | Fabric weight: lightweight | Sizes: XS-XXL
Best lightweight men’s long underwear: Lands’ End Men’s Stretch Thermaskin Long Underwear
- Wide size range
- Lightweight fabric
- Allows for movement
- Too thin for extreme cold
These long underwear are soft and stretchy to allow for less restrictive movement, according to the brand. They’re made from a poly-spandex blend that gives them a flexible feel, and are also designed with a sweat-wicking Thermaskin technology to keep your skin dry while wearing. The full elastic waistband keeps the thermal underwear in place, while the fabric has odor-controlling properties to keep you fresh at all times. These long underwear are available in both regular and tall sizes, and have a 4.4-star average rating from 107 reviews on Lands’ End.
Material: 92% polyester, 8% spandex | Fabric weight: lightweight | Sizes: XS-XXL
Best thermal set: Thermajane Long John Thermal Underwear Set
- Variety of styles
- Good value for money
- Slightly relaxed fit
This thermal underwear set is designed to be an everyday base layer, according to the brand. Each piece is lined with fleece for extra softness, and the set is highly rated with a 4.6-star average from 43,912 reviews on Amazon. The top is available in scoop neck, crew neck and V-neck styles, and both pieces can also be worn without a top layer.
Material: 92% polyester, 8% spandex | Fabric weight: lightweight | Sizes: XXS-3XL
Best active top: Baleaf Long Sleeve Thermal Shirt
- Has pocket
- Good for workouts
- Lightweight fabric
- Nothing to note at this time.
If you’re looking for a workout-friendly thermal base layer, consider this option from Baleaf, which has a 4.6-star average rating from 3,045 reviews on Amazon. This top is fleece-lined for added warmth, and has thumb holes to keep your sleeves in place while doing intense movement, according to the brand. The fabric is light enough to prevent overheating while wearing, and also has reflective elements to help with visibility during cold weather activities in the dark. The top also has a side zip-up pocket for added convenience.
Material: 87% polyester, 13% spandex | Fabric weight: lightweight | Sizes: XS-3XL
Best heavyweight: Arc’teryx Rho Heavyweight Bottoms
- Brushed fleece interior
- Zippered pocket
- Only for extreme cold
When you’re in need of heavy-duty warmth and layering, these Arc’teryx bottoms could be the answer. Their soft, brushed fleece interior traps heat for maximum warmth, and the smooth outer layer easily fits under other clothing pieces, according to the brand. The pants also have a laminated, zippered pocket that’s specifically sized to store your smartphone.
Material: 53% polyester, 38% nylon, 9% elastane | Fabric weight: heavyweight | Sizes: S-XXL
The best thermal accessories
“Your search for warm base layers shouldn’t be limited to just tops and bottoms,” says McClary. Thermal accessories can provide insulation and warmth for your head, hands and more — below, I rounded up some of the best thermal accessories, including expert picks and highly-rated options from top retailers.
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How to shop for thermal underwear
As you shop for your own thermal underwear, there are a few things you should keep in mind, including the underwear’s fabric weight and, when it comes to Merino wool, the temperature rating.
- Fabric weight: According to McClary, not all thermal underwear is the same warmth level, which is why she always checks the weight/density of the fabric before buying. “I look for keywords like lightweight, midweight, heavyweight or thermal weight,” she says. Lightweight options are best for cooler temperatures, whereas midweight thermal underwear works well with moderately cold weather. However in cases where a lot of extra warmth is needed, like below-zero temperatures, heavyweight underwear are best.
- Temperature rating: If you opt for Merino wool underwear, the temperature rating is a key factor. “Temperature rating is important because you want the appropriate amount of warmth for the temperatures you expect,” says Clower. The temperature rating corresponds with the fabric weight of Merino wool, which is measured in grams per square meter (GSM), according to the experts. McClary says she considers anything with a GSM of 100 or less to be lightweight warmth, anything from 100-250 GSM to be midweight warmth, and any thermal underwear with a GSM of 250 or higher to be heavyweight warmth.
Meet our experts
At NBC Select, we work with experts who have specialized knowledge and authority based on relevant training and/or experience. We also take steps to ensure all expert advice and recommendations are made independently and without undisclosed financial conflicts of interest.
- Ashleigh McClary is an avid hiking enthusiast and Backcountry Gearhead expert.
- Ben Clower is the marketing coordinator for Jax Outdoor Gear in Colorado, specializing in outdoor gear/product knowledge.
Why trust NBC Select?
I’m an associate SEO reporter for NBC Select, covering skin care, wellness, lifestyle and more, including stories on the best exercise bikes under $500 and the best travel mugs. For this article, I spoke to outdoor gear experts about the best thermal underwear, including their personal picks and advice on how to shop for base layers of your own.
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