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Thermal underwear goes by many names, from base layers to long underwear, and has more uses than we sometimes think. A good set of thermal underwear is essential for skiing, winter hikes and camping, but the best base layers can also be utilized for everyday activities such as chilly commutes, outdoor exercising and walking the dog. They’re an extra line of defense against the cold, and make a great addition to your (hopefully growing) loungewear collection.
The beauty of thermal underwear is its minimalism. The tight fit and heat-trapping technology in the best long underwear provide a lot of heat with just a little bit of fabric. This minimalism is why thermal underwear is so widely used in cold-weather sports, from skiing to winter running.
Although these pieces are fairly straightforward, there’s quite a bit of variation between options. Our buying guide below will help you find the best thermal underwear, as well as our favorite base layers for men that you can order right now.
Thermal Underwear Buying Guide
Our picks for the best thermal underwear include full sets, tops, bottoms and a onesie. Full sets are ideal, as you’ll be prepared for any weather condition or activity. But whether you choose a full set or a single piece, there are a few key considerations to keep in mind while shopping. Check them out below.
Purpose: The best thermal underwear is as versatile as possible, keeping you warm while commuting, sleeping in a tent and everything in-between. However, most base layers lean toward a certain activity — be it lounging, exercising or everyday wear. Consider how you’ll be using your layers before deciding on a set or piece.
Fabric: Many of the best modern thermal underwear pieces are made of high-quality merino wool, which keeps you warm while reducing sweat and odor. Some base layer fabrics also incorporate polyester to improve stretch, which is great for exercising.
Weight: A base layer’s weight refers to its warmth. A large portion of thermal underwear is ‘midweight’ or 250, as this type of base layer strikes a balance between warmth and slim comfort. For this reason, we’ve mainly stuck with midweight options for our picks (the exception being the lightweight Icebreaker 175 leggings).
Features: Keep an eye out for flies, pockets and vents while shopping for the best thermal underwear. Many wearers prefer minimal bells and whistles for a streamlined fit, while others like the convenience of a fly or pockets. Vents (typically in the armpits of base layer shirts) can be helpful for avoiding overheating while exercising.
1. Smartwool Merino 250 Baselayer Set
Smartwool’s 250 base layer collection offers some of our favorite thermal underwear because the pieces balance warmth, comfort and breathability. This means you can break them out for just about every cold-weather activity and even wear the long johns alone or under other pieces.
The secret to Smartwool’s long underwear is a high-quality 100 percent merino wool fabric. Merino wool is the gold standard of long underwear fabrics because it’s very warm, but also boasts natural sweat and odor-reducing capabilities. This means you won’t have to wash the Smartwool base layers as often as non-merino wool options, which is very convenient for backpackers and outdoorsmen.
The underwear also features flatlock seams which improve comfort by preventing chafing while you move. The fit is slim, allowing for easy layering, although the base layers (especially the shirt) can certainly be worn alone. Plus, a handful of color options lets you match your personal winter wardrobe style.
2. Patagonia Capilene Air Baselayer Set
These Capilene Air base layers from Patagonia are geared more towards activity than downtime, making them a good choice for intense hiking and backpacking.
The Capilene Air long underwear use 51 percent merino wool and 49 percent recycled polyester construction, which provides some extra stretch and breathability without sacrificing the warmth and anti-odor powers of merino wool. You don’t have to worry about chafing thanks to Patagonia’s seamless 3D construction, which adds to the underwear’s active character.
However, as with all activewear in 2021, the Capilene Air layers can certainly be worn for running errands or commuting in the cold thanks to the thermal underwear’s light, stretchy feel and odor-reducing powers. Seven color options for the shirt and two for the bottoms also give you some flexibility style-wise.
3. Carhartt Force Midweight Tech Baselayer Set
Carhartt is known for tough-as-nails workwear, and its Force midweight tech base layers stay true to the brand’s rugged reputation. They’re designed for just about any situation with a 100 percent polyester construction, although they’re comfortable enough to also pass as loungewear.
This polyester build translates to a stretchy feel and better range of motion while hiking, climbing or just doing yard work in the cold. It’s designed with flatlock seams and without annoying tags. And despite their lack of wool, the thermal underwear are still capable of wicking sweat, which is essential for a working set of long johns.
The shirt boasts a couple handy features. A streamlined zip pocket on the chest provides storage space for keys, a credit card or a pocket knife. On the sleeves you’ll find thumb loops, which prevent annoying ride-up under your other layers. The bottoms also have a fly for added convenience.
4. Columbia Omni-Heat Midweight Baselayer Shirt
If you’re in need of a high-quality thermal underwear shirt, check out this piece from Columbia. The shirt’s real power is Columbia’s thermal reflective Omni-Heat, which effectively traps body heat without any bulk. Another piece of tech is Omni-Wick, which pulls sweat away from your body to avoid a chill after hiking, exercising or working. The shirt also has an antimicrobial treatment to reduce body odor. Comfort hasn’t been forgotten with the Omni-Heat shirt either: Columbia used a flatlock construction and four-way stretch so you’re ready for any activity.
5. Under Armour Base 3.0 T-Shirt
This Base shirt from Under Armour is another great thermal underwear top. As you might have guessed from the brand, the Base shirt is made for exercise. This means you’ll be able to maintain your workout routine, even when the temperature drops.
The brand made it with a mix of 81 percent polyester and 19 percent elasterell, which offers serious mobility that can keep up with just about any activity. And, as with all the best activewear, the Base shirt effectively wicks away sweat.
6. Icebreaker 175 Everyday Leggings
The best pairs of leggings offer a snug, extra line of defense against the cold while exercising, working outdoors or just commuting. These leggings from Icebreaker do just that, providing lightweight warmth for a wide range of activities.
The Icebreaker 175s are made of 100 percent merino wool, which translates to top-of-the-line comfort, streamlined warmth and natural odor-reduction capabilities. They’re considered ultra-lightweight, but they’re still a game-changing upgrade for any winter wardrobe. The thermal underwear leggings also feature a fly for convenience.
7. Airblaster Ninja Suit
If you’re looking for full-body warmth, check out this Ninja Suit from Airblaster. The base layer suit is loaded with features that make the onesie a powerful tool for relaxation after a day of being out in the cold. You’ll find a 350-degree zipper around the waist (as well as a fly), while a full-length chest zipper makes it easier to take on and off. Plus, two front zip pockets provide a place for your phone, wallet and other gadgets.
As expected from a thermal onesie, the Airblaster Ninja is seriously warm (although it’s considered midweight). The suit is form-fitting (tighter than slim fit) for a very snug feel, but four-way stretch ensures mobility. The suit also has a seven-panel hood and thumbhole cuffs for added warmth.