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Thermal underwear goes by many names—long johns, base layers, long underwear, warm leggings, tights. No matter what you call it, a warm base layer plays a key role in keeping you warm in the winter. But not all thermal underwear is alike. Whether you’re braving the wilderness on a back-country snowshoe trek, ice fishing in sub-zero temps on a windy day, or simply shoveling your driveway, you need a pair of long johns that does the job—and stops winter in its tracks.

It can be easy to forget the importance of base layers, which are usually tucked away under flashier outerwear. Most thermal underwear is meant to be layered under snow pants or winter jackets, which function primarily as shells that deflect wind and moisture. But even the best snow pants or ski jackets won’t keep you warm on a cold day if you don’t have a comfortable, warm set of thermal underwear to match. Below is an-easy-to follow guide to choosing the best long underwear for your particular needs.

Best Thermal Underwear: Reviews & Recommendations

Best Merino Wool Base Layer: Smartwool Merino 250 Base Layer Bottoms

Best Merino Wool Base Layer

SmartWool is the most well-known maker of merino wool products—and for a good reason. Smartwool base layers are made of high-quality merino wool and are constructed with a consistently durable design. The 150 Base Layer Bottoms are the company’s lightweight long underwear option, making them an especially versatile selection. These thermal pants for winter can be layered under snow pants, but they’re also light enough to wear under your regular clothes. Whichever way you wear them, they provide impressively warm insulation at a reasonable price point. Smartwool’s products typically run a bit small. Consider ordering these one size bigger than you’d usually wear.

Best for Men: Le Bent Men’s Le Base Layer Lightweight 200 3/4 Bottom

Best for Men

Searching for the best men’s thermal underwear? Le Bent’s Men’s Light 200 ¾ Bottom is hands-down the best fitting thermal underwear for men. Le Bent is a ski company that boasts “definitive comfort in the outdoors for experiences that matter.” Though the company is not as well-known as competitors like Smartwool, you get what you pay for in terms of quality. These long johns offer a more relaxed fit than your typical thermal leggings. The long underwear is versatile and is as good for hunting in the winter as it is for skiing. It boasts a unique blend of merino wool and bamboo-derived rayon, a material known for its stretchiness. Another plus is the ¾ length of the leggings, which allows for a better fit with thermal socks and boots than full-length long underwear bottoms, which can create an uncomfortably thick layer and bunch up under winter boots.

Best for Women: 32 Degrees Womens Cozy Heat Baselayer Legging

Best for Women

32 Degrees Women’s Cozy Heat Base Layer is the best all-around thermal underwear for women. These long johns are tailored specifically to a woman’s body and provide a form-fitting but unconstricting fit. They’re made out of a blend of polyester and spandex. The spandex makes them extra flexible so the base layer feels snug and cushy. The thermal underwear has a stretchy elastic waistband that helps the product fit most body types. Unlike men’s thermal leggings, these long johns have a mid-rise waist, meaning they fit just below your belly-button. This thermal underwear is also soft—a brushed interior gives a velvety feel against your skin. Finally, the product boasts anti-odor and moisture-wicking qualities. These long johns are perfect for layering under pants and shorts, as well as wearing at yoga classes, when lounging around the house, or even as pajamas. They sell at a very reasonable price point.

Best Thermal Shirt: Carhartt Men’s Force Heavyweight Thermal Base Layer Long Sleeve Pocket Shirt

Best Thermal Shirt

Carhartt is renowned for its canvas workwear, but its thermal base layers are just as good. This thermal shirt is hands-down the most-heavy duty base layer of its kind. A blend of 81% polyester and 19% wool provides the best of both worlds. The polyester keeps the thermal shirt stretchy and versatile, while the wool provides a strong kick of insulation to keep you toasty. The shirt is thick, but the fabric blend helps regulate your body temp no matter how cold it is outside or how much exertion you put in. It’s great for snowmobiling in the bitter cold as well as wearing during a day of hard work on a bitterly cold day. The flat lock seams, concealed thumb loops, tagless labels are all nice touches that keep the shirt comfortable. The zip-pocket on the chest is another feature you don’t often find in a thermal shirt. It’s perfect for keeping small valuables secure.

Best Thermal Onesie: Duofold Men’s Mid Weight Double-Layer Thermal Union Suit

Best Onesie

Duofold’s Men’s Mid Weight Double-Layer Thermal Union Suit is a well-designed, all-in-one thermal underwear option. The 100% cotton inside layer is soft against your skin. But cotton has its limitations in extreme cold and when moisture comes into play, whether it’s from sweat or wet conditions. That’s where Duofold’s outer layer comes in. A blend of 65% cotton, 25% merino wool, and 10% nylon, the outer layer provides added insulation that makes the thermal underwear suit stand out among its competitors. The button-down front gives the product a classic look and makes it easy to put on and take off. This thermal underwear suit comes at an affordable price point.

Best for Exercise: Under Armour HeatGear 2.0 Leggings

Best for Exercise

Under Armour is an active-sportswear company with a good line of base layers. The Under Armour HeatGear 2.0 Leggings are the lightest base layers that the company offers. Made of 79% polyester and 21% elastane, the leggings are stretchy and won’t limit your mobility on active days. A stretch-mesh gusset and inseam panels deliver ventilation, and the material wicks sweat and dries quickly. These thermal legging also boast Under Armour’s compression technology, which offers extra support and improve oxygen flow in your legs. These leggings will not only help regulate your body temperature, they will also reduce soreness in your legs after an intense day in the backcountry.

Best Budget: TSLA Men’s Thermal Underwear Set

Best Budget

TSLA Men’s Thermal Underwear Set is a phenomenal thermal underwear option for its price point. For under $25, you get both a thermal shirt and thermal leggings. This two-for-one set matches the price of what would otherwise get you just one piece of similar quality thermal underwear. The long johns are made out of microfiber and spandex. While this doesn’t make for the warmest or highest quality long underwear, it’s mobile, versatile, and affordable. The thermal underwear set runs a bit small. Consider getting a size bigger than you’d usually get.

What To Consider When Choosing Thermal Underwear

It can be hard to sift through the fluff and find thermal underwear that gets the job done, but we’ve got you covered. There are five things you should consider before purchasing the best thermal wear for winter: fabric, fit, comfort, functionality, and cost. The best thermal underwear for you conforms correctly to your body, and will allow to do whatever you’ll be doing in the cold weather without getting cold—or overheating, which will cause you to sweat and then get even colder.


Thermal underwear is essentially a layer of fabric meant to help keep your body warm. That additional layer of insulation fits closely to your skin and keeps heat from escaping from your body. But thermal underwear is only as good as its fabric, so when you’re searching for long underwear, make sure to look at the fabric specifications.

Merino wool is the gold standard of base layers. It’s warm, lightweight, and wicks away moisture. Plain wool is warm but is usually thicker—and potentially itchier—than merino legging. Cotton is warm, but will draw body heat away from you when wet, so avoid cotton entirely. Synthetics offer good bang for buck, mobility for active days, and moisture wicking qualities, but they generally don’t provide the same level of insulation as wool long johns.


Thermal underwear should be form fitting. That means it should be skin tight, but not so tight that it restricts blood flow. That will simply make you cold and uncomfortable. Also, you can quickly wear out your thermal underwear if you get a size too small. But don’t get thermal underwear that is too baggy—it won’t insulate you very well. Thermal underwear should hug your body tightly without constricting your blood flow. For that reason, make sure to buy gender-specific thermal underwear.

Fabric will affect fit. Synthetic fabrics like spandex and polyester are especially forgiving and are good for women with curvier bodies. Other add-ins such as bamboo make for a stretchier material and a more comfortable fit.

Every brand’s thermal underwear fits differently. If you’re ordering online, considering buying several types to try on. Return the long johns that don’t fit well and keep the ones that do.


When most people hear the term “thermal underwear,” they think about warm leggings. These are certainly important, but to have the proper winter clothing, you need base layers for your entire body. You can buy a thermal underwear set that will seamlessly insulate your entire body, or you can buy a thermal shirt to match your leggings. If you opt for the latter approach, consider buying the same style thermal shirt as you do for your legs. That will ensure consistent warmth over your entire body.

When you’re wearing a set, make sure to tuck your thermal shirt into your leggings so that heat doesn’t escape from any gaps. Remember, base layers are meant to be layered. They often require one or two quality mid-layers on top of them before your outer jackets and pants. Layer up! Or to avoid that issue altogether, opt for a union suit—a warm one-piece that won’t let any cold air get between your skin and your outer layers.

Intended Use

Most thermal underwear products are designed to keep you warm. But if you’re working hard enough, you can easily start to overheat. Nothing’s worse than having your base layer turn against you when you’re on the grind. In this case, you should consider getting a lighter product that wicks moisture and keeps your body temperature well regulated. Look for lightweight thermal underwear made out of synthetic fabric.


The more you pay for thermal underwear, the better quality and fabric and advanced features you get in return. Paying for high quality long johns will undoubtedly pay off in the long run. Your long underwear will last longer and keep you warmer. That said, thermal underwear is a relatively simple piece of winter clothing. In the most basic sense, it is a layer of fabric that hugs your skin to give you added insulation. If you’re on a budget, there are less-expensive products that will still keep you warm and comfortable during cold weather activities.


Q: What is thermal underwear vs. base layer?

Thermal underwear and base layers are interchangeable terms for the same product. When you’re on the market for a set of thermal underwear, keep your eye out for products with “base layer” in the description, and vice versa.

Q: When should you wear thermal underwear?

In short, whenever it’s cold out. Thermal underwear can be worn for traditional outdoor winter activities such as hunting, ice fishing, and snowmobiling, but it’s not limited to them. Many of the best thermal underwear sets are thin enough to layer under your day-to-day clothes and will keep you warm when you’re working outside or going on errands.

Best Thermal Underwear: Final Thoughts

The best thermal underwear is a must-have part of your winter clothing outfit. It provides a crucial component of insulation that will help keep you warm on the coldest of days. A complete set of long johns or a long underwear suit is best for very cold conditions, while a thermal underwear shirt or a set of thermal leggings will suffice, depending on the activity.

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