The Best Thermal Underwear, Tried And Tested To Keep You Warm

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.