Stay warm and dry no matter the activity. From extra-warm wool underwear options to lightweight and eco-friendly picks, we’ve found the top base layers and best thermal underwear for women.
Base layers are simple, timeless, next-to-skin tools that help evaporate sweat. But the materials and designs have differences that impact long-term comfort, durability, and ultimate use. That’s why we’ve spent months seeking and testing the best long underwear for women across a scope of outdoor activities and conditions.
We tested these second skins while hiking, camping, rock climbing, hunting, ranching, farming, trail running, backcountry touring, downhill skiing, and snowboarding. We paid particular attention to warmth, wicking, and comfort. Secondarily we also considered value and durability.
And while there isn’t a single base layer that works for every body type or movement, we’ve highlighted a variety of the best options that should help you hone your own. These are the base layers that will hold up through a winter spent outdoors.
Best Base Layers & Thermal Underwear of 2020
Best Overall: Smartwool Merino 250 Base Layer Crew Top ($100) & Pant ($100)
The Smartwool Merino 250 will keep you warm all winter long. And, perhaps even more importantly, it will remain breathable throughout your outdoor adventures. The interlock knit makes for a comfortable next-to-skin feel, and the 100% merino wool construction naturally resists odors.
The pants have a side seam and curved back seam, which makes for a comfortable fit and cuts down on unwanted sagging. And the gusseted crotch allowed for a full range of movement while hiking.
Our tall testers appreciated the length, though shorter users may find them too long for comfort with ski boots. And it’s worth noting that pure merino wool layers do tend to be more fragile. Wash with care and wear under other layers for a long-lasting winter weather base layer.
Best Budget: Ridge Merino Aspect ($65)
This midweight merino wool base layer will keep your wallet happy. And it also happens to be more velvety than your average wool bottoms. Ridge Merino’s use of ultra-fine 18.5-micron fibers makes a big textural difference.
Our tester paired the Boy Short ($30) and bottoms (both 84% merino wool) beneath snow pants for alpine skiing and snowboarding, as well as backcountry touring.
She was impressed, saying, “I can’t believe I’ve gone 25 years without this extra, direct-skin layer of merino wool across my tush while I ski or ride. This underwear is soft. And I’m more comfortable and don’t get cold as easily, so my energy is better preserved.”
Both bottoms resisted odors, are ideal for endurance activity in 20- to 50-degree (Fahrenheit) temperatures, and work great for chilly lift rides. The tester also said, “I have muscular thighs and glutes, but the pant rise was high enough for the back and tummy — up to my belly button.”
Drawbacks: The leg bottoms don’t have strong elasticity, so we don’t recommend wearing them under jeans.
Best of the Rest
Best Seamless Long Underwear: Patagonia Capilene Air Bottoms ($129)
These seamless bottoms are the epitome of comfort. The wide waistband never digs into your side while still containing enough built-in elasticity to prevent falling down. The blend is a balance of 51% merino wool and 49% recycled polyester that dries quickly and resists odors.
“Even while camping out in my sleeping bag, the breathability of these bottoms is amazing,” said one tester, a ski patrol trail guide. While hiking and skiing, “I used softshell activewear snowpants over this base layer and was warm. This layer is perfect for thermal insulation and comfort beneath a soft or hard layer,” she said.
And you can feel good knowing these bottoms are made with recycled materials and that the seamless construction makes for virtually no manufacturing waste.
It’s also available in a Crew Neck Top and Hoody.
Best Made-in-USA Women’s Thermal Bottoms: Voormi 3/4 Length Thermal II Bottoms ($119)
If you prefer calf-length bottoms for ski and snowboard boots, this midweight pair is your mark.
“Voormi’s thermal bottoms are my choice every time for backcountry splitboarding, post-tour apres, and year-round camping,” said our tester, who took multiple hut trips with only this base layer bottom in both gripping-cold blizzards and sunshine. Despite back-to-back usage, the pants don’t reveal odor.
The fabric — a fine-micron wool construction with against-skin wicking yarns that pull and disperse moisture to the outer layer — is very soft, dries fast, and is extremely durable. The material and seams have held up through arduous usage beneath snow pants and jeans in all seasons.
Best Wool Base Layers: Kari Traa Akle Long Sleeve ($90) & Pant ($90)
To bar extremely cold temperatures, this heavyweight 100% merino wool Kari Traa set is a winner. We used this pant-and-shirt combo for backcountry splitboarding in subzero temperatures, socked-in ridgelines in Colorado’s steep San Juan Mountains, and inbounds skiing and snowboarding during 15-degree days.
“I tend to run hot but when it’s super cold — 10 degrees or lower. This is my No. 1 base layer, even if I’m sweating a ton while touring,” our tester said. “My skin is sensitive, so I can’t usually wear complete wool fabric. This isn’t the softest next-to-skin choice, but it doesn’t irritate.”
The seams provide an attractive, comfortable cut. Without non-wool fibers, the stretch isn’t supreme, but the freedom of movement is still fair. And the merino shields several days of odors like a champ.
Best Quick-Drying Tights: Stio Basis Power Wool Tight ($76 on Sale)
A professional outdoor adventure photographer tested these lightweight tights beneath snowboard pants in 30- to 40-degree temps at Keystone Resort.
The base layer showed no signs of sweat, and snow patches dried within 10 minutes. Its packability, ventilation, odor resistance, and movement are stellar throughout a ton of leg activity. And she even stayed warm on the lift rides.
We love the feeling of this merino wool-polyester fabric. It’s soft, smooth, and warm. Plus, the seams are comfortable. One tester did note, though, that they tend to sag in the crotch, bottom, and tummy areas as the day wore on.
All things considered, they’re an awesome design for lift-assisted and backcountry riding.
Best Next-to-Skin Shirt: Le Bent Le Base 200 Lightweight Crew ($85)
This could be the lightest, softest, stretchiest, most attractive base layer top we’ve ever worn. “I want to use it for everything outside and inside, even going out to dinner,” said our gear tester. She wore this crew neck shirt nonstop for several days of outdoor activity in the Colorado Rockies including frigid, windy trail runs, wintry walks, and sleeping.
The shirt’s buttery blend features bamboo rayon, merino wool, elastane, and four-way stretch. The arms and torso are long, so no skin is exposed in extreme temperatures or wind chill. And the fabric masked odor for multiple days despite sweaty activity. This is the perfect do-it-all layering piece during and post sport.
Best Odor Resist Thermal Underwear: Icebreaker BodyfitZONE 260 Long Sleeve Crew ($130) & Leggings ($130)
This heavyweight pair is made of merino wool and Lycra that’s incredibly soft, feels lightweight, and has a slim fit ideal for layering. The shirt’s dropped front and back hem helped it from riding up and exposing any skin. Temperatures ranged from 5 to 50 degrees with wind chill during night and day while testing.
One tester also helped pack out an elk in 15-degree temperatures but never overheated despite carrying the heavy load up a steep incline. The vents are thin but not located in high-friction areas, and their seams never chafed.
The best part? Even after wearing for 7 days straight, it stayed odor-free. The fabric never snagged and dried fast after high-perspiration adventure. The only drawback was a need to pull up the leggings a couple of times due to sizing, body shape, or elasticity.
Best Lightweight Set: Helly Hansen Lifa Merino Crew ($95) & Pro Lifa Seamless Pant ($110)
This top is perfect for all conditions. It’s not see-through, so you can wear it alone on warmer days and comfortably layer underneath on cooler days. At 10,000 feet and higher, these extremely lightweight base layers were put through the grinder while hunting and glassing for game in 6-degree lows between 6 a.m. and 8 p.m. and a max of 40-degree middays.
While both pieces are moisture-wicking, quick-drying, and offer good ventilation, the odor resistance didn’t last past a few days, and they’re not super stretchy.
“The arms don’t offer extra length for wrist coverage, and the leg openings don’t have elastic to hold them in place when I slide into my hunting boots,” our tester said. While the bottoms are not ideal without coverage — they’re too transparent — or warm enough for extremely low temperatures. We give it gold stars for softness and breathability.
Best Warmth-to-Weight Ratio: Obermeyer Discover 1/4 Zip ($89) & Tight ($79)
The lightweight Discover set is extremely stretchy, very quick-drying with good ventilation, and fair at moisture wicking. Both the top and bottom offer free-feeling movement while still being a protective outer layer and standalone piece of clothing. The odor resistance wasn’t prime, but that generally tends to be a challenge with synthetics.
The arms are lengthy with seamless, stylishly concealed thumbholes. In the tights, the location of the seams assists the structure while adding attractiveness. We also like that the fabric is soft enough to double as sleepwear. Note: The bottoms are high-rise.
We were particularly impressed with how warm they were for their weight. These are great fall and winter athletic base layers for cool-weather climbing, hiking, ski touring, winter camping, and curling up indoors on a winter day.
Best Recycled Fabric Long Underwear: Norrona Equaliser Merino Zip Neck ($89) & Longs ($79)
This base layer works hard and provides top-notch stretch, ventilation, ad odor resistance. Made from a blend of merino wool and recycled polyester, they dry quickly and feel soft against the skin.
Throughout a range of 25 to 70 degrees, the wool-synthetic blend provided excellent temperature regulation, even for trail running and weight lifting.
Extra seams in the pant’s crotch allow for wide movement, but the above-knee seams can be a nuisance while kneeling. These are meant for active pursuits. If you’re looking for something to stay cozy in at camp, go with a thicker set like the Kari Traa Akle.
How to Choose the Best Base Layer
First, start by imagining how you will use these base layers. Are you looking for something extra warm for relaxing around camp? Or will you be a working hard in the backcountry and need a breathable, fast-wicking layer? There’s no right or wrong answer, but knowing how you’ll use these layers will help narrow the field.
We love wool. It’s fast-drying, comfortable against the skin, and resists odors like a champ. That said, 100% merino wool tends to be less durable and susceptible to getting baggy throughout the day. Depending on your preference, you may prefer a wool-blend or straight synthetic materials.
Warmth & Breathability
This ties into end use. For extra-cold weather or more sedentary activities like ice fishing, sitting in the hunting blind, or relaxing around camp, you’ll want something warmer. In addition to trapping heat, it’s important that the layer breathe well and efficiently wick moisture. Freezing sweat will make you colder faster than a too-thin layer.
The Smartwool Merino 250 is a warm winter layer that breathes incredibly well. It’s our top pick for the dead of winter, but also for alpine pursuits in fall and winter. If you know you’re going to be busting it uphill on a bluebird day, then look for a lighter layer. Something like the Helly Hansen Lifa Pro set will be a key part of your layering system.
There’s nothing more annoying than ill-fitting base layers. From sagging to pulling to chafing, it’s important to find comfortable-fitting long underwear. Things to consider are softness against skin and tightness. You want a base layer to fit snugly against your body while allowing full range of movement.
It’s also important to look at length and seams. You don’t want gapping at the waist. Nor is it ideal to have too short sleeves or pants. Seams can cause chafing, so beware of your movement and potential trouble spots. If chafing is a constant problem, you may want to consider the seamless Patagonia Capilene base layers.
Thermal layers are an investment, so it makes sense that you want them to last. Synthetic layers are often more durable but can cause more trouble with retaining odors. Merino wool is naturally odor-fighting but tends to be more fragile. You’ll want to take care putting them on and use them mostly as true base layers underneath protective pants or other layers.
Have a favorite base layer we missed? Let us know in the comments for future updates to this article.