The colder months can be pretty darn brutal, but staying bundled up next to the fire all winter long isn't an option (though that does sound amazing, TBH). Whether you're working outside, puttering about indoors, going for a jog, or exploring snow country, thermal underwear is a must for staying comfortable during all sorts of cold-weather activities — even if it's just running everyday errands when it's below zero degrees out.
Gone are the days of bulky long johns, too: Slim yet warm is the name of the game with thermals these days, particularly as new fabric innovations continue to be released. Modern warmth technology fabrics like Thermachoice or Uniqlo HeatTech allow for thermal underwear to be higher performing and thinner, making them significantly more streamlined under clothes.
Here's what to consider:
Fabric
Material should be top of the list when shopping for thermal underwear. Look primarily for those that are moisture-wicking, as these fabrics move and dissipate sweat off skin evenly, preventing cold spots. (Remember: Staying warm means staying dry!)
Synthetic fabrics: Polyester, rayon (or viscose), nylon, polyamide, and polypropylene dry quickly, and are great for athletic wear or frequently used thermals, as they're more durable and elastic.
Merino wool: Though it takes longer to dry, merino wool is more flexible than traditional wool. Merino sheep are renowned for producing the softest, finest kind of wool in the world, which means your undies won't be short on comfort. Both synthetic fabric and wool also have higher levels of resistance against odor-causing bacteria.
Silk: Silk thermals are a famously comfy bet, especially if your activity level is on the less intense end of the spectrum — i.e. low-key evening dog walks or indoor lounging. It's a lightweight option that's great for moderately cool weather, seeing as it's also slower to dissipate moisture, as well as less odor-resistant.
Warmth technology: Textile innovations such as warmth technology fabrics (or "warmth tech") have also been dominating in recent years. These can vary widely, and are oftentimes the brand's own proprietary tech, which usually features a mix of standard synthetic fabric, silk, or merino wool, treated with cutting-edge add-ons like ceramic or other trademarked antimicrobial treatments. You can find a variety of these engineered textiles and blends out in the market — from those that incorporate coffee-bean shells (NanoStitch) to even milk proteins and amino acids, as in the case of Uniqlo's popular Heattech. Whichever engineering you go with is usually fairly effective in its own right, so choose the one that aligns with your intent of use.
Fit
Since base-layer thermals pretty much function as second skin, fit should also be a top priority. Look for those that are snug yet non-constrictive, with flatlock seams that don't chafe or irritate — particularly in sensitive areas like the inner thigh and groin areas. Ill-fitting thermals, whether too tight or too loose, can also create uneven heat distribution or air pockets that can render the layer ineffective.
Weight
Thermal underwear typically comes in three weights: lightweight, midweight, and heavyweight. These provide warmth against cool, cold, and freezing temperatures, respectively.
Also consider the warmth-to-weight ratio, which measures how much warmth a fabric provides against its own weight, with the goal being choosing the warmest at the lightest.
You do sweat during winter workouts, so the silver salt technology that’s integrated into this top’s fabric is clutch for odor control (as are the mesh panels under the arms).
The Italian yarn is also pretty freakin' comfy. It's stitched in a way that avoids chafing, yet provides more stretch and strength than your average pullover.
These merino wool leggings work either as a base layer or on their own to keep you warm in cold weather. But you can keep wearing them when it warms up — merino wool is naturally breathable.
Whether you're lounging by the fire or out on the slopes, these bottoms will keep you warm — and make you look good while doing so. The Nordic, Fair Isle-inspired design is perfect no matter where you find yourself this winter.
Plus, the superfine merino wool is quick-drying, so you’ll stay comfortable even if you spend half your time falling on your butt.
Saxx has been our absolute favorite men's underwear brand for years. But did you know they also make some of the comfiest and highest-performing base layers in our cold-weather kit?
The Viewfinder tight is our favorite piece in Saxx's base layer collection. These midweight bottoms are made with a sustainable blend of recycled polyester and merino wool, and they come well-equipped with the legendary BallPark Pouch. So, you can stay warm without overheating on backcountry ski laps or sunrise snowshoe adventures.
There are tons of high-tech fabrics that actually create warmth while you wear them, but this thermal top uses old-school graduated compression to promote circulation to the extremities. Translation: You won’t have to deal with cold, tingly hands during outdoor activities all season long.
Of all the layers we've tried, this midweight top is hands-down our favorite for skiing. The fabric feels magical next to skin, with flatlock stitching to minimize chafing, 4-way stretch for your most active adventures, UPF 50+ for sun protection, and thumbholes for easier layering.
For warmth and comfort, a drop hem tapered design stays tucked in to keep cold air off your backside, a custom contoured hood fits under or over a ski helmet, the Dual Layer Expedition Core adds insulation on your chest to keep your core warm, and vented armpits keep you from clamming up.
Ideal for both cold-weather runs and chilling on the slopes, this base layer features a soft-brushed interior that traps heat to keep you warmer, longer. That, along with four-way stretch construction, makes this the perfect thermal underwear for playing outside in the cold.
These long johns aren't just color-blocked: They're body-mapped for strategic warmth during cold-weather mountain activities that still make you break a sweat.
Wool terry over the knees and upper legs gives thin skin maximum warmth, while wool mesh over the front and back of the thighs allows for better ventilation and full range of movement. It's all made from a merino blend that wicks moisture away.
The brand's trademarked Rulu fabric is treated with a plant seed-based finish for optimum sweat-wicking performance, making this option great for chilly runs. Four-way stretch construction is courtesy of the nylon-Lycra blend, with thumbholes keeping things in place either on their own or under other layers. Oh, and it comes with a nifty zippered pocket in the seam!
Merino wool naturally wicks sweat during exercise, and keeps you warm while you’re taking a break. It’s also naturally antimicrobial, but unlike is the case with other wools, you can toss it right into the washing machine. So practical!
These midweight bottoms layer comfortably under insulated pants for adventures like skiing and hiking.
Step up your first-layer game with this super soft merino wool top. It’s made with an interlocking knit that helps regulate your body temperature, so you can stay warm without worrying about overheating on the coldest adventures.
Wear it as your only layer on cool-weather jogs, or use it as a base under your ski coat and head out for some backcountry laps. The seams are specially designed to avoid chafing when layered.
The fact that Uniqlo basically can't seem to keep their Heattech thermals in stock year after year is testament to their simple, everyday effectiveness.
A style that sits snugly and seamlessly under all your winter clothes, this crewneck thermal top is as straightforward as it gets: The proprietary Heattech fabric — a blend of polyester, acrylic, and rayon — wicks moisture back into fibers, converting kinetic energy into heat.
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Best Bottom for Layering
Carhartt Men's Force Midweight Classic Thermal Base Layer Pant
At just over 5 ounces, this thermal underwear is light enough to wear under your work pants on the most frigid winter days. Flatlock seams and stretch technology make doubling up not just bearable, but comfortable, too.
The fabric is also sweat-wicking and antimicrobial, so you don’t have to stress about wearing this pair straight from the office to an outdoor adventure.
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Best Bodymapped Top
Craft Sportswear Active Intensity Long Sleeve Crew Neck
Body-mapped clothes are designed to retain heat in some areas and release it in others — and it doesn’t hurt that the technical design makes this base layer super legit. The stretchy jersey fits like a second skin, and works best during high-intensity workouts like cross-country skiing, running, or cycling.
Patagonia is best known for its eco-friendly initiatives and extremely reliable clothing. This is the company's warmest, most breathable base layer, but without the heft you might expect. It's also treated with Polygiene permanent odor control, so it stays smelling fresh, even after an entire season of frequent wear!
These moisture-wicking tights were designed to keep you warm on the slopes. On bluebird days, when the sun is beating down on the snow, you’ll appreciate TNF’s FlashDry technology. It pushes moisture (read: your sweat) straight to the surface of the fabric where it can evaporate, keeping you dry and cool under all your gear.
This is a great technical base layer from Helly Hansen, with its classic fit and seamless construction allowing for an itch-free feel and maximum freedom of movement. You can try your hardest to cover it in sweat, but it's made of merino wool and equipped with Lifa Stay Dry technology. So, it breathes and wicks moisture like a pro.
Looking for a performance top for the winter? This midweight option is Ridge Merino's most comfortable merino wool zip-up pullover. The brand wrapped soft wool around a nylon filament, which provides extra durability in the craziest conditions. There's even extra length at the bottom to make sure it stays tucked in as you move.
This 100% merino wool base layer is the best way to add warmth without packing on any bulk. Wear these leggings under your hiking or skiing pants, or just to bed for a warm and cozy night’s sleep.
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Best for Warmer Nights
Hanes Men's X-Temp Thermal Waffle Pant with FreshIQ
If you're looking for a basic first layer, try this pair of cotton thermal pants. The waffle fabric is soft against your skin, and the flat seams won't rub at all. They're the perfect winter PJs, but they can even be worn under khakis for a brisk night on the town.
This four-way-stretch top has flatlock seams that feel nonexistent next to your skin. It also features Columbia's Omni-Heat thermal technology that reflects your body heat back onto you, helping to keep you warm when temperatures drop below freezing.
For especially cold workouts, start bundling up with heavyweight tights that still allow full range of motion. Like all good base layers, these are moisture-wicking and odor-resistant. Plus, they have flatlock seams to reduce irritation and chafing when you wear them under ski, snow, or hiking pants. They also include built-in UPF 50 to protect you from the sun.
Eunice Lucero-LeeSenior EditorEunice Lucero-Lee is the Senior Editor for Best Products, and is a beauty, astrology, and pop culture obsessive whose work has been featured in publications such as Cosmopolitan, Esquire, The Numinous, and woman&home, among many others.
Ashley MateoAshley Mateo is a writer, editor, and UESCA- and RRCA-certified running coach who has contributed to Runner’s World, Bicycling, Women's Health, Health, Shape, Self, and more.
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