When I moved from the East Coast to the Rocky Mountains years ago, I simply did not understand how to layer for warmth. I wore cotton long sleeve tees and was toasty when I stepped outside, but felt freezing once my body heat built up. Then I tried heavy wool base layers and long underwear, but those just made me overheat each time I’d pop inside a shop on a cold winter day.
Eventually, I learned the formula: Opting for technical base layers in the right weight helps you regulate heat better — that is, they keep you at the perfect temperature while you move between outdoor and indoor climates, even as you sweat.
Ultimately, sweat is your main adversary when trying to stay warm. As sweat cools on your skin, it pulls heat away from your body. In the summer, it’s great — but in the winter, it’ll chill you to your bones. make you feel way, way colder. The best base layers wick moisture while adding a layer of insulation to keep cold air out.
After years of testing outdoor gear, I’ve personally tried out dozens and dozens of base layers. I wear them almost everyday in the cold months: under my jeans, on trail hikes, or beneath my ski kit when I’m on the slopes. If you want to truly enjoy the outdoors this winter, here’s how to layer wisely.
Merino wool
Smartwool
One of the best base layer materials is Merino wool. Its dense fibers are naturally moisture-wicking, odor-resistant, and great at regulating your body temperature. These fibers work because they’re highly breathable and will stay warm even when they get wet from sweat or condensation. The knit naturally retains or releases hot air depending on the conditions, so it’s the best base layer for keeping warm.
There are a few downsides to Merino wool. Because Merino wool’s porous fibers have a more open knit to allow for greater airflow, they aren’t great at blocking out wind on their own. Another is that it dries slower than synthetic materials, so the latter will keep you warmer during seriously sweaty winter activities. It’s also less durable than a synthetic, so it develops holes faster from regular washing (which is why you should always air-dry your Merino wool base layers). And while Merino wool is much softer compared to other kinds of wool, people with a sensitivity to lanolin may find it itchy on their skin.
The best Merino wool base layers:
Smartwool Classic Thermal Merino Base Layer Bottom
This 208g midweight merino bottom base layer is seriously warm and well-constructed, with flatlock seams meant to be comfortable under baggy jeans or insulated snow pants. The flat, high waistband stays put and we love that they come in plus sizes, too.
Kari Traa Rose Half-Zip Base Layer Top
We love base layers from woman-owned Kari Traa for their fun prints and smart fabric mapping for a close-to-body bulk-free fit. This 100% wool midweight base layer is my go-to for winter skiing as it keeps moisture and heat in perfect balance, layers well, and has chafe-free flatlock seams so it’s comfortable all-day. The matching, high-waisted bottoms make for a very toasty combination.
Icebreaker Merino 260 Tech Thermal Leggings
“These Merino wool leggings function like long underwear. I wear them underneath jeans because they don’t add any bulk and have a comfortable stretchy waistband. I have these on whenever the temperature’s in the single digits,” says style editor Gabrielle Chase.
Synthetic base layers
Arc’teryx, Uniqlo
Synthetics like polyester, nylon, or polypropylene excel at moisture management and durability, making them ideal for high-intensity activities like running, skiing, and hiking in wet or humid environments. Synthetic base layers dry much faster than wool because they don’t hold moisture in their fibers.
However, this means synthetic base layers are less insulating than a Merino wool base layer. That said, many synthetic or blended base layers are crafted with an insulated backing, like a fleece grid or a waffle-knit construction.
Synthetics do tend to hold odors more than wool. Also, they can feel clammy under a non-breathable layer over them (like a thick fleece jacket) because the moisture sits on the fabric’s surface, rather than within the fibers as you’ll find with Merino wool. If you plan to sweat in your synthetic base layer, you should wear a breathable or moisture-permeable shell over it.
Find some of our favorite synthetic base layers in our Uniqlo HEATTECH review and Patagonia outdoor clothing review.
The best synthetic base layers:
Patagonia Capilene Midweight Crewneck
Highly-breathable and lofted, this midweight baselayer is made from 100% recycled polyester for great moisture control, and has a diamond-grid backing to trap warm air. It’s finished with an odor control treatment to keep you smelling fresh.
Arc’teryx Rho Hoody
For chill activities in cool temps, this slim-fitting, brushed polyester hoody offers warmth, stretch, moisture control, and a soft-to-the-touch finish. Its hood includes a full face balaclava.
Uniqlo Ultra Warm Legging
The Ultra Warm HEATTECH fabric is 2.25 times warmer than regular HEATTECH. Made from a polyester, acrylic, rayon, and Spandex blend, these are the perfect synthetic base layer leggings to wear under jeans, as they comfortably move with the body, are constructed to trap air for a soft feel and better insulation, and come at a great price.
Under Armour ColdGear Leggings
Highly compressive, ultra-warm, and made with a four-way stretch material, these midweight winter workout tights pull moisture away thanks to their nylon-polyester construction, and stay put while you move fast. Find them in petite, standard, and tall lengths.
Synthetic-wool blends
Smartwool, Halfdays
A synthetic-wool blend combines the best of both worlds: It has better durability wash-after-wash and dries faster in the moment than pure Merino wool — yet it still retains wool’s odor resistance and insulation.
These blends often have a Merino wool core wrapped in polyester to strengthen the fibers. Or, they come in a mix of Merino wool and Spandex for added stretch. Blends also sometimes incorporate semi-synthetics like bamboo viscose or Lyocell, which creates a softer handfeel and better breathability.
For people who find 100% Merino wool layers itchy, a synthetic-wool blend helps smooth out the texture.
The best synthetic-wool blend base layers:
Halfdays Fay Midweight Merino Top
Easily my favorite everyday base layer, the Fay Top (and matching bottoms) are made with a wool-bamboo blend that makes for a super breathable and very warm base layer for wearing solo around the house or under insulated ski pants. It also has a small amount of Spandex and polyamide which gives the fabric a slight bounce.
Janji Repeat Merino Long Sleeve (Women’s)
This soft, lightweight top is made for mountaineers. The fabric is a Merino-nylon blend made with volcanic ash, which enhances its odor-repelling and thermoregulating abilities. The thumb sleeves help it stay in place on winter runs.
Icebreaker Merino Blend 125 ZoneKnit Thermal Leggings
These bottoms are extra-high waisted with well-placed flatlocked seams for a comfortable fit. They’re made with 60% Tencel and 40% Merino wool, so they offer great sweat-wicking and light warmth in a design thin enough to layer with.
A Quick Guide to Base Layer Weights
Base layers come in three main weights, but the right weight depends entirely on what you’ll be doing and what else you’ll be wearing. Their weights are measured in grams per square meter, or “gsm.”
Lightweight
The thinnest base layer (100-150 gsm) offers maximum breathability while wicking sweat quickly and providing light warmth. This weight is ideal when you need moisture control more than heavy insulation. I reach for lightweight base layers a lot when I need an extra layer of warmth under an already-insulated wool sweater.
When to wear: Lightweight base layers are perfect on their own for cool fall hikes, or under your warm outerwear in mild winter temperatures down to about 32 degrees. Wear them if you plan to be moving around a lot, whether that’s on the slopes or hiking trails.
Midweight
Base layers weighing about 200-250 gsm offer a great balance of warmth and breathability. This weight is a great way to add warmth to a poorly-insulated jacket, or to stay super warm under insulated layers where you’ll be out sweating a lot.
When to wear: Opt for a midweight layer when you’re going to be active outside in very cold temperatures of about 32 degrees. Wear them under an insulated winter coat, or on their own if it’s only mildly cold. They’re also ideal under jeans on days you’re going from indoors to outdoors often. Midweight layers can also be worn under fleece jackets or heavier base layers in extreme cold.
Heavyweight
For very cold days, you want a base layer around 260+ gsm to give you maximum warmth. This weight still provides some moisture control thanks to the technical fabric, but the fibers are thicker and denser so it’s slower to pull sweat off your body. A heavyweight base layer focuses more on insulation and less on breathability.
When to wear: Choose a heavyweight base layer for freezing cold days where you won’t be moving or sweating very much, like on short walks, hunting trips, or winter camping excursions.
What to look for in the best base layers
Smartwool
By now, you know a great base layer needs to be moisture-wicking, well-insulated, and the right weight for whatever you’re doing outside. Besides these, here are a few more key features:
- Flatlock (and well-placed) seams: Bulky, obstructive seams can create friction and chafing when you move around. Always look for base layers that use flatlock seams. Ideal base layer tops should be with offset shoulder seams so there’s no rubbing under your backpack straps.
- Zoned construction: Many companies use targeted knit zones to add extra warmth in key areas (like your bum), and better breathability in others (like your armpits). This adds warmth where you need it, while shaving off extra weight in the areas you don’t.
- Gusseted design: Mobility is key when layering, whether you’re walking around the city or on a winter camping adventure. Having a gusseted crotch or underarm is especially beneficial for women to improve range of motion around curves, minimize chafing, and create a smoother silhouette.
- Neckline: Most base layer tops come in a crewneck, half-zip, or full hood. A crewneck is less visible as an underlayer and reduces heat if you’ll be wearing a scarf, buff, or turtleneck over the top. Half-zips and mocknecks are a great way to add some warmth without needing a scarf. A full hood protects your ears from wind, and locks in heat under a hooded shell jacket.
Base Layer FAQs
What is a base layer and why do I need one?
A base layer is the first layer of clothing you wear against your skin during cold weather. Its job is to keep you dry and warm by insulating heat and wicking sweat at the same time.
What materials are best for base layers?
Base layers typically come in Merino wool, synthetic fabrics (like polyester and nylon), or a blend of the two. Merino wool excels at moisture-wicking, odor resistance, and temperature regulation, while synthetics are often quicker-drying and easier to care for.
How do I choose the right weight for my base layer?
Your choice of base layers should also depend on your activity, the forecast, and other layers you plan to wear.
- Lightweight (100-150 gsm): Can be worn solo or layered on mild winter days.
- Midweight (200-250 gsm): For exercising in very cold weather.
- Heavyweight (260+ gsm): For low-activity outings in extreme cold.
