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Halfdays, Smartwool
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 sleeves and was toasty when I stepped outside, but felt freezing once my body heat built up. I wore too heavy of wool base layers and found myself overheating when 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 the biggest enemy of staying warm. When sweat sits on your skin, it pulls heat away from your body. In the summer it's great, but in the winter, it'll make you feel way, way colder.
The best base layers work to pull the moisture off your skin, allowing you to stay warm, while also adding a layer of insulation to keep the external chill out.
After years of both living in the Colorado mountains and testing outdoor gear as a product journalist, I've now tried out dozens and dozens of base layers. I wear base layers most every day in the winter, under my jeans walking around town, on winter hikes, or under my ski kit on the slopes. Of everything I've tested, these are some of my favorite base layers for feeling comfortable and warm.
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 temperature. These fibers work largely because they're highly breathable and 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 staying 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:
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:
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:
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 the bum), and better breathability in others (like the underarms). 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.
- Neck line: Most base layer tops come in a crew neck, half zip, or full hood. A crew neck 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.
