Think of base layers—aka long underwear or long johns—as the foundation for your cold-weather layering system. Choose them wisely, and you’ll stay warm and dry whether you’re cross-country skiing, winter camping, or skiing and riding in a resort. Choose poorly, and you might be drenched in sweat or shivering (or both!), no matter what else you’re wearing. Base layer tops can be worn alone or under a shell or insulated jacket, while bottoms are almost always meant to go under soft-shell or hard-shell winter pants (though some can double as standalone leggings).

Base layers come in a variety of different materials and weights, each best suited to a particular kind of winter recreation. Here’s how to choose the best base layers for all your activities.

In this article, you’ll learn:

• The pros and cons of different base layer materials
• How to choose the best base layer thickness
• What base layer features to consider

Materials Matter

One of the biggest base layer shopping decisions goes right down to the fibers used. Generally, base layers can be sorted into two camps: natural materials (primarily merino wool, but also plant-based rayon and hemp) and synthetic materials (polyester, nylon, spandex).

Wool & Other Natural Materials

Let’s get one thing straight: Today’s wool is not itchy. Made from ultrafine fibers sourced from Merino sheep, merino wool is soft and comfortable against the skin, never abrasive. And some of the plant-based natural materials, like bamboo-derived rayon or wood-based lyocell or modal, feel even cushier. Other reasons to love these fabrics: They’re breathable and great at moisture management, wicking sweat away fairly quickly. Though wool retains moisture within its fibers, it still feels dry against the skin. And merino wool is naturally antimicrobial, preventing the growth of odor-causing bacteria for an impressively long time.

On the con side, merino wool isn’t as durable as synthetic fabrics, and often pills with abrasion (like under pack straps). It’s slow to fully dry and tends to be the most expensive option.

Worth noting: Some wool production has been associated with mistreatment of the sheep that grow it. In response, some brands have developed standards for their suppliers to ensure the humane treatment of the sheep and systems to trace their wool back to the ranches for accountability.

Synthetics

The typical synthetic fabric is made from polyester or nylon, often with a bit of spandex added for stretch. These fibers excel at wicking and drying quickly. They’re more durable than natural fibers, and usually cost less. But they’re notorious for getting stinky fast (some have anti-odor treatments, which help). Synthetic fabrics are also derived from petroleum, but more brands are using recycled fibers to reduce the environmental impact of their layers.

Blends & Combinations

Some base layers use both natural and synthetic fibers to take advantage of the best qualities of each. One common construction sandwiches a synthetic layer to a wool layer—one on the inside, one on the outside—to deliver fast wicking with odor control. Another wraps a wool fiber around a nylon core for added durability.

How Low Can You Go?

Another major factor in base-layer shopping is the garment’s thickness, or weight. This plays a big role in how warm the base layer is. Not surprisingly, thicker fabrics tend to be warmer than thinner ones, but your level of exertion matters a lot in this equation, too. Also consider if you usually run hot or cold; what’s perfect for one person at 20°F might be way too warm for another. Base layers are typically classified as one of three weights:

Lightweight

Thin and light, these base layers are ideal for warmer weather (making them great for three-season use) and/or high-exertion aerobic activity. Think skate skiing or trail running. Lightweight base layers work well on their own, and they dry faster than thicker ones.

Midweight

Medium-thickness base layers excel for moderate activities in cool weather—like resort skiing or hiking—or more aerobic activities in colder temps.

Heavyweight/Expedition Weight

The plushest, heaviest base layers are made for exceptionally cold weather and more moderate activities (like winter camping). They’re usually too hot for very active wear.

Features

Base layers tend to be fairly simple in design, but there are a few useful features to look out for:

Zippers

Some tops include a quarter- or half-length zipper from the neck down, making the layer more versatile. Zip it up to your chin in colder weather, or unzip it when you start to heat up.

Thumbholes

These holes let you pull a shirt’s sleeves halfway over your hands, adding warmth. Make sure to check the fit: Too-tight thumbholes are uncomfortable to wear.

Hood

Close-fitting base layer hoods add a significant amount of warmth, as well as sun protection. 

Pockets

Some base layer tops and bottoms feature zippable pockets on the shoulder, forearm, or backside. These can be handy for storing keys or ID but remember that they’ll only be accessible if you’re wearing the layer on its own.

Boot Cut

Skiers love this calf-length design for base layer bottoms because it won’t bunch up uncomfortably under ski or snowboard boots.

High Waist

Bottoms that come up to the belly button or higher fit comfortably under hip belts and harnesses, and they prevent the dreaded patch of exposed skin between your pants and top when bending over. 

All articles are for general informational purposes.  Each individual’s needs, preferences, goals and abilities may vary.  Be sure to obtain all appropriate training, expert supervision and/or medical advice before engaging in strenuous or potentially hazardous activity.