A solid base layer is part and parcel of staying warm in the backcountry. As the garment closest to your skin, it insulates you from the cold and wicks away sweat—all in the name of helping you regulate your body temperature.
But with all the base layer options available (materials, cuts, styles and weights), finding the best one for you can feel trickier than staying warm on a wet day in the mountains. It’s all dependent on the activity, how hot or cold you run and, well, personal preference. We asked backpackers, skiers, runners, parents and our very own employees to share their favorites available at REI—so the next time you head outside, you can trust your layer to pull its weight.
What is a base layer and why do I need one?
Base layers sit next to your skin and are responsible for helping you regulate your body temperature by wicking moisture, thereby helping you stay dry and warm. Base layers come in tops and bottoms in a variety of styles (T-shirts, long -sleeves, quarter-zips, tank tops, long bottoms and more) and are typically made from wool or synthetic fabrics, like polyester. Base layers shouldn’t be made from cotton, because cotton doesn't efficiently wick away moisture from your skin, takes a long time to dry and is a poor insulator.
What is a layering system?
A layering system usually involves wearing a base layer, mid layer (often a fleece) and top layer (often a down jacket) with a weatherproof shell, according to Tim Brown, senior product manager at REI Co-op. The idea is to stay warm and dry without overheating.
“Being able to remove layers as you start to exert more energy is paramount to performance and safety,” he says. “Preventing overheating and sweating in cold conditions can prevent [you from becoming hypothermic] when you stop your chosen activity.”
What base layer weight should I choose?
We listed lightweight and midweight base layers for this guide because they’re best for aerobic activities in cold weather. Lightweight base layers are particularly helpful if you plan to spend time outside on a warmer day or you run hot. It might seem counterintuitive to wear base layers in warm weather, but base layers wick sweat, which can prevent chafing and blisters. Some brands also offer heavyweight base layers for extra-cold activities like winter camping or ice fishing.
What is merino wool?
Merino wool is high-quality wool that comes from merino sheep, which originated in Spain. Soft, breathable, moisture wicking and naturally odor resistant, it’s an ideal fabric for active base layers.
When should I choose synthetic base layers over wool base layers?
It’s really up to you. Brown from REI Co-op says synthetic materials are popular for people who run hot and sweat a lot, as well as for people who plan to spend a lot of time in wet climates. This is because synthetic fibers dry more quickly than wool. Synthetic products also tend to be a bit cheaper than wool products, but they tend to pick up bad smells a bit more quickly. Wool is best for active days—say, backcountry skiing—where regulating your body temperature during climbs and descents is paramount.
How do I care for my base layers?
In general, you should wash your merino base layers as infrequently as possible. (Most people can get three to four wears out of a merino wool garment before needing to wash it because of the odor, but this will vary depending on the person and the specific layer.) When you do wash your merino gear, consider handwashing it with a wool-specific soap. Avoid products containing bleach or fabric softeners, as these can damage and weaken the wool fibers, causing them to warp when they dry.
Synthetics are a bit sturdier and can be machine washed, although you’ll want to turn your garment inside out to keep it from pilling. Turn the washing machine to a gentle or delicate cycle on warm. Hang your garments to dry unless otherwise directed by the tags on your gear.
To pick the best base layers, we asked our crew of REI Co-op staffers for their favorite base layers sold at REI. We then read through dozens of customer reviews on REI.com and spoke with several outdoors experts to get a sense for what matters most in a good set of base layer tops and bottoms. Then, we narrowed our list down to five base layer tops and bottoms.