You are intimate with your long underwear in a way that far exceeds your relationship with other outdoor gear. It hugs your body and keeps you cozy at night. It siphons your sweat and keeps you dry while you play. And unlike your hiking partners, it never complains when you stink. So isn’t it worth finding a next-to-skin friend that is warm, comfortable, and made for the activities you’ll do together?

FEEL THE HEAT Your body constantly warms a thin layer of surrounding air. Long underwear is designed to trap that air—and its accompanying heat—next to your skin. The more air pockets contained within the fabric, the more warmth it will trap. Thus a thicker layer of fabric will, as a general rule, be warmer than a thinner one. But look closely at the weave; tiny air pockets are often incorporated into the fabric, increasing warmth without adding weight (think of the classic waffle-weave).

SNUGGLE UP A base layer should fit flush against your skin but not be so tight that it restricts motion. The goal is to eliminate the cooling “bellows effects,” which is caused by gaps between the fabric and your body. If you are wearing a looser-fitting layer, these gaps will be constantly compressed and re-expanded, forcing out the warm air and drawing in cooler air to replace it. Tops should be long enough to tuck into your pants and eliminate any waistline gaps, even when you lean forward.

GET A GOOD FEEL Most long underwear these days is made from some form of polyester, though a few alternative fabrics are available. Polyester absorbs very little water, dries fast, and (unlike polypropylene) generally does not become permanently stinkified with regular use. Manufacturers often coat the threads to enhance the material’s moisture-wicking ability; Patagonia’s Capilene coating is perhaps the best known example. If you prefer a natural, non-petroleum-based fabric, consider silk, which has the softest touch, or merino wool, which insulates well even when damp. Avoid cotton; you don’t want a moisture-sponging, slow-drying fabric that might lead to hypothermia.

MANAGE MOISTURE You sweat all the time. In cooler conditions, it’s critical to move that moisture away from your body, where it will otherwise suck up body heat like a clammy vacuum cleaner. Any synthetic fabric will wick water away, but it’s more crucial not to sweat so much in the first place. If you are active, even in conditions well below freezing, you will rapidly generate enough heat to stay warm with minimal insulation. If your base layer is too warm, you will sweat more profusely—and be that much colder when you stop. As a general rule, except in the most frigid conditions, opt for a lightweight base layer during periods of exertion and a thicker layer for times of prolonged inactivity.

DRESS WITH STYLE Long underwear tops come in two basic styles: crew neck or zip neck. The latter is warmer, extending farther up your neck, but make sure the top isn’t uncomfortably tight against your trachea, especially if you’re going to have several more layers on top of it. Some sleeves are stitched longer and include an integrated thumbhole that allows you to pull them over your lower hand, a nice feature. But be watchful with longer sleeves in wet conditions; water will absorb through any exposed cuffs and wick upward along the sleeve.

Courtesy of the Appalachian Mountain Club