The deep chill has arrived early this season. A good base layer (a.k.a. long underwear) helps combat it and provides the foundation for any good winter layering system. A veritable blizzard of options, styles, and materials are available, however, which can make it difficult to select one that's right for you. Focus instead on these three key aspects and you'll be happier and warmer this frosty season.
Fit snug as possible without causing constrictive.
Form fitting, yes. Cotton, no. Image: Wikimedia
A base layer serves two primary functions. First, it traps your body heat (and the warm air it generates) directly next to your skin. Second, it moves heat-sucking moisture (your sweat) away from your skin and into your outer layers.
To most effectively accomplish these, a good base layer should fit flush against your skin everywhere. The goal is to trap maximum body heat and 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.) A snug fit has the added bonus of providing direct contact for maximum moisture transfer as well.
Your base layer should not fit so tightly, however, that it restricts motion or is uncomfortable. It should go without saying that the only way to find the base layer that best fits your body shape is to try on multiple styles and brands.
Avoid the waist gap.
One of the most common problems in a base layer system, especially for taller people, is the dreaded waist gap. You know, the crack that opens up above your lower back when you bend forward and your upper layer pulls away from the lower.
Eliminate this prime cold air opening by purchasing an upper base layer that is long enough to securely tuck in to your bottoms—and stay tucked. Taller individuals should look for brands that offer tall sizes, which provide extra length in the torso and arms. (I'm 6 feet 5 inches and 190 pounds; every base layer I own is a size large-tall.)
Polyester or wool? Yes. Cotton? Absolutely not.
Don't wear cotton long underwear. It absorbs moisture like a sponge and, once damp, it rapidly sucks away body heat.
Polyester is the go-to fabric for many (including me). It absorbs very little water, transfers moisture effectively, and dries quickly when it does get damp. (My favorite base layer fabric, hands down, is Polartec Power Stretch.) Merino wool garments are another good (but spendy) option, such as those offered by Smartwool.
Remember that fit almost always trumps fabric tech. An inexpensive, properly fitting base layer will usually be warmer than a poorly fitting base layer made from the fanciest, most expensive fabric you can buy.