Thermal underwear is a set of clothing, most often a long sleeved shirt and pair of long pants, that is meant to be worn underneath the outer clothing on cold days. They are also sometimes called “long underwear” or even “long johns.” This type of underwear is very popular among men and women who work outside or pursue sports and other outdoor activities in cold climates and during the winter time. Some kinds of thermal underwear even have multiple layers of different kinds of fabrics for extra protection against the cold and for wicking away sweat while working hard or playing sports.
For maximum insulation, thermal underwear should be worn under other warm clothes that trap heat against the body instead of letting it escape into the air. On days where it is chilly but not extremely cold, a thermal shirt can be worn underneath a regular long or short sleeved shirt, or even alone as a shirt by itself, since it is often cut and shaped like a normal shirt instead of an undershirt. The tapered shape of most thermal underwear pants, while optimal for trapping heat and keeping the wearer warm, does not lend a very flattering appearance, so they are usually worn only underneath other long pants and not underneath shorts or as pants by themselves. The exception to this is when the wearer chooses to sleep in thermal underwear at night, taking advantage of the extra warmth that it provides.
An alternative to the standard long pants and long sleeved shirt set is a full body suit made out of thermal material. This is worn in the same fashion as the individual pieces of underwear and serves the same purpose. The lack of separation around the middle may even give it a slight advantage at trapping heat against the body since there is not a gap for the warm air to escape through. Some people prefer the separate pieces to the full body suit, however, because it takes less effort to get into and out of them to change clothes or use the restroom.
Many people who frequently wear thermal underwear recommend trying on a set underneath clothing, and then testing different movements to make sure the material will not bunch up, roll, chafe, or otherwise cause discomfort after a long day of wear. Buying a slightly smaller size might help to reduce slipping and bunching, as well as make it easier to fit the underwear underneath blue jeans and other non-stretch outerwear. However, be careful when choosing a smaller size because some brands might shrink slightly when washed and dried.