Thermal underwear--the long sleeved thin, waffled weave shirt and pants worn under regular clothes--is very effective in keeping people comfortable in cool and cold weather. It is popular for people who spend a good amount of time working or playing outside in cold weather and prevents them from having to wear a heavy coat that could restrict movement.

Thermal underwear adds two more layers of clothing to the wearer's normal wardrobe. As it is made up from two-ply fabric, it traps more body heat than a single layer of clothing could. This extra body heat insulates against cold air and prevents heat loss. When worn under regular clothes, thermal underwear can keep a person warm for many hours in a cold environment. Heavy cotton or wool thermal underwear should be worn in very cold weather.

Thermal underwear that is made of polypropylene fabric retains more heat than other fabrics and is better for active people who do sports or work outside. In addition to holding a good amount of heat, it also works to keep sweat away from the skin. This type of thermal underwear "breathes," which retards body odor. It keeps the wearer dry and warm by wicking moisture away from the skin. In the same way a candlewick will pull oil from a lamp and up the fabric to be lit and burned, the fibers in the fabric absorb the moisture on the skin and pull it away from the body. This prevents the wearer from becoming cold, soggy, and frozen.