Information on Thermal Underwear

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Information on Thermal Underwear
Cold-weather activities require good thermal wear. (snowmobiles image by Przemyslaw Moranski from

Enjoying cold-weather sports or performing outdoor chores is more pleasant with the right base layer. Thermal underwear, sometimes just referred to as thermals or thermal wear, is now referred to as the base layer. Whether natural fibre or man-made, thermal wear's main purpose is the same: warmth. Choosing the right base layer depends on the activity and, with all the choices available, may seem daunting.


Sir Benjamin Thompson, Count Rumford (1753-1814), credited with inventing thermal underwear, discovered that the air trapped in the weave of fabrics produced heat insulation. Later discoveries developed theories about fabric's ability to wick perspiration away from skin to conserve warmth. Modern thermal underwear works on a combination of both theories.

How it Works

Fabric traps body heat, forming a warm layer next to the skin. The ability of the fibres to absorb moisture and pull it away from the skin--wicking--keeps skin dry, increasing heat retention. Early thermals relied on waffle-type weaves--known as thermal weaves--to trap air that retained body heat. Modern technical base layers combine trapped heat and wicking to achieve results.


Originally made of natural fibres like cottons, wools and silks, early thermal wear had drawbacks. Cotton absorbs, but does not wick. Once damp, it loses thermal properties and creates an environment for microbe growth, producing odours. Wool, especially fine wools like merino and cashmere, absorbs up to 33 per cent of its weight, but irritates delicate skin. Silk thermals are thin and soft, but do not wick well.

Later generations added wicking properties from man-made fibres like acrylic and polyesters. Modern thermal wear may be made entirely of man-made fibres or a blend of natural and man-made.

Polypropylene, an excellent fibre for cold-weather aerobic activity like cross-country skiing or jogging, wicks perspiration away, then dries quickly: simply hand-wash, hang, and it's dry in minutes. It retains warmth even when wet. Blended or bonded polypropylene/natural fibre options are available.

Newer Options

Recent additions to thermal fabrics include crystals like tourmaline, believed to conduct magnetic and far-infrared energy, and metallic threads. Synthetic antimicrobial properties and scent guards improve functionality, and incorporating Spandex improves fit and circulation.

Only want natural fibres? The race for eco-friendly, renewable products offers surprising new choices in thermals. Bamboo fibres wick moisture much like man-made fibres and are antibacterial. To achieve the technical properties of high-end thermals, charcoal is added for odour absorbency.

Technical Information

Marketed by temperature-range or base number, technical thermal wear needs depend on the activity and the individual. Internal body heat production determines initial and continuous warmth, so temperature ranges are inaccurate and serve only as a guide. Base ratings (base 1, base 2, etc.) are determined by activity level and are suggestions, since actual heat retention depends on personal metabolism.

Thermal Fashion

Technical thermal wear has become more stylish. Seaming, piping and bright colours give wearers a broad range of choices. Thermal lingerie, with lace trim, smooth finishes and delicate designs, is making a foray into the fashion world.

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