Ask Fuzzy: How does thermal underwear work?

Thermal underwear's main job is to fight evaporative cooling.
Thermal underwear's main job is to fight evaporative cooling. Photo: Rebecca Hallas

Q How can thermal underwear keep you warm when it is made of such thin fabric?

A We lose body heat in a number of ways. These include conduction, where heat is transferred from a warmer solid object to a cooler solid object - anyone who has sat on a metal bench in winter will have experienced this first hand (or first backside); convection, where heat is transferred from your body to a liquid or gas, as experienced on a windy winter day or diving into a pool in summer; and evaporative cooling, where the process of a liquid changing state into a gas takes energy (in the form of heat) from its surroundings.

Fleecy jumpers work as insulators - or convection-heat-loss-fighters. They keep the air close to the skin in place, so that once your body warms it up, the pesky wind won't blow it away and the rate at which it leaks out into the cool outside air is slowed. Double-glazed windows work on similar principals.

The word 'loft' can be used to describe how thick the insulating air space created by clothing is. Puffer jackets trap lots of air, so they have a high loft, and keep you warm.

So how does super-thin thermal underwear work?

Although thermals provide some insulation, their main job is to fight evaporative cooling. Here's a quick example of evaporative cooling: blow across the back of your hand, then lick it, and blow again. The wet feels much cooler. It's because as the water evaporates, it draws energy (in the form of heat) away from your skin. Thermals 'wick' (or draw) sweat away from the skin, minimising evaporative cooling.

Wool, wool-bamboo blends and synthetic fabrics like acrylics have excellent wicking properties in your quest to keep dry and warm while working outdoors. Wool can absorb more than 30 per cent of its weight in water before it starts to feel wet.

■ Response by Alice Ryder, Fuzzy Logic

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