Base layers - a top, leggings or shorts - are worn next to your skin. The purpose of this critical layer is to keep your skin dry - and therefore warm - when you exert yourself and begin to sweat.
A base layer should be breathable, transporting moisture vapor away from your skin. It should also ‘wick’ perspiration from the surface of your skin.
What to look for in a base layer
The best base layers are made from wool or a synthetic polyester blend - both are excellent at wicking. Synthetic base layers are quicker-drying and can be more durable. On the other hand, they offer virtually no added insulation. Wool traps body heat effectively, and continues to insulate even when wet. It’s breathable too, helping to regulate your core body temperature.
Avoid wearing cotton as a base layer. It’s extremely good at absorbing moisture, but it doesn’t allow liquid to evaporate quickly, so it stays wet - which can make you dangerously chilled in cold weather.
Choose a soft fabric - if you’re going to wear it against your skin all day, it better be comfortable. Ultra-fine wools such as yak wool or merino lack the itchiness of traditional sheep wool.
Make sure your base layer fits well. Since the moisture needs to make contact with the fibers in order to be pulled away from the skin, a tighter, more contoured fit means better wicking.
If you’re planning a trip and want to pack light, opt for an odour-resistant fabric. Synthetic garments tend to smell dreadful after an intense bout of exercise. Wool is naturally anti-bacterial, so stays smelling fresher for longer.