The base layer is the layer next to the skin and is the most important element because it forms the foundation, or base, of warmth and moisture control. A damp base layer is the main source of body chill. Wet clothing against the skin will transfer heat away from the body, at a rate up to 25 times faster, than one that is dry.
Many of today’s base layer fabrics are made from artificial fibres. Some fabrics only wick moisture away from the body when in contact with skin. Others will wick from the microclimate between skin and fabric. Modern fibres are designed to dry quickly and used to suffer from odour retention. The newest fabrics are treated to reduce the odours left after prolonged usage. More traditional fibres are making a comeback with the ‘reinvention’ of Merino wool base layers. Merino is very warm and comfortable to wear and resists odours very well naturally without chemical treatments. An important aspect of base layers is that the fabric is lightweight and sturdy so it doesn’t restrict movement or be uncomfortable to wear. Base layers come in different weights and the one you choose depends on the level of activity, prevalent weather conditions and how you react to the cold yourself. People with poorer circulation will need a heavier base compared to those who “run hot”.
In general, lightweight base layers are for higher levels of activity and moderately cool temperatures. Medium weight layers offer the greatest versatility, warmth and wicking when undertaking mid-level activities or cooler temperatures. Heavy or expedition weight layers are used for higher warmth when activity is low. Since the base layer is the most critical layer, and largely a personal comfort level issue, having several weight options available is a good idea.