Base Layer
A base layer is the garment worn against the skin, chosen for moisture management rather than for warmth.
Its job is to move sweat away from your skin so you do not get chilled when you stop moving. Get this layer wrong and no amount of insulation or shell performance above it will make you comfortable.
What it must do
Wick moisture along the fibre surface and away from the skin, dry reasonably fast, and stay comfortable when damp. It should fit close, because a loose base layer does not contact the skin and therefore does not wick.
Fibre choice
Merino resists odour for days, which reduces how many you carry, and dries slowly. Synthetic dries fast and lasts longer, and smells within a day of hard use. Blends of merino with around 15 percent nylon capture most of both. See merino vs synthetic.
Weight
150 to 200 GSM is the general travel range. Lighter for hot climates, heavier where the base layer is doing real insulating work rather than only moisture management.
What not to use
Cotton. It absorbs water into the fibre, stays heavy, and actively cools you as it evaporates. It is the single worst choice next to the skin in any condition where you might sweat or get wet.
Related terms
- Merino vs synthetic base layers: choosing the fibre
- GSM: the weight bands
- Mid layer: what goes over it
- Glossary: all terms