Key takeaways
- A base layer is the next-to-skin layer that wicks sweat and regulates temperature.
- Keeping skin dry is what keeps you warm in cold and comfortable in heat — the foundation of layering.
- Materials: merino wool (comfort, odor resistance, warm-when-damp) vs synthetic (fast-drying, durable, cheap).
- Avoid cotton — it absorbs sweat, stays wet, and chills you ('cotton kills').
What a base layer does
A base layer is the innermost layer of clothing, worn next to the skin. Its job is to wick sweat away from your body and help regulate temperature. Keeping your skin dry is the whole game: dry skin stays warm in the cold and cooler when you’re working hard. That’s why the base layer is the foundation of any layering system, with mid and outer layers built on top.
Materials
- Merino wool — comfortable, odor-resistant, warm when damp; pricier and slower-drying.
- Synthetic (e.g., Capilene) — wicks aggressively, dries fast, durable, cheap; less odor resistance.
- Cotton — avoid it: it soaks up sweat and chills you (‘cotton kills’).
For a cold, active day a hiker wears a snug midweight base layer that pulls sweat off the skin on the climbs — so when they stop and cool down, they’re dry rather than clammy and chilled in a damp cotton shirt.
Choosing one
Pick the material for your priorities (merino for comfort and freshness, synthetic for drying speed and durability) and the weight for the conditions — lightweight for warmth and high output, midweight as an all-rounder, heavyweight for cold. Fit should be snug to wick effectively.
The bottom line
The base layer is the foundation of staying comfortable outdoors: it wicks sweat off your skin so you stay dry, and therefore warm or cool as needed. Choose between merino (comfort and odor resistance) and synthetic (drying speed and durability) to match your activity, pick a weight for the conditions — and never, ever use cotton.
Frequently asked questions
What is a base layer?
A base layer is the innermost clothing layer, worn against your skin to wick perspiration away and help regulate your temperature. It's the foundation of a layering system: by keeping your skin dry, it keeps you warm in the cold and cooler when you're working hard, while mid and outer layers add warmth and weather protection over it.
Merino wool or synthetic base layer?
Merino wool is comfortable, naturally odor-resistant, and stays warm when damp; synthetic (like polyester) wicks more aggressively, dries faster, is more durable, and costs less. Choose merino for comfort and multi-day freshness, synthetic for hard aerobic efforts and budgets, or a blend that combines the strengths.
Why shouldn't you wear cotton as a base layer?
Cotton absorbs and holds sweat instead of wicking it, so it stays wet against your skin, stops insulating, and rapidly draws away body heat — dangerous in cold conditions, hence the saying 'cotton kills.' Wool and synthetic base layers wick moisture and keep performing when damp, which is why they're the standard.
Sources
- Layering systems — The Mountaineers
- Clothing for the trail — American Hiking Society