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Guide
Active Base Layers
A base layer sits next to your skin, fits close and is made of synthetic fibre, merino wool or a blend of the two. If you sweat hard, go synthetic. If you are out all day or spend time standing in the cold, go merino.
Key points
- Choose synthetics for high effort; consider merino or blends for longer days.
- Choose a close fit without pressure at the neck, underarms or waist.
- Icebreaker lists 150 g/m² for versatile use and 200 g/m² for cooler days.
Which base layers for which activity?
| Use | Choice | Limit |
|---|---|---|
| Running, gym, Nordic skiing, the climb on a ski tour | ChoiceSynthetic (polyester, polyamide) | LimitSmells sooner, no warmth when wet |
| Hiking, hut-to-hut treks, resort skiing, everyday wear | ChoiceMerino wool | LimitHeavier, needs a wool wash |
| Mountain days with a pack, ski touring | ChoiceMerino blend | LimitFibre content varies widely: read the label |
| Running, gym, ball sports | ChoiceSports bra | LimitOnly supports in the right size |
| Everyday wear, easy sessions with little sweat | ChoiceCotton | LimitSoaks up sweat and stays wet: wrong for the hills |
Types compared
Synthetic
Wicks sweat, for running, gym and Nordic skiing.
- Advantage: Wicks sweat
- Drawback: Smells sooner
Merino wool
Warm when damp, for hiking, the piste and everyday.
- Advantage: Warm when damp
- Drawback: Heavier
Merino blend
Wool and synthetic, for mountain days and ski tours.
- Advantage: Combines wool and synthetic
- Drawback: Fibre content varies: read the label
What to check before you buy
| Question | In short | Why |
|---|---|---|
| Merino or synthetic? | In shortDepends on sweat | WhySynthetics wick sweat reliably and dry fast: right for running, Nordic skiing and steep climbs. |
| How warm should a base layer be? | In short150 to 240 g/m² | WhyWith merino, fabric weight tells you the warmth. Icebreaker rates 150 g/m² as an all-season weight, 200 g/m² for cool days and 240 g/m² and up for cold. |
| How tight should a base layer fit? | In shortClose, not tight | WhyClose-fitting with no folds, because only fabric that touches the skin can move sweat away. |
Frequently asked questions
Do you wear normal underwear under a base layer?
Yes, but not cotton. Odlo recommends boxers or briefs under long bottoms and a sports bra under the top, for reasons of hygiene. Both should be synthetic or merino too.
Why do synthetics smell sooner than merino?
Wool binds odour molecules inside the fibre, where bacteria do not thrive, as Australian Wool Innovation explains.
Long or 3/4-length bottoms for skiing?
3/4-length for downhill skiing. They stop above the cuff of the ski boot, so only your ski sock sits inside the boot and no hem presses on your shin.
How can I tell how much merino is in it?
Check the sewn-in label. Regulation (EU) No 1007/2011 requires every fibre to be listed with its percentage by weight.
More from the magazine
Merino wool: benefits and limitsMerino wool turns up in lots of outdoor clothing. We look at its properties, its strengths and its limits across different activities.
SAXX: performance underwear from VancouverA fishing trip off Alaska, a damp drysuit and a problem men rarely talk about: that is how SAXX began. Since 2006, the Vancouver brand has tackled it with a patented hammock design. Now at SportFits.
Compression in sport: what the research saysCompression clothing promises faster recovery and better performance. We looked at what the studies actually back up and what is just marketing.
Last updated: October 2026
Everyday
For everyday wear, pick a merino wool base layer of around 150 g/m²: it stays fresh over several wears. If you hardly sweat, the simpler synthetic top does the job too.
- Merino around 150 g/m²
- low odour
- flat seams
Running
For running, pick a thin synthetic base layer, because it wicks sweat reliably and dries fast. Dress as if it were 10 °C warmer: you will only feel chilly for the first few minutes.
- Thin synthetic
- close fit
- supportive sports bra
- flat seams
Hiking
For hiking, wear merino or a merino blend, which stays warm when damp. The air cools by about 6.5 °C per 1,000 m on average: a wet cotton T-shirt leaves you cold on the summit.
- Merino or blend
- warm when damp
- spare top in the pack
Winter sports
For the piste, pick a merino base layer of 200 g/m², or 240 g/m² and up on cold days, with 3/4-length bottoms. For Nordic skiing and ski touring, go for thin synthetics, because you sweat hard.
- Piste: merino 200+
- Nordic: synthetic
- 3/4 bottoms for boots
- long sleeves
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