Layering
Base Layers: Merino vs. Synthetic vs. Cotton (One of These Can Hurt You)
The cheapest layer in your system has the biggest say in whether you stay warm, because it manages the thing that actually makes you cold: your own sweat. Wet fabric conducts heat away from skin dramatically faster than dry — the base layer's whole job is keeping you dry from the inside. Get this layer wrong and no parka on earth fixes it; get it right and a modest mid layer suddenly performs like a much warmer one.
Merino wool
Fine merino fibre (17–19 micron — nothing like scratchy old wool) is the default recommendation for a reason:
- Warm across a range: comfortable from mild to deeply cold without swapping. The fibre absorbs moisture vapour into its core while the surface stays dry-feeling, so it buffers temperature swings instead of amplifying them.
- Insulates when damp: wool's crimped fibre keeps trapping air even holding moisture. Merino can absorb around 30% of its weight in water before it even feels wet against skin.
- Odour resistance that isn't marketing: wearable for days; lanolin and fibre chemistry suppress the bacteria that make synthetics smell. On a five-day trip, one merino top genuinely replaces three synthetic ones.
Weaknesses: dries slower than synthetic, costs more (€60–100 a top), and durability is only fair — thin merino wears through at pack-strap points, and a 150 g/m² top can develop holes within a season of hard pack use. Merino/nylon blends (typically 80/20 to 65/35, or "core-spun" merino wrapped around a nylon filament) fix most of the durability problem for a small warmth cost, and they're what most people should actually buy. Smartwool, Icebreaker and Ortovox all sell blends alongside pure merino for this reason.
Weight guide: 150–200 g/m² for active days, 230–260 g/m² for cold, low-output days. Heavier than 260 is a mid layer wearing a base-layer costume. If you only buy one, 190–200 g/m² covers the widest range: warm enough at -5 °C (23 °F) standing around under a mid layer, not sweltering at 10 °C (50 °F) on the move.
Synthetic
Polyester or polypropylene base layers wick faster and dry fastest of anything — the right call for genuinely high-output activity (winter running, ski touring) where you produce sweat faster than merino can shift it.
- Pros: fastest drying, cheapest (€20–40), most durable, easy care.
- Cons: smells after a single hard day; static-prone; some feel clammy at rest.
The odour thing is structural: polyester's surface is a bacterial buffet. "Anti-odour" silver/zinc treatments help and then wash out — after 30–50 washes, most treated tops smell like untreated ones.
Two distinctions worth knowing inside the synthetic category:
- Polypropylene vs polyester. Polypro absorbs almost no water at all (under 1% of its weight, against roughly 0.4–4% for polyester) and is the classic Nordic choice for the coldest active use — but it holds odour even worse and melts at lower dryer temperatures. Polyester is the sensible default.
- Grid fleeces like Polartec Power Grid or Power Dry put channels on the inside face that speed wicking and venting. A grid-backed synthetic is the single best base layer for ski touring and winter running — the channels move sweat sideways and out faster than any flat knit.
Cotton: the safety paragraph
Cotton absorbs up to 25× its weight in water, holds it against your skin, and keeps conducting your heat into it for hours — it cannot wick, cannot insulate wet, and dries slowest of all. Around town that means being clammy and miserable. In backcountry cold, a cotton base layer is a hypothermia accelerant; "cotton kills" is taught in mountain rescue courses, not invented by outdoor brands. A cotton t-shirt under your parka is the single most common reason warm coats "don't work".
This includes the sneaky cotton: most "thermal" long johns from supermarkets are cotton or cotton-rich blends, and so are ordinary hoodies and jeans. A cotton/poly 50/50 blend is better than pure cotton and still worse than either real option. Check the label — anything over about 30% cotton doesn't belong against your skin in the cold.
Head-to-head
| Merino | Synthetic | Cotton | |
|---|---|---|---|
| Warmth when damp | Good | Fair | None |
| Drying speed | Fair | Excellent | Terrible |
| Odour after 3 days | Fine | Biohazard | Bad |
| Durability | Fair (good blended) | Excellent | Good |
| Comfort at rest | Excellent | Fair | Fine until you sweat |
| Price | €€€ | € | € |
Match the layer to the day
The material question is really an output question:
- Commuting and errands (walk, sit, walk): merino, 190–260 g/m². Output is low, the odour resistance means fewer washes, and the at-rest comfort matters more than drying speed.
- Hiking and resort skiing (moderate, variable output): merino or grid synthetic, 150–200 g/m². Either works; sweaty people should lean synthetic.
- Winter running, ski touring, anything that soaks a shirt: synthetic, light (under 150 g/m²). You'll out-sweat merino's buffering, and synthetic's drying speed wins the moment you stop moving.
- Multi-day trips without laundry: merino, no contest. Day three in polyester is antisocial.
Fit and care
Base layers work in contact with skin — snug, not compression-tight, no gaping at waist or wrists. A base layer that billows can't wick because wicking requires contact; if you can grab a fistful of loose fabric at the stomach, size down. Sleeves should reach the wrist bone with arms extended, and the hem should tuck and stay tucked.
Care rules that actually change lifespan:
- Wash merino inside-out on a wool/gentle cycle at 30 °C, with wool wash or any mild detergent. Modern merino is machine-washable; the dryer is what shrinks and shortens its life — hang it instead.
- No fabric softener, ever, on anything technical. It coats fibres and kills wicking on synthetics and merino alike. This one habit ruins more base layers than wear does.
- Wash synthetics cold and hang-dry to slow the odour buildup; an occasional soak in a sports wash (Nikwax BaseWash, Grangers Active Wash) strips the bacterial film that normal detergent leaves behind.
- Don't over-wash merino. Airing it overnight restores it for days more wear — that's the whole point of paying for wool.
Quick answers
Is merino itchy? At 17–19 micron, no — that's finer than the human itch threshold (roughly 22 micron). If a wool top itches, it's coarser wool or a very sensitive neckline; try a different brand before writing off the fibre.
Are €25 merino tops from discount chains any good? Often yes for the fibre, but check the weight (frequently a flimsy 140–150 g/m²) and expect seams and cuffs to fail sooner. A mid-price blend usually outlasts two cheap pure-merino tops.
Do I need merino bottoms too? Below about -5 °C (23 °F) or on low-output days, yes — legs lose real heat and jeans over bare skin is the classic cold-legs setup. For high-output use, legs sweat less than torsos, so lighter or synthetic bottoms are fine.
What about silk? Lovely at rest, very light, but fragile, slow-drying and mediocre at wicking. A niche choice for travel and sleepwear, not for effort.
The honest recommendation
Own both: merino (blended, ~200 g/m²) for everyday winter and multi-day use, synthetic for the sweatiest sessions. And donate the cotton thermals — whoever ends up with them deserves to know they're pyjamas.
