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.

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.
• Insulates when damp: wool's crimped fibre keeps trapping air even holding moisture.
• Odour resistance that isn't marketing: wearable for days; lanolin and fibre chemistry suppress the bacteria that make synthetics smell.

Weaknesses: dries slower than synthetic, costs more (€60–100 a top), and durability is only fair — thin merino wears through at pack-strap points. Merino/nylon blends fix most of the durability problem for a small warmth cost.

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.

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.

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".

Head-to-head

Fit and care

Base layers work in contact with skin — snug, not compression-tight, no gaping at waist or wrists. Wash merino inside-out on wool cycle, no fabric softener (it coats fibres and kills wicking on synthetics too), and skip the dryer for merino.

The honest recommendation

Own both: merino 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.