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What a base layer is actually doing for you

Everything in your hill kit hangs off the layer touching your skin. Get the base layer right and the rest of the system — fleece, shell, spare insulation — quietly does its job. Get it wrong and you'll be shivering at a cairn while your mate fiddles with a zip.

The job is simple: move sweat away from your skin, hold a thin layer of still, warmed air next to your body, and not stink so badly at the end of three days that the tent becomes a diplomatic incident. On a British hill you're dealing with damp air, wind chill, and a temperature that can swing ten degrees between the car park and the summit. You'll sweat on the climb and cool down fast the moment you stop for a butty. The base layer is the buffer between those two states.

Wool and synthetics do the same job in different ways, and the right answer depends on how you walk, how long you're out, and how much you mind smelling like a wet dog.

Wool: merino and its modern blends

Modern merino is nothing like the itchy school jumpers of memory. Fine fibres — look for 17.5 to 19.5 micron — feel smooth next to skin, and the natural crimp traps air brilliantly. A 190–200gsm long-sleeve merino top is the classic British winter base layer, warm for its weight and comfortable under a shell for hours.

• Warmth when damp: wool absorbs moisture into the fibre rather than sitting clammy against you, and it still insulates reasonably when wet — not the miracle some marketing suggests, but genuinely better than polyester.
• Comfort range: merino feels pleasant from about 2°C up to 15°C, and you can wear it in a bothy without feeling soggy.
• The weak point: pure merino wears through at armpits, shoulders and where rucksack straps rub. Blends with nylon or a little polyester last far longer for a small loss in softness.
• Cost: expect £50–£90 for a decent 200gsm merino crew, more for a hooded version.

Synthetics: the fast-drying workhorses

Polyester, polypropylene and their blends with a touch of elastane are the default for good reason. A 120–160gsm synthetic top wicks hard, moves sweat outwards quickly, and dries astonishingly fast — an hour on a windy fence post, or twenty minutes of body heat if you're still moving.

• Best for high output: fast fell running, summer longs, or a big climb where you know you'll be soaked and then keep moving.
• Weak when wet and stopped: a soaked synthetic top sucks heat out of you fast once you stop. Pack a dry spare for the summit break, always.
• Cost: £20–£45 covers most good options, and they shrug off abuse.

Fit matters more than fabric here. A synthetic top needs to be close to the skin at the back and shoulders, or it just sits there while you marinate.

Odour: the biggest practical difference

Wool resists the bacteria that cause smell, and that advantage is enormous on multi-day trips. Three days in the same merino top in a damp Welsh valley is unpleasant but survivable. Three days in polyester and you'll be sleeping with it outside the tent, whatever the weather.

Antimicrobial treatments on synthetics help for about a season, then fade. If you regularly do bothy trips, wild camps or back-to-back days out, wool or a merino-heavy blend is worth the money purely for the social and personal comfort.

Drying, weight and packability

Synthetics win on drying, no argument. Wring them out and they're nearly wearable; a damp merino top in a cold tent may still be damp the following morning. If your plan involves washing kit in a sink and wearing it the next day, synthetic is far more forgiving.

For weight, a 150gsm synthetic is lighter than a 200gsm merino for similar warmth, but the gap is smaller than most people assume. Where merino really loses is packability — it's bulkier and heavier in the rucksack as a spare. A sensible compromise for a week-long trip is one merino top worn, and one light synthetic as the dry spare for stops and sleeping.

Cost, durability and choosing yours

Synthetic is the cheaper entry point and lasts well, though it pongs. Merino costs more, can pill and wear quicker, but you need fewer of them because you can wear one for days. Blends — roughly half merino, half synthetic — sit in the middle and often make the best all-round British hill top.

• Winter hill days and multi-day camping: 190–200gsm merino or merino blend, long sleeve, plus a synthetic spare.
• Summer walking and fast days: 120–150gsm synthetic, cheap enough to own two.
• Changeable spring and autumn: 150gsm merino blend — warm enough at dawn, not stifling at noon.
• Care: wash wool at 30°C with a wool detergent, never tumble dry. Wash synthetics cool, and skip fabric softener — it clogs the fibres and kills the wicking.

Buy one good merino top and one cheap synthetic, use them for a season, and you'll know exactly which suits the way you walk. That's better than any spec sheet.