Smartwool Classic Thermal Merino Base Layer Bottom
250 g/m² merino, full-length
The single best sleep-warmth upgrade above 4,000 m and it doubles under trousers on a cold pass; at 250-weight it is too warm to walk in on any ordinary day.
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Do not buy this if: Low-altitude treks. Below 3,000 m you will never take them out of the bag.
Not every route needs it. These are the ones where our guides would put it on your list.
Everything else we recommend in base layers, including the ones we think suit a different trekker.
Best overallIcebreaker Merino 200 Oasis Long Sleeve Crew
Merino at 200-weight is the one true trekking base layer — warm damp, warm dry, and it does not smell; it is also three times the price of synthetic and it will eventually hole at the shoulder where the pack rubs.
Best syntheticUnder Armour Tech 2.0 Long-Sleeve
Dries in an hour and costs a fraction of merino, which makes it the better buy on a short trek — it will smell by day three and nothing you do changes that.
