The shirt you wear for eight days without anyone minding
Icebreaker 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.
Skip it ifA four-day trek where you can wash and change. Synthetic is cheaper, tougher and dries faster; merino earns its price on long trips without laundry.
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The rest of the shelf
Best sleeping layerWhat you change into at the lodge and sleep in
Smartwool Classic Thermal Merino Base Layer Bottom
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.
Skip it ifLow-altitude treks. Below 3,000 m you will never take them out of the bag.
Best syntheticThe one that dries by dinner when merino does not
Under 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.
Skip it ifA fortnight without laundry. That is exactly the case merino is for.