Base Layers, Merino vs Synthetic, Explained
Merino base layers resist odor and handle stop and start activity well, while synthetic base layers dry faster, cost less and stand up better to rough use.
By Adolph GreenContributing writer.
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The short answer
A base layer is the snug layer worn against your skin. Its job is to move sweat away and keep you comfortable under the other layers.
- Merino wool: resists odor, so it suits multi-day trips and stop and start activity such as hiking.
- Synthetic (polyester, nylon, polypropylene): dries fast, costs less and shrugs off abrasion, so it suits hard, sweaty effort such as running and cycling.
- Cotton: skip it. It holds moisture and stops insulating when wet.
How each fabric handles sweat
The two fabrics deal with moisture in different ways. Icebreaker, a merino brand, explains that merino takes moisture into the fiber itself, while synthetic fabric moves sweat along its surface. Alpkit, which sells both kinds, adds that polyester and nylon fibers absorb no water, so a synthetic top does not get heavy with sweat, and the more open the knit, the more it breathes.
Merino vs synthetic at a glance
| Merino | Synthetic | |
|---|---|---|
| Odor | Resists odor buildup | Tends to hold odor with repeated wear |
| Drying | Holds moisture inside the fiber | Fast drying |
| Toughness | Less robust | Resists snags, tears, abrasion and pilling |
| Price | Higher | Generally lower |
| Care | Cold, gentle wash | Wash often, regular detergent |
| Best for | Mild to cold weather, hiking, multi-day trips | Hot weather, running, cycling, climbing |
Icebreaker cites a reviewer who found synthetic tops can keep an odor even with antibacterial technology, and Alpkit rates such treatments as less effective than merino. Alpkit notes polypropylene wicks fastest and weighs least but can retain odors, and that smooth synthetic fibers suit people who react to wool.
Fineness and fabric weight
Merino is generally finer than 22 microns, according to the Woolmark Company; Icebreaker says its hiking base layers use 17.5 to 18.9 micron fiber. Finer fiber feels softer next to the skin.
Many merino makers also print a fabric weight such as 125, 200 or 300. Icebreaker describes the lower numbers as better for hard effort, since they move heat and moisture faster, and the higher numbers as better for standing still in the cold.
Blends
Blends mix the two. Alpkit lists 80/20 merino to synthetic and 50/50 as common ratios, and uses a "corespun" yarn: a nylon and spandex core wrapped in merino, still 86 percent merino by its count. The synthetic part adds strength, stretch and faster drying; the tradeoff is less of merino's odor resistance.
Fit
A base layer should sit close to the skin. Alpkit's guide warns that a loose top does not wick well and a too tight one restricts movement. Flat seams help under pack straps.
How we count
The chart groups base layers on Wiki Shopping by the fabric their product names mention, on October 26, 2026. Each item counts once, in the first match of merino, other wool, fleece, cotton or a named synthetic fiber. 60 of the 79 "fleece" names say fleece lined, which describes a brushed inside rather than the fiber, so check the fabric details on each listing. 155 of the 270 names give no fabric. Browse the full range from clothing, shoes and jewelry. Counts change as listings are added or removed.