Smartwool Men's Classic Thermal Merino Base Layer 1/4 Zip

Smartwool Men's Classic Thermal Merino Base Layer 1/4 Zip

One of Smartwool's heaviest base layer tops, updated with a refined slim fit and improved seam construction for skiers, hikers, and runners who want the ventilation advantage of a quarter zip in a 100% Merino wool thermal layer. The same natural temperature regulation, odor resistance, and flatlock seam construction as the crew with the added ability to dump heat fast on high-intensity efforts without stopping to remove a layer.

Why We Carry It
It's the base layer we recommend for high-intensity cold weather athletes who want a premium Merino thermal with a quarter zip for on-the-fly ventilation management during variable effort levels on the skin track, trail, or slope.

Highlights

• 100% responsibly sourced Merino wool in an interlock knit — Natural sweat management, temperature regulation, and odor resistance without the chemical treatments that synthetic base layers rely on for similar performance. Merino's inherent crimp traps air for insulation while the fiber's moisture-wicking properties move sweat away from the skin for a comfortable next-to-skin feel across varying effort levels.
• 10-inch quarter zip for on-the-fly ventilation — Open it up on sustained climbs and high-output efforts to dump heat and moisture without removing a layer, then close it down when the effort drops or the temperature falls. The single feature that makes a thermal base layer significantly more versatile across variable conditions and effort levels.
• Heaviest base layer top in the Smartwool lineup — A substantial Merino thermal weight suited for skiing, cold weather hiking, and running where maximum warmth and moisture management both matter throughout a full day in the cold.
• Slim fit that follows body contours — An updated fit designed to follow the natural contours of the body for a close, low-profile layer that doesn't bunch under a midlayer or shell and maintains skin contact for effective moisture transfer.
• Flatlock seam construction with offset shoulder and side seams — Seams stitched flat and positioned away from the highest-contact zones for a smooth, chafe-free interface under a shell or midlayer on long days where standard seam construction would cause irritation under a pack harness or helmet.
• Natural odor resistance across multiple days — Merino wool's inherent antimicrobial properties resist odor buildup without chemical treatments that wash out over time. A meaningful advantage for multi-day hut trips, ski tours, and backcountry objectives.

Best For

Skiers, ski tourers, cold weather hikers, and runners who want a premium 100% Merino wool thermal base layer with a quarter zip for ventilation management during high-intensity cold weather activity where both warmth and moisture control need to show up across a full day of variable effort.

Specs