10 Best Thermal Underwear 2025 in the United States

The Merino.tech Merino Wool Base Layer set for men is made from 100% superfine merino wool, known for being soft and comfortable without causing itchiness. This set comes in three weights—lightweight, midweight, and heavyweight—with the described option being the heavyweight at 320 g/m², making it well-suited for very cold conditions. The fit is designed for daily wear, avoiding bunching or overheating, which helps keep you comfortable under other clothes.

One of the strongest points of this product is its natural moisture-wicking and odor-resistant properties, keeping you dry and fresh even during active use. The fabric breathes well, helping regulate body temperature across different seasons. It includes a pair of Merino wool hiking socks, adding value to the package. Durability-wise, while merino wool is naturally resilient, the recommendation to hand wash and lay flat to dry means it may require more care than synthetic options. Machine washing is mentioned but hand washing is preferred to maintain quality longer.

This set provides a high-quality, all-natural thermal underwear option that balances warmth, comfort, and odor control, especially suitable for outdoor activities or chilly weather. However, those seeking very low-maintenance or budget options might find it less convenient due to care needs and premium material costs.

The Merino.tech Merino Wool Base Layer Women Set stands out for its high-quality 100% Merino wool construction, making it a great choice for anyone seeking comfort and functionality in thermal underwear. Available in three weight options—lightweight (165 g/m²), midweight (250 g/m²), and heavyweight (320 g/m²)—it offers versatility for different weather conditions. The midweight version, which includes a top, leggings, and hiking socks, is ideal for moderate climates and active wear.

The superfine 17.5-micron wool ensures a soft, itch-free experience, catering to those with sensitive skin. Its excellent moisture-wicking and natural odor-resistant properties keep you dry and fresh throughout the day, which is perfect for extended outdoor activities or daily wear. The set is also breathable, regulating body temperature to maintain comfort in varying conditions.

However, this product requires special care; it is recommended to hand wash and lay flat to dry, which could be inconvenient for some users. The durability is generally good, but like all wool garments, it might be prone to pilling over time. The product comes from China, which might be a consideration for those preferring locally-made goods. As a bonus, the packaging is attractive, making it a suitable gift option. This set is best suited for women looking for reliable thermal underwear that can handle both everyday use and outdoor adventures while providing superior comfort and performance.

The Cuddl Duds Thermal Underwear set for women is designed to keep you warm and comfortable during cold weather. Made from a soft, heavyweight blend of polyester and spandex, it provides a cozy fleece lining inside with a smooth, stretchy exterior that moves with you. The spandex stretch ensures a snug but flexible fit, making it easy to wear under clothes or on its own.

It features moisture-wicking fabric that helps pull sweat away from your skin to keep you dry, which is great for active days. The thermal set also offers protection from UV rays, which is a nice bonus if you’re outside for long periods. The comfortable design includes thoughtful touches like thumbholes and a tag-free label to reduce irritation.

While the material blend supports some odor resistance and durability, it's mainly polyester-based, so it may not be as long-lasting as natural fibers like wool. The set is lightweight enough to avoid bulk while still providing good insulation. This thermal underwear is best suited for women looking for a versatile, warm base layer that balances comfort, stretch, and moisture management for everyday cold weather use.