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 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.

The Merino.tech Merino Wool Base Layer Mens Set stands out with its use of 100% superfine merino wool, known for being soft and itch-free, ensuring all-day comfort. This set includes both a top and bottom in midweight (250 g/m²), making it versatile for varying temperatures. Additionally, the package includes a bonus pair of merino wool hiking socks, adding extra value.

The natural moisture-wicking and odor-resistant properties of merino wool make this thermal underwear an excellent choice for staying dry and fresh during active use. Its breathability helps regulate body temperature, providing comfort throughout the year. Despite being machine washable, hand washing and air drying is recommended for extended longevity, which might be inconvenient for some users.

The fit is designed to prevent rubbing, overheating, or bunching, contributing to comfort. This set would be particularly beneficial for men needing reliable and comfortable base layers for outdoor activities or everyday wear in cooler climates.