10 Best Thermal Base Layers 2025 in the United States

The Merino.tech Merino Wool Base Layer Set for men is a strong choice if you're looking for warm, comfortable thermal underwear. Made from 100% superfine merino wool, it promises softness without the itchiness common in wool products. The midweight fabric (around 250 g/m²) strikes a nice balance, offering warmth without overheating, which makes it suitable for various activities and weather conditions. Its natural moisture-wicking and odor-resistant qualities help keep you dry and fresh throughout the day, which is great if you plan to wear it for outdoor activities or long hours.

The included merino wool beanie adds extra value and warmth, especially for cold days. The fit is designed to avoid bunching and rubbing, enhancing comfort during movement. One thing to keep in mind is the care: while the product description suggests hand washing and air drying for best results, the official care instructions mention machine washing, so you might want to be cautious to maintain the garment's quality over time. This set is imported and seems well-suited for everyday wear as well as outdoor use in cooler climates.

If you want a natural, breathable base layer that handles moisture well and stays comfortable, this set is worth considering, especially with the bonus beanie included. For those preferring easier care or a lighter or heavier weight, exploring other options might be beneficial.

The WEERTI Thermal Underwear for Men offers reliable warmth with its soft fleece lining, ensuring comfort throughout cold days. Made primarily of polyester with a bit of spandex, the fabric is lightweight yet stretchy, allowing for easy movement without feeling tight or restrictive. The four-way stretch fit is ideal for those who want base layers that move with them, whether active outdoors or lounging at home.

This material is breathable and wicks away sweat quickly, helping you stay dry and fresh even during physical activities. While the fleece lining enhances warmth, it may feel a bit bulky under very tight outerwear. Additionally, since it’s mostly synthetic fabric, some might prefer natural fibers like wool for better odor control or moisture management. The set is machine washable and easy to care for.

This thermal underwear serves well as a base layer for cold weather, suitable for everyday use, sports, or sleepwear. It is a strong choice if you want cozy, stretchy, and moisture-wicking long johns but may not be ideal if ultra-lightweight or natural material base layers are your priority.

The Merino.tech Merino Wool Base Layer Women Set is designed for women looking for warmth and comfort during cold-weather activities. Made from 100% superfine Merino wool, the fabric is very soft and comfortable without the itchiness that some wool products have. It comes in three weights—lightweight, midweight, and heavyweight (320 g/m²)—with this set being the heavyweight option, making it a solid choice for very cold conditions. The fit is intended to be snug but not restrictive, helping with layering without bunching.

One nice bonus is the inclusion of Merino wool hiking socks, which adds extra value. The wool naturally wicks away moisture and resists odors, so you stay dry and fresh even during active use. Because it’s wool, it requires delicate washing and air drying to maintain its quality, which may not suit those wanting low-maintenance gear. While heavyweight wool offers warmth, it might feel too warm for milder days or high-intensity activities.

This set suits outdoor enthusiasts and anyone needing reliable insulation in cold weather, but it is less ideal for casual or warm-weather wear.