We tasked 14 evaluators with outdoor winter activities like hiking, running, skiing, and snow shoveling to test various thermal undergarments. Our recommendation is the Ridge Merino Aspect Midweight Merino Wool Base Layer Long Sleeve Shirt, available for both men and women. This shirt stands out for its versatile design suitable for both casual wear and outdoor activities, providing warmth and breathability. One of the advantages of choosing this model is the hassle-free online purchasing process, as Ridge Merino offers free returns within the US, including Alaska and Hawaii.
In this article, when we mention “thermal underwear,” we are talking about base layers that help regulate body temperature by managing moisture while engaging in physical activities. If you are searching for traditional cotton long johns more suitable for lounging indoors rather than outdoor activities, you can find those in our pajamas guide.
Our Choice
This merino-blend shirt, available in basic colors, is breathable, durable, and can be worn alone or as a layer. While it comes in a wide range of sizes, larger sizes could be a welcome addition.
This is the women’s version of the same shirt, with a more tailored fit.
The Ridge Merino Aspect Midweight Wool Base Layer Long Sleeve Shirt, designed for both men and women, is our top choice due to its breathability, merino blend, and understated style. It’s a versatile shirt that transitions seamlessly from city to trail, suitable for running, skiing, backpacking, work, and even casual dinners. Moreover, it comes at a reasonable price point.
While the fit of this shirt is on the slimmer side, it still offers comfort without being excessively tight, making it suitable for various activities beyond outdoor performance. With a range of basic colors, it can easily complement your existing wardrobe.
The merino wool in this shirt effectively manages perspiration in cold weather, while the addition of nylon enhances durability and shape retention even after continuous use. Ridge also provides a crew-neck version of this shirt with a quarter zip for both men and women, along with options featuring a neck warmer for women and a full hood for men.
Although this shirt offers a broader size range than many competitors, there’s still room for expansion, particularly in larger sizes. In case you need to exchange for a different size, Ridge Merino stands out as the sole company among our recommendations to offer free returns, including a return label to facilitate a correct fit.
The Aspect shirt is machine-washable, but due to its predominant merino wool composition, air-drying is recommended to preserve the fibers and prolong the garment’s lifespan.
Budget Option
This synthetic option is ideal for individuals who prefer non-merino wool base layers and seek easy-care garments. While it resembles traditional “underwear” more than our top pick, it offers a slim fit and can be layered for additional warmth.
Like the men’s version, the women’s shirt is a slim fit.
The REI Co-op Midweight Base Long-Sleeve Base Layer Top, available for men and women, stands out as the best synthetic alternative to our top pick. It offers a lower price point and appeals to those looking for a softer fabric, albeit with slightly lesser odor control and moisture management compared to our top choice. While it may resemble traditional underwear more closely, it might be less versatile in terms of styling.
This garment, constructed from synthetic fibers, boasts faster drying times than merino wool due to its reduced water absorption. However, the midweight nature of the fabric is slightly heavier than our top picks, with only marginal improvements in drying time during intense physical activity. While some synthetics may offer superior breathability compared to merino, the REI Co-op Midweight shirt showed slightly lower breathability than our leading choice.
Since this shirt lacks the natural odor resistance of merino wool, it may require more frequent laundering compared to our top pick. Nevertheless, the REI Co-op Midweight shirt can be safely machine-dried for added convenience.
REI offers an extended return policy, allowing members a 365-day window and non-members a 90-day return window—among the most generous in the market. However, for those unable to visit a physical REI store for returns, a fee may apply for mailing labels.
The REI Co-op Midweight shirt provides a broader size range for both men and women compared to our top pick, with tall options available for men.
Also Exceptional
This thick, 100% merino wool top offers extra warmth compared to our other picks, making it ideal for individuals seeking additional insulation and prioritizing comfort over performance during sweaty activities.
Although labeled differently, this garment mirrors the men’s version but with a tailored silhouette.
The Minus33 Midweight Merino Wool shirt, available for both men and women, offers a thicker option compared to the Ridge Merino Aspect and the REI Co-op Midweight base layers. The fabric of the Minus33 Midweight shirt performs better in situations involving less sweat, such as daily wear, static outdoor activities like hunting or fishing, or skiing on very cold days where additional layers are necessary for warmth. While slightly pricier than our top choice, this shirt provides enhanced insulation.
Being the only 100% merino wool option among our recommendations, the Minus33 base layer may not be as durable as those made with a merino-nylon blend (like the Ridge Merino Aspect) or a synthetic fabric (such as the REI Co-op Midweight). However, the thicker fabric of the Minus33 shirt helps offset these shortcomings. Similar to the Ridge Merino Aspect, air-drying is advised to extend the lifespan of the garment.
With a looser fit compared to our other picks, the Minus33 shirt may not offer as effective moisture management as more form-fitting options like the REI Co-op and Ridge Merino selections. Nevertheless, this shirt’s casual fit makes it suitable for everyday wear.
Minus33 offers the widest size range across both men’s and women’s options among our picks, accommodating up to a 61-inch chest size (6X) for men. However, the women’s version only goes up to a 47-inch chest size (3X).
Minuns33 boasts a shorter 30-day return window, without offering free returns. However, with widespread availability at various retailers, exploring different purchase options online or visiting a physical store may provide more favorable return policies.
