While our affection for wool apparel is well documented, we’re particularly fond of it in fall and winter climates for its warmth and versatility. A good base layer can be used day after day, on its own or under a shell, and consistently provides comfort and thermoregulation in a variety of conditions without accumulating odor which is common with synthetic fabrics. Here are some various styles and thicknesses to get you through cold training days.

Ibex 24 Hour Short Sleeve Crew ($90)
Fabric composition: 100% Merino Wool. 165 g/m2*

The 24 Hour lineup is part of Ibex’s Ultralight category of year-round performance wear that work equally well as standalone pieces or as base layers in cold conditions. There are multiple styles to choose from—women have tank, short-sleeve and long-sleeve options in either a regular crew neck or scoop neck styles, while men have short and long-sleeve crew shirts as well as a short-sleeve polo model. The entire collection is constructed from MECool fabric, which is 100% merino wool with a performance finish that makes the material cooler to the touch than regular merino. This is particularly helpful if you are wearing wool in warmer than average conditions, and we found the short-sleeve crew had an exceptional temperature range of comfort with very little bulk. The 24 Hour tops have a regular fit through the torso that accommodates most body shapes, and the fabric has a nice feel even after multiple days of use. Our female testers appreciated the contoured sleeves (on the tee version) and curved hemline along with the scoop neck option. Available at www.ibex.com.

WoolX Explorer (Men’s) and Hannah (Women’s) Base Layer ($125)
Fabric composition: 100% Merino Wool. 230 g/m2

Without question, the most luxurious tops we’ve tested this year come from WoolX. The fabric has a silky smooth feel that evokes high-end cashmere. Even better, as opposed to some luxury wool garments that sacrifice performance elements for the sake of comfort, we’ve found these tops perfectly capable of handling frosty 20-milers. Perhaps the biggest challenge in shopping for WoolX garments is finding equivalency between men’s and women’s items, as the product lines for each gender have different names. We tested the men’s Explorer and women’s Hannah tops, which have the same thickness and fairly identical construction aside from princess seams on the Hannah. Their weight and insulating capacity come from density rather than bulk, and both have a somewhat form-fitting silhouette for easy layering under a jacket, but the fit isn’t as restrictive as a standalone layer. One thoughtful design element is the 3” foldable cuff, which increases the compression when folded back and pushed up on your forearm. Available at www.woolx.com.

Ridge Merino Natural Merino Tencel Tee ($60)
Fabric composition: 60% Merino Wool, 40% Tencel Lyocell. 150 g/m2

It can be argued that the only thing better than a naturally sourced super fabric (wool) is combining it with a second naturally sourced super fabric. In this case, a wood pulp-derived fabric called Tencel adds softness while increasing breathability and thermoregulation. The resulting material has a plush feel that mimics the smoothness of synthetic fabrics while maintaining the softness of natural wool. Tencel also has inherent anti-bacterial capacity, so a blended fabric garment doesn’t reduce the odor resistance of pure wool. This ultralight tee is our top choice for staying comfortable in warmer conditions or indoor gym use, but also provides strong insulation on colder days. Women don’t have a tee option, but there is a three-quarter sleeve equivalent with the same material construction. Both men’s and women’s versions have a slightly roomy fit through the torso and a long hemline for easy tucking, with the women’s version adding a drop-hem on the backside. Available at www.ridgemerino.com.

Tracksmith Brighton Base Layer ($88)
Fabric composition: 52% Wool, 28% Nylon, 20% Polyester. 145 g/m2

The most innovative shirt in this test group also turned out to be the one we used the most for its overall comfort and versatility. The Brighton tops are designed as form-fitting garments that gently wrap your body—but not in a compressive way—and have extensive stretch capacity to allow a full range of motion. Engineered merino mesh is perforated to optimize ventilation from your core, while a closed knit on the sleeves keeps your extremities warm. Because it is form-fitting, it slides effortlessly under a second layer, but we’ve worn it as a standalone layer for temps into the high 30s. The close fit pulls moisture off your skin more easily so the fabric gets wet faster than traditional tops, but because it’s merino wool, the insulating capacity remains solid. The fabric itself is exceptionally soft, and seamless construction eliminates potential sources of chafing. The sleeves are cut slightly longer than usual, and the hem is also on the long side. Otherwise, the classic Tracksmith style is on display with a small accent logo at one wrist, and a stealth sash across the front. The standard version of this shirt is long-sleeved for men and women, and men have a short-sleeve option available. Find all of them at www.tracksmith.com.

Paka Apparel Everyday Baselayer ($75)
Fabric composition: 80% Tencel, 20% Royal Alpaca. Fabric weight not available.

Although this garment has a relatively low quantity of wool, it is noteworthy because the wool comes from alpacas rather than sheep. Compared to merino wool, alpaca wool fibers have a hollow structure and are much lighter in weight, allowing similar heat retention in a much thinner fabric. Indeed, these shirts for men and women are by far the thinnest in our test group but have equal insulating capacity to heavier weight merino wool equivalents. Alpaca wool also has a lower water retention rate than merino wool, and has exceptional durability, so these shirts will retain their fit and performance features over the long term. A high percentage of Tencel fiber gives these tops additional softness and moisture-wicking ability, and both materials together maintain the same microbial resistance and odor control found in merino wool. Fit through the torso is narrow but not form-fitting, and men’s and women’s versions are similar to the point of being indistinguishable, even with the same four-color options. Available at www.pakaapparel.com.

Smartwool Classic Thermal Merino Base Layer Crew ($115)
Fabric Composition: 100% Merino Wool. 250 g/m2

The heavyweight option of our test group provides the best insulation and is made with an interlock knit process that lets it maintain a relatively low profile against the body. The thicker fabric still maintains good breathability but will be snug for those with muscular upper bodies. Shoulder panels eliminate seams at the top, and flatlock seams minimize any possible chafing in the torso. These tops were our choice for base layers in frigid winter conditions, but having said that, another primary reason we love these shirts is the amazing range of color options available: 26 different colors and patterns for women and 18 for men. Available at www.smartwool.com.

*Grams per square meter, a standard measurement for merino weight. Higher numbers mean thicker and heavier garments. Frequently these measurements are described as one of four categories: Ultralight (less than 150 g/m2), Lightweight (155 to 190 g/m2), Midweight (195 to 250 g/m2), and Heavyweight (greater than 250 g/m2), but there is some inconsistency among companies with these categories.