Table of contents
A base layer is an essential component of a layering system, tasked with providing cozy warmth and wicking moisture away from the skin. The primary differentiator between models is the material—typically merino wool, polyester, or a blend of the two. Below, we break down the full range of base layers in 2026 that have all been tested, reviewed, rated, and photographed by our team. As you compare options, be sure to use our sort tool to rank models based on your priorities, whether that’s warmth, breathability, durability, comfort, odor resistance, weight, or price.
Base Layer Superlatives
Smartwool Intraknit Thermal Merino ($150)
Smartwool Classic Thermal Merino 1/4 Zip ($130)
REI Co-op Midweight ($55)
Ridge Merino Aspect ($80)
Outdoor Research Echo Hoodie ($85)
Patagonia Capilene Thermal Weight Zip-Neck ($115)
Kari Traa Rose Light Half Zip ($140)
Our Base Layer Picks
With a merino-polyester blend and mesh ventilation, Smartwool’s Intraknit Thermal is ideal for fast-paced activities in cold weather. It’s one of the only merino base layers we’ll wear when we plan to sweat. However, the price is high, and not everyone will love the skin-tight fit.
With a merino-polyester blend and mesh ventilation, Smartwool’s Intraknit Thermal is ideal for fast-paced activities in cold weather. It’s one of the only merino base layers we’ll wear when we plan to sweat. However, the price is high, and not everyone will love the skin-tight fit.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
53% merino, 45% polyester, 2% elastane
Fabric weight
200 g/m²
Weight
7.3 oz.
Variations
Crew, quarter-zip
Pros
Cons
One of our primary gripes about merino base layers is that they’re too warm for rigorous activities and don’t handle sweat well (they grow bogged down with moisture and are slow to dry). Not so with Smartwool’s Intraknit Thermal, thanks to its high polyester content and mesh ventilation throughout. The added tech really works: The Intraknit Thermal is highly breathable and quick to dry, making it one of the only merino base layers we’ll reach for when heading out for sweat-inducing winter sports like running and cross-country skiing.
Another benefit of Intraknit’s construction (53% merino, 45% polyester, 2% elastane) is that it's noticeably more durable than base layers with a higher merino content. Our crew neck has yet to form thin spots or holes despite heavy use beneath a fanny pack and backpack.
The Intraknit Thermal isn’t as warm as Smartwool’s Classic Thermal, but our tester nevertheless thought it was an ideal base layer for backcountry skiing on cold days when paired with a light synthetic puffy and an uninsulated shell. The stretchy base layer fits very snugly—not everyone’s favorite style (we prefer more relaxed-fitting layers), but great for moisture-wicking and layering underneath a streamlined jacket. And thankfully, the Intraknit Thermal is longer at the back and stays tucked under ski pants. For milder or colder conditions (respectively), Smartwool also offers the Intraknit Active (52% merino, 21% lyocell, 21% polyester, 6% elastane) and Intraknit Thermal Max, which features 100% merino with mesh vents. For more, read our review of the Smartwool Intraknit Thermal.
Smartwool's Classic Thermal is an exceptionally warm, 100% merino base layer for winter conditions. It's soft, cozy, and odor resistant. Given its heavyweight construction, it can quickly become too warm; because of this, we only recommend it for low-output activities in cold weather.
Smartwool's Classic Thermal is an exceptionally warm, 100% merino base layer for winter conditions. It's soft, cozy, and odor resistant. Given its heavyweight construction, it can quickly become too warm; because of this, we only recommend it for low-output activities in cold weather.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% merino wool
Fabric weight
250 g/m²
Weight
10.5 oz.
Variations
Crew, quarter-zip, hoodie
Pros
Cons
Smartwool is a big name in merino wool, and the Classic Thermal is their warmest base layer offering. Built with 100% merino, the Classic Thermal offers all the benefits you’d expect from this miracle wool: it packs in a ton of warmth for its weight, has a silky-soft hand feel, and doesn’t hold onto body odors (one of our main gripes with synthetic materials). It's also among the warmest base layers we’ve tested; with 250 grams per square meter of merino, it’s ideal for cold winter days at the resort or in the hills. Smartwool offers this heavyweight base layer in crew, hoodie, and quarter-zip versions, and we prefer the latter—it’s helpful to be able to open a vent with this level of warmth.
But buyer beware: A thick merino base layer like the Classic Thermal can quickly go from warm to hot, especially if you’re adventuring in mild temperatures or generating a lot of body heat. Overheating is decidedly not a fun feeling, and to make matters worse, merino doesn’t dry out quickly. Don’t get us wrong, the Classic Thermal is a fantastic layer and is really well made—but we only recommend it for cold conditions and low-output activities. If you plan to sweat in single-digit temps, the ventilated Smartwool Intraknit Thermal Max or polyester-blended Intaknit Thermal are better options. For more, read our review of the Smartwool Classic Thermal Merino.
The Co-op’s Midweight is an affordable, no-frills base layer that’s built to last. But while it gets the job done for casual users, it’s not a standout for comfort, breathability, or warmth.
The Co-op’s Midweight is an affordable, no-frills base layer that’s built to last. But while it gets the job done for casual users, it’s not a standout for comfort, breathability, or warmth.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
92% polyester, 8% spandex
Fabric weight
220 g/m²
Weight
9.7 oz.
Variations
Half-zip
Pros
Cons
REI is best known as a retailer of outdoor goods, but it also has a strong lineup of in-house products that offer consistently good value. At just $55, their Midweight Long-Sleeve is one of the most affordable base layers on this list, yet it still delivers excellent performance for resort skiing, snowshoeing, and other winter uses. Its simple design—a plain crew neck with thumb loops—is at its best when worn as part of a layering system. That said, it’s classy and casual-looking enough that you could also pair it with your favorite jeans.
Despite its 220-gram fabric weight, the REI Midweight offers about average warmth for a midweight base layer and doesn’t compete with merino offerings of the same weight (even the 190-g Arc’teryx Rho is much warmer). But the stretchy polyester feels substantial, and the generous length makes the top easy to tuck in (although some users complain that the shirt is too trim at the hips). Those with sensitive skin might find the fabric ever so slightly scratchy—it’s certainly not as soft as synthetic options like the Arc’teryx Rho or Patagonia Capilene Thermal. On the bright side, the Midweight is highly resistant to abrasion and pilling. For a deeper dive, read the REI Co-op Midweight review.
The Aspect is a high-quality merino base layer that’s more affordable than much of the competition. Well-designed thumb loops and a drop hem contribute to a classy vibe, and the addition of nylon helps with durability, structure, and dry time. However, inventory can be spotty.
The Aspect is a high-quality merino base layer that’s more affordable than much of the competition. Well-designed thumb loops and a drop hem contribute to a classy vibe, and the addition of nylon helps with durability, structure, and dry time. However, inventory can be spotty.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
84% merino, 16% nylon
Fabric weight
180 g/m²
Weight
8.3 oz.
Variations
Quarter zip, turtleneck (w), hoody (m)
Pros
Cons
Ridge Merino is a great one-stop shop for merino clothing, offering consistently lower prices than big-name brands like Smartwool and Icebreaker. Their Aspect collection is home to midweight tops, bottoms, gloves, hats, and more, each featuring the same 180-gram merino-nylon blend. This combination of materials offers the best of both worlds: You get the warmth and odor resistance of merino (plus some, albeit limited, temperature regulation and moisture-wicking), while the nylon boosts durability and adds structure. It also helps speed up the Aspect’s dry time if you sweat out your base layer, which is one of the shortcomings of merino wool.
The Aspect Midweight offers just-right warmth for mild to cool winter conditions, and it doesn’t cause overheating like we’ve experienced with heavyweight layers like the Smartwool Classic Thermal. However, for warmer temperatures or big climbs on the skin track, the predominantly merino build might still be a little too much. Our favorite use cases for the Aspect are resort skiing and wearing it around our house in the winter—it’s just that soft, and the long cut and discrete thumb loops feel chic and thoughtfully designed. We’re partial to the women’s High Neck version, and men can find extra coverage with the Balaclava Hood option. For a deeper dive, read our review of the Ridge Merino Aspect.
A lightweight and airy base layer that doubles as a sun hoody, the Echo is a great next-to-skin layer for activities that prioritize breathability and moisture management above warmth. But it’s not as soft as merino, and the material is prone to snagging and pilling.
A lightweight and airy base layer that doubles as a sun hoody, the Echo is a great next-to-skin layer for activities that prioritize breathability and moisture management above warmth. But it’s not as soft as merino, and the material is prone to snagging and pilling.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% polyester
Fabric weight
Unavailable
Weight
5.2 oz.
Variations
Crew, quarter-zip
Pros
Cons
If you were to stand at any trailhead and poll passing hikers about their favorite base layer, chances are the Outdoor Research Echo Hoodie would be mentioned more than most. The Echo has achieved iconic status due to its lightweight construction, which provides a flowy fit alongside excellent breathability and moisture management.
We’ve never reached for the Echo with warmth in mind—instead, it’s a thin-as-can-be barrier between the skin and the midlayer or shell, which is great for comfort and breathability. Thanks to its UPF 15 fabric, it also doubles excellently as a sun protection shirt. Regardless of the season, the Echo is an excellent match for mild to warm temperatures and heart-pumping activities.
At $75, the Echo is more affordable than most base layers, but for good reason—it’s a simple and thin design with nothing to write home about in terms of tech or features. As with many Outdoor Research products, its build quality is average, and the fabric is prone to pilling or forming snags with minimal effort. However, it’s a versatile, lightweight option, and even manages to (mostly) keep odors at bay with an ActiveFresh treatment. If the hoody isn’t your jam, check out the other Echo styles, which include a tank, short-sleeve, long-sleeve, and quarter-zip.
Don’t trust the name: This base layer provides light to midweight warmth and has a massive range to keep you comfortable whether you’re moving or at rest. However, it’s on the expensive side for a synthetic base layer and can’t match the warmth of merino.
Don’t trust the name: This base layer provides light to midweight warmth and has a massive range to keep you comfortable whether you’re moving or at rest. However, it’s on the expensive side for a synthetic base layer and can’t match the warmth of merino.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
92% polyester, 8% spandex
Fabric weight
129 g/m²
Weight
6.2 oz.
Variations
Crew
Pros
Cons
Despite “thermal” being in its name, Patagonia’s Capilene Thermal Weight is one of our favorite midweight base layers, and it’s almost thin enough to find its way into our lightweight category. However, the level of warmth it provides far exceeds its weight. We love the Capilene for hiking, resort and backcountry skiing, and climbing—activities when we need a bit of added warmth but want our base layer to breathe and manage moisture well, too. Our most memorable day out in the Capilene Thermal Weight was on an early-summer climb of Washington’s Mt. Stuart, during which we wore the top all day— over a dozen miles of hiking and 2,500 feet of climbing—without once overheating.
We’ve put a lot of days into our Capilene, and it’s fared well despite heavy use, with just a few patches of pills at the cuffs. Of course, you don’t get the unbeatable warmth and coziness of merino, but the Capilene Thermal Weight’s waffled polyester is still very soft and has an airy rather than suffocating feel. What’s more, the HeiQ Pure odor control actually works, so you don’t have to worry about stinking up your polyester. For more, see our review of the Patagonia Capilene Thermal Weight.
Designed specifically for women, the all-merino Rose Light offers just-right warmth for resort skiing and other low-output winter activities. Unfortunately, it does not come in a men’s design.
Designed specifically for women, the all-merino Rose Light offers just-right warmth for resort skiing and other low-output winter activities. Unfortunately, it does not come in a men’s design.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% merino wool
Fabric weight
230 g/m²
Weight
8.5 oz.
Variations
Crew
Pros
Cons
Kari Traa is a women’s-specific apparel brand, founded by three-time Olympic medalist Kari Traa. Its designs are decidedly feminine and crafted for female bodies, but anyone is welcome to partake. After testing a few Kari Traa base layers, we think the Rose Light strikes the best balance for most winter enthusiasts (and particularly resort skiers). The midweight all-merino layer is neither too thick nor too thin for changing mountain conditions. Our tester even took it for a spin in the backcountry, using the quarter-zip as a vent to dump heat on the climb. For context, we’ve also tested the standard Rose base layer, which—despite being advertised as only 20 g/m² heavier than the Rose Light—is too warm for most conditions.
Kari Traa’s offerings have a classy Nordic feel that look just as good at après as they do on the slopes. And they’re well built, too: The Rose Light’s zipper is subtle and so low-profile it’s almost imperceptible, and all of the stitching is clean and low-bulk (we do wish there were thumbholes). Like all 100% merino base layers, the Rose Light is exceptionally warm but takes longer to dry than synthetic alternatives, which makes it less suitable for sweat-inducing activities like cross-country skiing and winter running. But for an every-day-of-the-season resort base layer that’s stylish and cozy, it’s one of our favorite options. For a deeper dive, read our Kari Traa Rose Light review.
A smooth exterior, brushed fleece interior, and highly stretchy design make the Rho LT more comfortable than most synthetic base layers. It also excels in wicking moisture away from the skin and dries quickly. However, the cozy fleece is hot in mild conditions.
A smooth exterior, brushed fleece interior, and highly stretchy design make the Rho LT more comfortable than most synthetic base layers. It also excels in wicking moisture away from the skin and dries quickly. However, the cozy fleece is hot in mild conditions.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
84% polyester, 16% elastane
Fabric weight
190 g/m²
Weight
7.9 oz.
Variations
Zip neck, hoody
Pros
Cons
We’re used to Arc’teryx throwing curve balls, but the Rho Crew Neck still caught us off guard with its innovative design. While many base layers follow a similar formula, the Rho charts a new course with its unique and highly elasticized fleece, which features a brushed interior and smooth face. The exterior resists abrasion and even sheds precip, while the interior is super soft and brilliantly moisture-wicking. Merrow seams keep pace with the stretchy fabric and allow for great freedom of movement, and the fit strikes an ideal not-too-tight, not-too-loose balance. Added up, the Rho is a really fun layer to wear, whether underneath a shell or on its own.
We commonly wear the Rho while ski touring and cross-country skiing in the dead of winter. The base layer is adept at shedding heat and—unlike merino alternatives—doesn’t get bogged down with sweat. To address the issue of odor build-up, Arc’teryx added a Polygiene treatment, which actually works. But the Rho will be too hot in spring-like conditions, especially if you’re working hard; in this case, you might want to reach for a base layer with mapped ventilation (like the Smartwool Intraknit Active) or a lighter-weight design. That said, the Rho is nevertheless exceptional for a range of conditions and activities, and is one of the base layers we reach for most. For more, read our in-depth review of the Arc'teryx Rho.
A wonderful option for high-output activities, the Summit Series Pro 120 is lightweight, breathable, and quick-drying. It also offers significant warmth for its thin construction, making it versatile for a range of temperatures and activity levels. However, not everyone will love the skin-tight fit.
A wonderful option for high-output activities, the Summit Series Pro 120 is lightweight, breathable, and quick-drying. It also offers significant warmth for its thin construction, making it versatile for a range of temperatures and activity levels. However, not everyone will love the skin-tight fit.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% polyester
Fabric weight
120 g/m²
Weight
6.7 oz.
Variations
None
Pros
Cons
Let us introduce you to one of our favorite synthetic base layers for high-output use, The North Face’s Summit Pro 120. This super lightweight polyester top (the women’s small checks in at just 4 oz.) offers much more warmth than you’d expect from its thin build. It also breathes exceptionally well. TNF accomplished this feat with a proprietary DotKnit fabric that features small perforations through which air can sneak through; it also helps that the Summit Pro has a skin-tight fit with no room for air to stick around. That said, do keep in mind that most users will prefer to size up.
The Summit Pro is a prime example of why we’re not completely partial to merino base layers, especially for rigorous activities in mild temps. We wore the long-sleeve (often without a layer overtop) throughout many days of traversing the North Cascades in early summer conditions. It was comfortable in a wide range of temperatures and output levels, wicked sweat away from our skin, and quickly dried out. In the same situation, a merino base layer would have developed a collection of holes and abrasions, overheated, and grown heavy with sweat. To learn more, read our review of The North Face Summit Series Pro 120.
The Oasis is a versatile merino base layer that offers ideal midweight warmth for most winter activities. It's a great option if Smartwool’s Classic offerings are too light or heavyweight for your needs.
The Oasis is a versatile merino base layer that offers ideal midweight warmth for most winter activities. It's a great option if Smartwool’s Classic offerings are too light or heavyweight for your needs.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% merino wool
Fabric weight
200 g/m²
Weight
7.6 oz.
Variations
Half-zip
Pros
Cons
Icebreaker and Smartwool compete as specialists in merino wool. The 200 Oasis falls into a similar category as Smartwool’s popular Classic All-Season and Classic Thermal—and in terms of weight, it falls squarely in between (200 g/m² compared to the Classic tops’ 150 and 250 g/m²). Considering our primary gripes with the Smartwool Classic lineup—we’ve found the All-Season to be too fragile and the Thermal too warm—the 200-gram Oasis is an ideal alternative for most winter scenarios, especially for those seeking pure merino. It’s very well constructed, too—Icebreaker is known for its high-end build quality, which includes flatlock seams, gusseted underarms, and very soft wool. Moreover, our tester appreciated its fit and drop-tail hem.
However, the weight and pure merino composition of the 200 Oasis isn’t ideal for warmer weather or strenuous activities. It also shows its wear faster than synthetic materials, and the price is relatively high. But if you’re looking for a base layer in that sweet spot between the Smartwool Classic All-Season and Thermal, Icebreaker’s best-selling base layer could be worth it for its versatility. For more, read our review of the Icebreaker Merino 200 Oasis.
The Rock'N'Wool is an all-merino base layer with a very playful style. The finely woven merino is very soft, provides exceptional warmth, and effectively resists odor build-up—but will need to be babied.
The Rock'N'Wool is an all-merino base layer with a very playful style. The finely woven merino is very soft, provides exceptional warmth, and effectively resists odor build-up—but will need to be babied.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% merino wool
Fabric weight
185 g/m²
Weight
6.7 oz.
Variations
None
Pros
Cons
Ortovox’s 185 Rock’N’Wool is one of the most eye-catching base layers around, mixing 100% merino wool with a playful asymmetric style and fun colorways. However, despite its appearance, the 185 Rock’N’Wool has a fairly basic design, featuring a no-frills crew neck and finely woven merino wool that feels particularly high-quality. Our tester had sworn off merino after a scratchy experience with an REI base layer, and was blown away by the smooth and “kind” feel of the Ortovox piece. For winter-ready warmth that’s best worn under a midlayer or shell jacket, it’s well worth consideration.
The primary downside to the 185 Merino Rock’N’Wool is price—at $110, it’s $10 more than Smartwool’s competing Classic All-Season. You do get a bit more warmth (185-gram vs. the All-Season’s 150-gram) and the all-natural feel of 100% merino, but we worry about the Rock’N’Wool’s ability to resist wear over time and don’t recommend it for use without a layer overtop. On the other hand, we were able to wear the base layer for multiple days of hard use without detecting any odor, which is remarkable. For a deeper dive, read our Ortovox 185 Merino Rock'N'Wool review.
The Lifa Merino features a two-piece construction that layers merino over top of polypro, resulting in an affordable cold-weather base layer for rigorous activity. However, the polypro isn’t cozy next to the skin and the merino outer is prone to wear.
The Lifa Merino features a two-piece construction that layers merino over top of polypro, resulting in an affordable cold-weather base layer for rigorous activity. However, the polypro isn’t cozy next to the skin and the merino outer is prone to wear.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
57% merino, 43% polypropylene
Fabric weight
Unavailable
Weight
9.7 oz.
Variations
Half zip, hoodie
Pros
Cons
Typically, base layers with both merino and synthetic content feature a blended construction that weaves the two fibers together, a technique that serves to increase the wool’s durability and moisture management. Helly Hansen’s Lifa Merino takes a different approach to mixing materials, instead layering merino on top of polypro in a two-piece construction. This style of “blending” isn’t as techy as what we see from other brands, such as Smartwool’s Intraknit series, but it nevertheless achieves a similar outcome. The Lifa Merino offers the warmth and breathability of wool alongside the moisture-wicking and fast dry time of polypro, making it a viable cold-weather base layer for rigorous activities like resort or backcountry skiing, running, and hiking.
But in contrast to other merino-synthetic base layers, its construction is not quite as functional. The next-to-skin polypro isn’t as soft as merino and retains odors, while the 100% merino outer layer is prone to wear. We’ve found ourselves wondering if Helly Hansen has the ordering wrong—shouldn’t the merino be next to the skin instead?—but even this scenario has its flaws (next to the skin, the merino would get bogged down with sweat). As it stands, the Lifa Merino has the same blatantly synthetic next-to-skin feel as Helly Hansen’s budget Lifa Stripe, but for $55 more. However, you do get the added warmth boost of merino, and the Lifa Merino is still more affordable than most performance blends.
Helly Hansen markets the Lifa Merino as “fitted,” but our tester wore his standard size and found it more of a regular fit. In his words, you “could wear it out in a pinch and it wouldn’t look like you just went cross-country skiing.” For more, read our Helly Hansen Lifa Merino review.
The Merino 185 is REI’s 100% merino base layer. The 185-gram weight is a nice middle ground between light and heavyweight alternatives. However, it’s not as soft as we’d prefer and won’t hold up to heavy use as well as merino-nylon blends.
The Merino 185 is REI’s 100% merino base layer. The 185-gram weight is a nice middle ground between light and heavyweight alternatives. However, it’s not as soft as we’d prefer and won’t hold up to heavy use as well as merino-nylon blends.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% merino wool
Fabric weight
185 g/m²
Weight
8.2 oz.
Variations
Half-zip
Pros
Cons
The REI Co-op Merino 185 is about as basic as it gets for a merino base layer. This crew neck (which is also available in half-zip and short-sleeve versions) features 100% merino in its purest form: no nylon blend, no elastane, and no ventilated mesh panels. The 185-gram weight slides nicely in between Smartwool’s standard offerings (150 and 250g) and is a great match for most winter days.
Compared to alternatives from Smartwool and Ridge Merino, REI’s merino is just a tad scratchier next to the skin. Due to its underarm gussets, it has more lines of stitching down the arms, which can result in extra pressure points and chafing. It’s also vulnerable to thinning and forming holes due to its pure merino build—but if you care for the layer gently, launder it carefully, and commit to wearing it under a mid or outer layer, it should last for a long time. Added up, the REI Merino is a solid low-cost entry point for a merino base layer. For a deeper dive, read our REI Co-op Merino 185 review.
The ZoneKnit is designed to breathe, with perforations that promote airflow when your body heat rises. However, the all-merino build absorbs sweat and is slow to dry, so the 200 ZoneKnit is not our first choice for high-output use.
The ZoneKnit is designed to breathe, with perforations that promote airflow when your body heat rises. However, the all-merino build absorbs sweat and is slow to dry, so the 200 ZoneKnit is not our first choice for high-output use.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% merino wool
Fabric weight
200 g/m²
Weight
8.8 oz.
Variations
Half zip
Pros
Cons
The 200 ZoneKnit is meant for action. This base layer takes the midweight warmth of Icebreaker’s 200-gram merino and puts a spin on it, adding perforations in key areas (at the back, under the arms, and on the lower half of the torso) for more airflow. The idea is that the ZoneKnit can dump heat more effectively than the Icebreaker 200 Oasis, without compromising on the benefits of a 100% merino build.
We think the 200 ZoneKnit is overall more versatile than the Icebreaker 200 Oasis, although it’ll cost you $45 more for the added breathability. However, despite its intentions, it’s not our first choice for active use. Merino’s biggest shortcoming for sweat-inducing activities is that it gets bogged down with moisture and doesn’t dry out quickly. What’s more, merino can go from warm and cozy to too hot in a hurry, especially if you’re generating a lot of body heat—and the 200 ZoneKnit’s vents only partially address this issue. If you want to stick with merino for high-output activities, we recommend Smartwool’s Intraknit series instead. These active-use base layers feature a significant amount of polyester, which is crucial for regulating temperature and moisture. To learn more, read our review of the Icebreaker 200 ZoneKnit.
The Lifa Stripe is an iconic layer from Helly Hansen for just $50. It’s all function here: The polypro material feels somewhat plasticky and isn’t particularly cozy, but it moves sweat away from the skin and dries quickly. Unfortunately, the fit isn’t refined, and there’s no odor-resistant technology.
The Lifa Stripe is an iconic layer from Helly Hansen for just $50. It’s all function here: The polypro material feels somewhat plasticky and isn’t particularly cozy, but it moves sweat away from the skin and dries quickly. Unfortunately, the fit isn’t refined, and there’s no odor-resistant technology.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
100% polypropylene
Fabric weight
Unavailable
Weight
5.3 oz.
Variations
None
Pros
Cons
If saving money is your top priority, check out the Helly Hansen Lifa Stripe. Helly Hansen has been making this base layer for as long as we can remember—growing up, we proudly wore a well-loved hand-me-down Lifa Stripe that was riddled with holes. Today’s updated Lifa Stripe is made with polypropylene—a synthetic material that, like polyester, efficiently directs sweat away from the body and dries quickly. Unlike most modern base layers, the Lifa Stripe doesn’t make much effort to feel particularly soft or cozy; in the words of our tester, it has that “classic synthetic polypro feel.”
The Lifa Stripe is a lightweight layer, but it fits the bill for mild conditions and those who run warm or plan to build a sweat. Be aware that it lacks odor-resistant treatment, whereas ours has accumulated a collection of smells that persist despite washing. In terms of fit, we found the collar to be a bit tight and wished the hem were longer; however, we appreciated the slim, not-too-snug style. It’s also worth noting that for $15 more, Helly Hansen makes the Lifa Active Stripe, which adds a second layer of polyester to the outside for even better moisture management. For a deeper dive, read our review of the Helly Hansen Lifa Stripe.
Compare Base Layers
With a merino-polyester blend and mesh ventilation, Smartwool’s Intraknit Thermal is ideal for fast-paced activities in cold weather. It’s one of the only merino base layers we’ll wear when we plan to sweat. However, the price is high, and not everyone will love the skin-tight fit.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
The Aspect is a high-quality merino base layer that’s more affordable than much of the competition. Well-designed thumb loops and a drop hem contribute to a classy vibe, and the addition of nylon helps with durability, structure, and dry time. However, inventory can be spotty.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Don’t trust the name: This base layer provides light to midweight warmth and has a massive range to keep you comfortable whether you’re moving or at rest. However, it’s on the expensive side for a synthetic base layer and can’t match the warmth of merino.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
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The base layers above represent our small group of selects, but we’ve researched and tested a much larger pool of products. You can view all the base layers we've reviewed via the link below.
Our Testing and Team
With more than four decades of outdoor gear review experience among our founding team and an ever-growing crew of testers with diverse backgrounds and equally deep knowledge, you can trust the authority behind our reviews. Our base layer article is especially fun, as we tapped our entire testing team—from backpackers and hikers to trail runners, skiers, and climbers—to get a wide range of input across conditions and output levels. The result is a thoughtful guide featuring our top recommendations, spanning the full spectrum of use cases. But they’re just a snapshot of the products we’ve tested: we’ve fully reviewed 18 base layers thus far, and our team’s collective testing experience spans thousands of hours of wear and well over 50 products.
Highlights from Better Trail’s base layer testing:
- Our Leavenworth, Washington-based tester Matt Leitzinger put the Ortovox Rock’N’Wool through its paces over the course of a full ski season, including a single-day push to ski the dwindling East Cascade Glacier on Bigalow Peak in the Chelan-Sawtooth Range.
- Ashley McNeish alternated between the REI Co-op Midweight and Merino 185 during a 40-plus-day season at Mt. Bachelor, dialing in the nuances between synthetic and merino performance.
- Our co-founder Jenny Abegg wore The North Face Summit Series Pro 120 on a seven-day traverse of the North Cascades. She spilled Heather’s Choice salmon chowder on it the first night—thankfully, the local bears didn’t take notice.
- Jenny also rotated between the Smartwool Intraknit Thermal and TNF Summit Series Pro 120 during a full season of nordic skiing near her home in Leavenworth, working through what materials perform best during high-output efforts.
- The Outdoor Research Echo has quietly built a resume across our team, from the Enchantments Traverse in Washington to the Huemul Circuit in Patagonia.
Our Ratings Explained
Our overall rating considers the product’s complete picture, encompassing price, build quality, performance (based on intended use), durability, sustainability, and more. At the high end of the rating scale are standout base layers that offer something special, whether they’re the crème de la crème or budget designs that provide a savvy combination of features for the price. If you ask us, readers should look for a base layer with an above-average overall rating—there’s simply little reason not to. That said, base layers with lower overall ratings are still notable enough to have made our list; for various reasons, these might be compelling to users with specific design preferences or those who get out only a few times a year.
Where you aim to land on our warmth scale will depend on your intended use. Ultra-warm heavyweight base layers, such as the Smartwool Classic Thermal, are a great match for resort skiers who venture out in cold conditions (roughly 20˚F or below) and maintain relatively low output—more often than not, these are merino base layers that are slow to dry and not particularly breathable. Midweight base layers, which include merino, synthetic, and blended varieties, offer the most versatile warmth for winter and shoulder season conditions. Finally, lightweight base layers—often made of polyester—are ideal for active use in cool weather or cutting the chill on a summer evening.
Mid-to-heavyweight merino base layers, ideal for resort skiing in temperatures around 20˚F or below. Fabric weights around 200 to 250 g/m².
Midweight base layers with merino, synthetic, or blended construction. Ideal for high-output use in cold conditions or lower-output use in temperatures up to around 40 degrees Fahrenheit. Fabric weights around 150 to 200 g/m².
Lightweight base layers for mild-to-warm conditions. Fabric weights below 150 g/m².
If you participate in an activity that substantially raises your heart rate (like winter running, cross-country skiing, and ski touring), you’ll want a base layer that can breathe. Base layers with an above-average breathability rating effectively remove heat, wick moisture from the skin, and don’t get bogged down with sweat. In general, high breathability correlates with synthetic materials, which aren't as insulating as merino (meaning you’re less likely to overheat) and are quicker to dry, although there are a few partial-merino outliers with thoughtfully designed ventilation. On the lower end of our breathability scale are 100% merino base layers. These offer great warmth for low-output activities like groomer skiing and winter camping, but they’ll quickly become too hot if your body temperature rises.
Highly breathable and ideal for warm weather or rigorous activity. These base layers are generally made of thin polyester or are vented merino blends.
Moderate breathability that gets the job done for resort skiing or other moderately sweat-inducing activities. Most all-merino base layers fit into this category.
Middling breathability due to thick merino construction. These should only be worn during low-output activities in cold temperatures.
The durability of a base layer is largely dependent on its materials. There’s a lot to love about merino wool, but it’s also relatively fragile: If not mixed with synthetic materials, it will grow thin and become riddled with holes and pills over time. Thin merino wool is the worst culprit—some of our merino hiking shirts have even amassed large thin spots and holes from a backpack rubbing at the shoulders. When mixed with a bit of nylon or polyester, merino has a better chance of success. And finally, synthetic base layers are among the most durable, especially those with abrasion-resistant faces to complement their soft interiors. However, keep in mind that no base layer can compete with a midlayer or outer layer in terms of durability—it was not designed for heavy external use.
Suppose you only wear your base layer under a midlayer or ski jacket. In that case, you can get away with products that score low in our durability metric. Fabrics (especially merino) will have a much longer lifespan when consistently protected from rubbing, abrasion, and sharp objects. On the other hand, some people prefer to wear their base layer on its own, such as a long-sleeve hiking shirt, to ward off the chill while climbing, or around the cabin after a day of skiing. If this is you, consider a base layer with a higher durability rating.
Very durable and can handle consistent abrasion. Great options for standalone use. Primarily synthetic base layers.
Thick merino and merino-synthetic blends. Can be worn on their own but are vulnerable to thinning out or forming holes. Wear primarily as a layering piece.
Thin merino with no synthetic content; should be predominantly paired with a mid or outer layer. Very likely to form thin spots or holes.
“Comfort” is a fairly straightforward term, but we factor in a slight nuance when rating it. To start with the obvious: How does the base layer feel against your skin? Merino is known for its soft hand feel and is generally quite a bit cozier than synthetic alternatives, which can have some “bite” to them. However, some varieties of merino are itchier than others. Next, how does the base layer feel while moving vigorously? For active use, we’re a big fan of base layers with some extra stretch, and features like gusseted underarms and drop-tail hems can also contribute to comfort while moving.
Comfort can closely correlate with getting a proper fit, so we’re sure to call out our fit impressions where applicable. Most base layers are given a “trim” or “slim” designation, but in our experience, they still run the gamut from skin-tight to loose and flowy. For more on fit, see our section on fit and sizing below.
Cozy comfort with a soft next-to-skin feel and unhindered mobility. Usually thick merino.
Middling comfort and/or fit, generally due to the scratchy, catchy feel of polyester.
Scratchy or snaggy fabric and poor fit.
If you’ve ever worn an old polyester base layer on a week-long backpacking trip, you’ll know why we’re calling out odor resistance as an important purchasing consideration. One of the attributes of synthetic materials is that they retain odors—just an hour of hard work can make a synthetic base layer smell like a gym locker room, and it may persist despite thorough washings. In contrast, merino is almost completely effective in resisting odors, even throughout days of use (think of it like a cat: it’s practically self-cleaning). However, the gap between the two types of materials is closing thanks to effective odor-resistant treatments such as Polygiene, HeiQ Fresh, and ActiveFresh.
If we’re able to wash our base layer in between uses (for example, after a day trip to the local ski hill), we have no problem opting for a design with a lower odor-resistance rating. However, if we’re heading out for a long backpacking trip or a week-long hut trip with no access to laundry, we want a base layer on the high end of the odor-resistance scale. In fact, in these scenarios, we’ll reach for a merino base layer ten times out of ten.
A merino base layer that resists all odor build-up. You could wear this base layer all season and it still won’t stink.
Holds onto stink but not between washings. Most 100% synthetic base layers fit into this category.
Starts to smell immediately when you put it on, even between washings.
Our sustainability rating for base layers considers seven distinct criteria. At the product level, we’re looking for recycled materials, Bluesign-approved materials, responsibly sourced wool (if applicable), and Fair Trade Certified production. On the brand level, our criteria include whether the company has a dedicated resale or recycling program, uses recycled or reduced packaging, monitors greenhouse gas emissions, and publishes an annual impact report. We also consider the degree to which our criteria are met; for example, 10% recycled materials is not the same as 100% recycled materials. Note that not every sustainability effort by a brand or product fits neatly into our categories. In these circumstances, we may adjust a rating based on these factors. You can find details on these factors in the sustainability drop-down under each product above.
These base layers are among the most sustainable on the market and earn our Better Trail Certified distinction. They often meet most of our sustainability criteria for this category.
These base layers are a good, middle-of-the-pack option for sustainability. They often meet half or more of our criteria for this category.
These base layers are the least sustainable and have significant room for improvement. They usually only meet about a quarter or less of our criteria.
Anatomy of a Base Layer
Base layers are simple pieces of clothing, generally composed of one type of material (or material blend) and sewn together with flatlock seams that minimize pressure points and chafing. Manufacturers will also typically offset the shoulder seams to the front or back to keep them from causing pressure underneath a pack’s shoulder straps.
Base layers are generally composed of merino wool, a synthetic material like nylon or polyester, other natural fibers like lyocell or bamboo, or a combination of some or all of the above. Each type of material has its strengths and weaknesses, which we break down below.
Merino Wool
Merino wool is a popular material for base layers, socks, and underwear, and for good reason. You’ll know a base layer is merino the moment you put it on—it’s just that much softer than any synthetic material, and immediately insulating, too. We’ve been blown away by how much warmth this all-natural material provides, even in the thinner varieties. Merino wool also breathes decently well and wicks moisture away from the skin (although not as well as synthetic materials), which is precisely what you want from a base layer. Finally, it is miraculously odor-resistant. Merino base layers, including the Ortovox 185 Rock'N'Wool, simply don’t get stinky with body odor (we’ve tested this to its full extent), which is an incredible feature to have in a next-to-skin layer.
However, merino wool is not a super strong fiber and can wear down or form holes when subjected to rubbing, sharp objects, or heavy use. Lightweight merino is the worst culprit, but even heavyweight constructions have this issue. For this reason, manufacturers often blend merino with a synthetic material (generally nylon or polyester), which significantly increases abrasion resistance. This construction is on full display in the Smartwool Intraknit Thermal. What’s more, merino is not a great match for high-output use: It quickly becomes too hot, and it retains sweat rather than sluffing it away (this can become a real issue in cold temperatures). We love merino for slower-paced activities like resort skiing and winter camping, but synthetic base layers are generally a better fit for rigorous pursuits like winter running, nordic skiing, and ski touring.
Synthetics
Many base layers are made from synthetic materials, such as nylon and polyester. Synthetics are generally more affordable than merino—for example, the Helly Hansen Lifa Stripe will run you a mere $50, while a merino base layer like the Icebreaker 200 ZoneKnit is $150. They also breathe better than merino, don’t absorb as much moisture, and dry out very quickly. For this reason, they’re our top recommendation for sweat-inducing activities, mild conditions, or those who run warm. Finally, synthetics are quite durable and can handle abrasion and heavy use, making them a great fit for those who wear their base layers without a mid or outer layer overtop.
That said, there are a few notable downsides to synthetics. Nylon and polyester are nowhere near as cozy or warm as merino, and they also tend to absorb odors and retain them—even after a thorough washing, your synthetic base layer might still smell like you’ve been sweating in it for a week. Relatively new technologies like Polygiene, HeiQ Fresh, and ActiveFresh have made significant improvements to the odor resistance of synthetic materials, but they still can’t match the resistance of merino. For more on the differences between the two primary base layer fabrics, see our article Merino Wool vs. Synthetic Base Layers.
Other Natural Fibers
Merino isn’t the only natural fiber used in base layers; we’ve also started to see rayon and lyocell, most notably in offerings from Smartwool and Le Bent. These materials are incredibly soft and enjoyable to wear—both have a buttery soft, almost silky feel that you’ll be raving about to all your friends. Like merino, they’re also fairly breathable and highly odor-resistant. However, lyocell and rayon aren’t nearly as warm as wool; they also absorb moisture and don’t dry out particularly quickly. As a result, these natural fibers are only a good fit if you don’t need too much insulation (in mild conditions, for example) or if you don’t plan on sweating.
Blends
Manufacturers often blend merino with polyester or nylon to give you the best of both worlds: The warmth, softness, and odor resistance of merino alongside the fast dry time and durability of synthetics. And the proof is in the pudding: Our base layers with a merino-synthetic blend—including one of our favorites, the Smartwool Intraknit Thermal Merino—have lasted longer and managed sweat better than those with pure merino.
Similarly, we've been impressed with the durability, comfort, and quick-drying performance of base layers made with Nuyarn. This relatively new tech spins merino wool around a nylon core to maximize the benefits of the two materials. In early testing of products from the Artilect and Ibex, Nuyarn has lived up to its billing. However, the real evidence will come from how long they last—we'll report back with the results.
If you’re perusing the internet for base layers, you’ve likely noticed product names are often accompanied by a number. This is the fabric weight, and it can give you a good idea of the base layer’s warmth. Manufacturers specify fabric weight in terms of grams per square meter (g/m² or gsm)—in other words, the weight (in grams) of a 1-meter by 1-meter piece of fabric. Looking at the base layers in this article, weights can range from around 120 g/m² (for The North Face’s Summit Series Pro 120) to 250 g/m² (for the Smartwool Classic Thermal) or more.
However, fabric weight isn’t the perfect measurement of a garment’s warmth. The material matters, of course—gram for gram, merino is much warmer than nylon or polyester. Additionally, you’ll want to consider factors such as fit, added technology (manufacturers are constantly trying to make materials lighter yet warmer), and extra coverage like a hood or zip-neck.
Base layers are available in several styles, including crew neck, quarter-zip, and hoody. A crew neck is the most no-frills option, with little to no features (you might get thumb loops). This design is great for layering underneath a mid or outer layer, with minimal bulk and no pressure points.
Quarter-zip (or half-zip) styles feature a collar that extends partially up the neck (usually not quite as far as a turtleneck) and a zipper that extends to the sternum. This adds a bit of spice to the simple crew-neck look, while also providing more options for ventilation or warmth at the neck.
Finally, base layers with hoodies feature a tight-fitting scuba-style hood with a high neck and a small opening around the face. These are a great option for stormy days and frigid temperatures, especially given the extra coverage at the back of the neck and around the face. We love wearing a hooded base layer under our ski helmet while resort skiing or under our climbing helmet on cold days in the alpine.
The base layer is a simple piece of gear designed to be worn next to the skin and barely see the light of day. However, some features can come in handy if you do plan on wearing your base layer on its own (i.e., without a mid or outer layer overtop). Thumb loops are common: These keep the sleeves in place and provide extra warmth across the wrist and back of the hand—and most are so streamlined that you won’t know they’re there if you choose not to use them. Some base layers feature a chest pocket that can fit chapstick or a credit card (and not much more), including the men's Arc'teryx Rho LT. Finally, a scuba hood is a nice addition to protect the face and the back of the neck from the elements.
How Should I Size My Base Layer?
Base layers are trim-fitting by nature, which is important for layering purposes. In effect, they’re meant to act as an extension of your skin. However, there are nuances to be aware of with the “trim” or “slim” fit designations: Some base layers completely hug the body, while others hang more freely. You can discern the difference by looking at photos (both ours and those of the manufacturers) and paying attention to the fabric composition—if you see elastane or spandex, that’s one clue that the base layer may be skin-tight. If you don’t want a too snug fit, it can be a good idea to size up.
We generally prefer looser base layers if we’ll be wearing them without a layer overtop—while hiking, climbing, or at the ski lodge. They’re simply more comfortable and are more likely to stay tucked into our ski pants. On the other hand, snug-fitting base layers do a slightly better job of wicking moisture away from the skin; however, in our experience, the differences are barely perceptible. Both styles keep excess fabric to a minimum and are built to accommodate a snug midlayer overtop. Your final decision will come down to style and comfort.
Your Questions Answered
Broadly speaking, a layering system includes a base layer, a midlayer, and an outer layer. The base layer is the next-to-skin layer; its key functions are warmth, moisture-wicking, and providing a comfortable barrier between your skin and more technical-feeling fabric.
Midlayers are a broad category that includes fleece, down, and synthetic insulated jackets. This layer (or these layers, if you wear more than one) offers the most warmth. Fleece and synthetic jackets are our favorite choices for active use (they breathe well and aren’t too lofty to wear underneath a shell), while down is a great option for casual environments, maximizing warmth, or if you’re not wearing an outer layer.
Finally, the outer layer provides wind and moisture protection. Most outer layers are shell jackets, including ski shells, hardshells, and rain jackets. Some outer layers include built-in insulation (like an insulated ski jacket)—if this is the case, you might be able to drop the midlayer.
For more on this topic, read our article on How to Layer for Skiing.
You've come to the right place—as winter endurance sport enthusiasts, we have many opinions on this.
The best base layer for high-output use is one that breathes well, wicks sweat away from the skin, doesn’t grow heavy with moisture, and dries out quickly. Merino wool checks the first two boxes but falls short in the latter two, while synthetic materials like nylon and polyester excel at all of the above.
With that said, we strongly prefer synthetic base layers for sweat-inducing, blood-pumping activities in cold weather, including running, ski touring, and cross-country skiing. From experience, we love the Arc’teryx Rho for cold conditions, The North Face’s Summit Series Pro 120 for cool conditions, and the Outdoor Research Echo Hoodie for mild conditions. For more on this topic, check out our article When A Merino Wool Base Layer Isn't the Answer.
On our list above, the warmest base layer is the Smartwool Classic Thermal. The Classic Thermal is crafted from 100% merino, renowned for its exceptional warmth and cozy feel. The other factor to consider is the merino’s weight; in this case, the Classic Thermal uses 250-gram merino, which is heavyweight compared to most base layers (for example, the Smartwool Classic All-Season uses 150-gram merino and thus is not nearly as warm). If you want to go even warmer, consider a merino base layer with a heavier gram-weight, such as the 100% merino, 400-gram Woolx Glacier Long Sleeve Crew.
A lot of hype has been generated around merino wool, with phrases like “miracle material” and “nature’s gift” being tossed around. We understand the excitement: Merino is impressively warm for its weight, stays warm when wet, wicks moisture off the skin, and doesn’t hold onto stink. These factors make it an exceptional material for base layers, socks, and underwear, especially if you’re venturing out in the cold and planning to go a long time between washings. For cold days at the ski resort, traveling, and multi-day backpacking trips, merino is always our first choice.
But merino comes with its fair share of caveats. For one, you’ll need to exercise care when washing it; namely, merino wool needs to be air-dried, or it may shrink. It’s also far less durable than most synthetic materials and is prone to forming holes, especially in high-wear spots like the back (under a backpack) and cuffs. And if you’ve ever tried working out in merino, you’ll know that it’s not the best material to sweat in. As we’ve detailed above (see the question regarding high-output use), merino actually gets warmer when wet and doesn’t dry out particularly quickly. For high-output activities like running, ski touring, and cross-country skiing, we always reach for a synthetic base layer, or, at the very least, a merino-synthetic blend like the Smartwool Intraknit Thermal.
The base layers in our article range from $50 to $150—that’s a pretty sizeable spread. As with most things, you get what you pay for. The cheapest base layers are the least insulating, and the range starts with exclusively synthetic offerings. These base layers don’t provide a lot of insulation and are known to retain odors, which—while it doesn’t impact performance—is a notable downside in our opinion.
Around $75 is where we see our first merino base layer: the Ridge Merino Aspect. This is a great value and our pick for the best low-cost merino offering. The $75 to $100 range is where most folks should focus their research. Here, we find premium synthetic base layers, such as Arc’teryx Rho LT and The North Face’s Summit Series Pro 120, alongside simple midweight merino designs like the Icebreaker Merino 200 Oasis ($105) and REI Co-op Merino 185 ($80). If you want to treat yourself or anticipate needing a little extra warmth or performance, look at the top-tier offerings. Here, we find merino-synthetic blends like the Smartwool Intraknit Thermal and ultra-warm designs like the Smartwool Classic Thermal.
Yes—manufacturers like Smartwool recommend washing merino wool on a gentle cycle in cool or lukewarm water, using a mild detergent. When it’s time to dry, lay your base layer flat and let air do the work. You can also tumble-dry merino items on a low heat setting, but be aware that machine drying can shrink merino (especially when done at too high a temperature) and cause premature thinning and pilling, particularly with thinly woven varieties.
Before you wash your merino, it’s also worth asking if it actually needs a refresh. Merino’s odor-resistant qualities are impressive, allowing you the luxury of putting in more sessions between washings than you could with synthetic clothing. Washing your merino less can help extend the lifespan of the product.
Most of the time, manufacturers offer base layers in matching tops and bottoms. Base layer bottoms aren’t as essential as tops—keeping your core warm is simply more important than keeping your legs warm—but we always wear a pair while skiing, alpine climbing, or playing in the snow. If you plan to wear your base layer bottoms while skiing, consider a ¾-length pair like the Smartwool Classic Thermal Merino ¾-Length Bottom.
At a macro level, it’s a bit challenging to compare merino and synthetics categorically. Merino is derived from sheep, so, as with any animal product, it is accompanied by endless debate and a string of factors, including feed, land impact, ethical treatment of animals, and more. That said, several recent studies have shown that grazing sheep can lead to carbon sequestration, and, being a natural fiber, merino biodegrades relatively quickly. On the other hand, synthetic materials are merely another form of plastic, and their production can lead to greenhouse gas emissions and potentially hazardous work environments. The microfibers of nylon and polyester have also been found throughout the wastewater system and do not degrade naturally.
For both materials, there are a few bright spots, including responsibly sourced wool and recycled synthetic materials. If you choose to purchase a merino base layer, look for certifications like the Responsible Wool Standard and ZQ-certified wool. These certifications ensure the ethical treatment of sheep, as well as proper land management and the health and safety of workers (you can learn more in our article on Responsible Materials). If you choose to shop for synthetic base layers, keep an eye out for recycled materials, which promote the circularity of plastics, keeping items like plastic bottles and fishing nets out of our landfills and oceans.
Awesome question. Our sustainability team conducted a thorough analysis of this category, identifying seven criteria for synthetic base layers, six for merino, and eight for merino-blend (also known as hybrid) options—encompassing both product- and brand-level practices. Impressively, the Patagonia Capilene Thermal Weight and both REI Co-op base layers (the synthetic Midweight and the Merino 185) earned perfect 5/5 sustainability scores. Icebreaker was close behind, losing points only for its lack of Bluesign-certified materials, while the Ortovox Rock’N’Wool was docked for not participating in recycled or reduced-packaging initiatives. To learn more, use the Sort By tool above to filter by sustainability or click on individual products for a full breakdown of their sustainability reports.
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