Updated:
Jan 19, 2025Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
53% merino, 45% polyester, 2% elastane
Fabric weight
200 g/m²
Weight
6.35 oz.
Variations
Crew, quarter-zip
Pros
Cons
Smartwool Intraknit Thermal Merino
For this season's top models, see our guide to the Best Base Layers.
While many base layers are made of either 100% polyester or 100% merino wool, Smartwool’s Intraknit Thermal features a blended construction—53% merino wool, 45% polyester, and 2% elastane. The midweight material checks in at 200 grams per square meter, an above-average thickness among both merino and synthetic base layers. As a result, the Intranit Thermal offers a nice dose of warmth for cold winter days, although it’s not quite as insulating as 100% merino base layers, such as the Smartwool Classic Thermal (250 g/m²) or the Icebreaker 200 Oasis (200 g/m²). But on the other hand, it won’t overheat as much as these 100% merino offerings, thanks to its blended material and mesh ventilation. This makes it an ideal base layer for active use on cold winter days.
Last winter, I consistently reached for the Smartwool Intraknit Thermal for skate skiing in temperatures anywhere from 10 to 35 degrees Fahrenheit. I generally layered a lightweight synthetic jacket, such as the Arc’teryx Proton Lightweight, over it, or, in milder temperatures, simply wore a wind jacket. Admittedly, my layering system usually made for a “be bold, start cold” morning, but I always warmed up quickly once I began moving. Another Better Trail tester wore the Intraknit Thermal underneath a light synthetic puffy and shell while resort skiing; she admitted she was cold, but believed it would be ideal for uphill travel while backcountry skiing. For a slightly warmer version with similar mesh ventilation, check out the 100% merino Intraknit Thermal Max Hoody.
I tend to steer clear of merino base layers when I anticipate building a sweat: merino absorbs sweat, doesn’t dry quickly, and—through a chemical reaction known as sorption—actually heats up when wet. The combination of these factors leads to an overheating, slightly claustrophobic effect. But because the Intraknit contains 45% polyester, it breathes well, does not readily absorb sweat, and dries quickly. Furthermore, the body-mapped mesh ventilation enhances this breathability. While a 100% synthetic base layer will dry out faster and breathe better, the Intraknit is an ideal middle ground for cold days, making it the only merino layer I’ll reach for during rigorous, heart-pumping activities.
Merino is known for its fragile nature, so the Smartwool Intraknit Thermal stands out with its partial-polyester build. It can’t match the durability of many synthetic base layers, but it’ll have a longer life than 100% merino wool layers of similar weight. In fact, after two years of consistent winter use under a midlayer or shell, my Intraknit has just one hole. I wouldn't want to expose it to sharp rocks or equipment and thus never wear it on its own—and given its skin-tight, underwear-resembling fit, I don’t want to!
The Intraknit earns a high score in comfort owing to the instant coziness you’d expect with a merino layer. The skin-tight silhouette, however, isn’t for everyone; it was too tight for both my taste and that of our Better Trail tester. The compression resulted in a slightly suffocating sensation akin to a relentless, stuffy hug, which is contrasted with the airy freedom afforded by other base layers (such as the Arc'teryx Rho). Further, I’d frequently need to tug the hem down, as it would ride up around my waist. Luckily, the Intraknit is still stretchy enough to be unrestrictive while moving.
Merino is renowned for its exceptional odor resistance, attributed to its antimicrobial properties, the natural sheep wax known as lanolin, and its distinctive texture. Although the Intraknit is only 53% merino, it’s been highly stink-resistant even after prolonged activity. In fact, it’s just about on par with the 100% merino base layers I’ve worn. This makes it a great option for multi-day backpacking trips or hut trips, when you’ll be wearing the Intraknit for multiple days on end without laundering.
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Longevity
Choosing durable outdoor gear and keeping it in use for longer is one of the best ways to reduce environmental impact. Our proprietary longevity rating assesses factors like overall build quality, materials, fabric denier, component durability, and real-world performance. A green check indicates that we expect the product to be long-lasting relative to its peers, a yellow check mark indicates average longevity, and a red X indicates a product that may have a limited lifespan.
Responsible Wool
Responsible wool is most commonly certified by standards such as the Responsible Wool Standard (RWS) and ZQ Wool, which promote sustainable land management, animal welfare, and responsible farming practices, though some brands occasionally use other equivalent third-party certifications. A green check mark indicates a product uses responsibly sourced wool, a yellow check indicates the brand sources responsible wool for the majority of its products but does not specify it at the product level, and a red X indicates the product does not use responsibly sourced wool.
Recycled Materials
Recycled materials are prevalent in outdoor gear, with nylon, polyester, wool, and down among the most common, but the composition varies widely. A green check mark indicates the product is made with a substantial amount of recycled materials (100% recycled or the core fabric that makes up most of the product is recycled), a yellow check means it contains a moderate to small amount of recycled materials, while a red X means there are no recycled materials in the product.
Bluesign Approved
Bluesign Technologies, based in Switzerland, operates a third-party textile management system that ensures materials are manufactured to strict environmental, chemical, and worker safety standards. A green check mark indicates a product is either a Bluesign Product (contains at least 90% Bluesign-approved fabrics and 20-30% Bluesign-approved accessories) or features a significant amount of Bluesign-approved materials. A yellow check mark means it uses some Bluesign-approved materials but less than the aforementioned category, and a red X means there are no Bluesign-approved materials in the product.
Responsible Manufacturing
This criterion evaluates a brand’s commitment to fair wages, safe working conditions, and reducing environmental impact through certifications and programs like Fair Trade Certified, Fair Wear Foundation, Worldwide Responsible Accredited Production (WRAP), and the Fair Labor Association, while also recognizing brands that manufacture primarily in the U.S. or Europe under strict labor and environmental regulations. A green check mark indicates a brand meets our responsible manufacturing criteria, while a red X means it does not.
Recycled and/or Reduced Packaging
Packaging can add significant waste to outdoor gear purchases, so many brands work to reduce its impact by using recycled materials, incorporating Forest Stewardship Council-certified paper products, and minimizing plastic and paper use. A green check mark indicates a brand uses recycled or reduced materials across all of its packaging, a yellow check indicates moderate or limited use of recycled or reduced packaging, and a red X indicates the brand does not make either of these efforts.
Resale and/or Recycling Services
Resale and recycling programs help keep outdoor gear in circulation longer and out of landfills, ranging from trade-in resale platforms that offer store credit to take-back initiatives that recycle products at the end of their usable life. A green check mark indicates a brand offers both resale and recycling programs, a yellow check mark indicates it offers one or the two, and a red X indicates it offers neither.
Carbon Footprint Tracking
This criterion evaluates whether a brand measures, reports, and works to reduce its greenhouse gas emissions using established frameworks such as the Higg Index, Greenhouse Gas Protocol, or The Change Climate Project. A green check mark indicates a brand publicly reports greenhouse gas emissions data, sets clearly defined reduction targets, and uses established tracking frameworks such as Higg or The Change Climate Project. A yellow check mark indicates the brand tracks emissions and outlines reduction goals but provides limited data, lacks science-based verification, or does not clearly report progress. A red X indicates the brand does not appear to track greenhouse gas emissions or provides so little information that its efforts cannot be verified.
Annual Impact Report
Annual impact reports provide transparency and accountability by outlining a brand’s sustainability efforts across areas such as material sourcing, greenhouse gas emissions, waste, water use, supply chains, and packaging. A green check mark indicates a brand publishes a detailed, brand-specific impact report that closely aligns with our sustainability criteria and leaves little room for ambiguity. A yellow check indicates a brand provides some relevant sustainability reporting but lacks detail in key areas or is covered only briefly within a parent company report. A red X indicates a brand does not regularly publish an impact report.
Tester: 5’6” 140 lb.; 36" chest; 29" waist
Size tested: Women’s medium
At 5'5", I tested a women’s small in the Smartwool Intraknit Thermal, which is my usual size. I found it markedly tight, as did our Better Trail tester, who’s an inch taller and wore the medium. While neither of us loved the constrictive fit, it was ideal for tucking under wind jackets or lightly insulated jackets. The length in the arms is fine, but we disagreed over the functionality of the hem length; I wished it were longer and often found myself pulling it down, but our tester found it sufficient, noting she liked that it was longer at the back and that the layer stayed tucked.
Icebreaker 200 ZoneKnit ($150): All-Merino for Active Use
Like the Intraknit Thermal, Icebreaker’s 200 ZoneKnit is designed for high-output activity. But in contrast to the Intraknit’s polyester blend, it’s made with 100% merino and features perforations at the back, lower torso, and under the arms. Those in search of an all-natural solution for rigorous activities will appreciate the ZoneKnit’s extra venting, but its all-merino build falls short of the Intraknit’s blend in terms of breathability, dry time, and durability. If you plan to build heat and sweat, the Intraknit is the better pick. To learn more, read our Icebreaker 200 ZoneKnit review.
Arc'teryx Rho ($100): High-Quality Polyester
Merino is deservedly popular, but synthetics like polyester are arguably better for sweat-inducing activities thanks to their superior breathability and quick dry time. We used the Arc’teryx Rho (men's: Rho LT), made from 84% polyester and 16% elastane, alongside the Intraknit Thermal for a season of cross-country skiing. The Rho is noticeably softer, has a more relaxed fit that will appeal to a broader audience, and doesn’t absorb sweat (the Intraknit gets wet with sweat, but nowhere near to the extent of an all-merino base layer). It’s also $30 cheaper and has a premium build quality. To be clear, the Rho is not designed for high-output use; yet, we regularly reach for it over the Intraknit Thermal for rigorous activities in cold temperatures. For more, read our in-depth review of the Arc'teryx Rho.
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 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
A smooth exterior, brushed fleece interior, and highly stretchy design make the Rho 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
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. Not everyone will love the snug fit, but it's perfect for layering under a jacket and heading for the backcountry. This is one of the priciest base layers on the market, but you get what you pay for.
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