The Arc’teryx Rho Merino Crew Neck LS ($120) is a warm, exceptionally comfortable merino base layer that performs best in cold conditions and at moderate output. With its plush feel, chill-stopping warmth, and high odor resistance, it’s one of the best winter-weight base layers we’ve tested. It balances insulation and breathability well, and though the fit might run a bit relaxed for some, it’s a standout for versatile cold-weather layering.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
Fabric(s)
95% merino wool, 5% elastane + nylon
Fabric weight
240 g/m²
Weight
10.2 oz.
Variations
Crew, zip-neck
Pros
Cons
Arc’teryx Rho Merino Crew Neck LS
For this season's top models, see our guide to the Best Base Layers.
The Arc’teryx Rho Merino Crew Neck LS has become my go-to base layer when the temperature drops. I’ve worn it consistently over the past five months for shoulder-season trail running, winter hiking, and resort skiing, and it shines in cold conditions. The Rho Merino worked well as a standalone layer for trail runs with temperatures in the high 40s and low 50s, and it’s equally at home as a ski base layer under a midlayer and a shell when things get really cold—down into the 20s and below.
I used it extensively in the Alps in late fall, including hiking in snowy conditions around Chamonix (layered under a Black Diamond Access Down Hoody), and early-season ski laps on the Zermatt glacier with a wool midlayer on top. Compared to synthetic base layers like the Patagonia Capilene Thermal Weight, the Rho feels noticeably warmer and cozier. In terms of merino competitors, it’s slightly less warm than the Smartwool Classic Thermal (250 g/m²), but it offers better insulation than lighter options such as the Ortovox Rock’N’Wool (185 g/m²) or the Smartwool All-Season (150 g/m²). For use across late fall and all of winter, it’s right in the sweet spot.
Despite its warmth, the Rho Merino manages moisture impressively well. I’ve sweated heavily in this layer while trail running, resort skiing, and hiking, and it consistently wicked moisture away from my skin without ever feeling clammy. That said, it’s not the base layer I’d choose for high-output ski touring or shoulder-season efforts; it’s simply too warm, and—unlike synthetic materials—merino doesn’t dry quickly once wet. All told, the Rho Merino is a cold-weather, moderate-output specialist.
If you use the Rho in the right conditions, though, it rewards your selection. One standout moment came during a vertical kilometer run in Chamonix. I was pushing hard uphill and sweating heavily, then transitioned into much colder, snowy, and windy conditions at the summit. After throwing on a wind shell, I stayed warm on the descent despite how much I’d sweated on the way up. That kind of stay-warm moisture management is where this layer really excels.
After roughly five months and 40 days of use, the Rho Merino is holding up exceptionally well. Its main fabric makeup is 95% merino wool and 5% elastane. Merino is not a particularly durable fiber, but Arc’teryx reinforces it with a nylon core, which gives it a noticeable durability advantage over 100% merino base layers like the Ortovox Rock’N’Wool or the Smartwool Classic All-Season. There are no signs of thinning, pilling, or seam issues so far.
One notable incident I experienced involved running directly into a metal signpost during a trail run in Chamonix. My wind shell tore, but the Rho underneath was completely unharmed. This feels like a piece that will last for many seasons with proper care, though I’ll certainly try to avoid metal signs in the future.
The Arc’teryx Rho Merino is the most comfortable base layer I’ve ever worn. The fabric is plush, soft, and stretchy, with none of the scratchiness sometimes associated with merino. The material’s 5% elastane adds just enough stretch to allow unrestricted movement without the fit feeling loose or sloppy. I’ve worn it for long runs, full days in the mountains, and plenty of casual time at home, and it never becomes irritating. It simply disappears once it’s on.
I found the fit of the Rho Merino more relaxed than a traditional skin-tight base layer, closer to a well-loved long-sleeve T-shirt than a compression shirt. Personally, I love this feel—it’s far more comfortable than the tight, athletic cuts of many technical base layers, like The North Face’s Summit Series Pro 120 or the Smartwool Intraknit Thermal. I suspect sizing down would yield a more performance-oriented fit, but as tested, the comfort is unmatched.
This is where the Rho Merino (and, to be fair, other majority merino base layers) truly separates itself from synthetic competitors. This base layer’s odor retention is virtually nonexistent. I’ve worn it for multiple runs, hikes, and travel days without washing, and it never developed that persistent stink that’s basically unavoidable with synthetics.
For ski trips, travel, or multi-day outings where laundry isn’t an option, this is a huge advantage. Compared to synthetic base layers that seem to smell permanently after a few uses, no matter how well you wash them, the Rho Merino is a revelation—and it’s made me a full convert to wool for traveling and cold-weather layering.
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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: 6’2”, 175 lb.; 41" chest, 31" waist
Size tested: Men’s large
I typically wear a men’s large, and the Arc’teryx Rho Merino in that size fits like a standard, non-technical long-sleeve T-shirt. The sleeves run slightly long, which initially made me consider sizing down, but the shoulder width, chest, and torso length were spot-on, so I stuck with the large. In practice, the sleeve length wasn’t an issue.
If you prefer a tighter, more technical base-layer fit, consider sizing down. All in all, the Rho has a relaxed, comfortable cut that prioritizes wearability and movement over compression. And no matter whether or not you size up, it should slide effortlessly beneath whatever layers you throw on top.
Smartwool Classic Thermal Merino ($125): $5 More Warmth
The 250-gram, 100% merino Classic Thermal Merino is about as heavy as base layers get—slightly beefier than the Rho’s 240 g/m²—and can comfortably take you into the teens (and likely lower) as part of a winter layering system. The tradeoff is versatility: the Classic Thermal can feel like overkill for anything but the coldest days, limiting its range across different activities. And while it’s exceptionally soft—arguably even cozier than the Rho—it lacks a nylon core, making it less durable over time. If you’re mostly skiing cold resort days, the Classic is a great pick; for broader use, the Rho is the more well-rounded option. For more, read our review of the Smartwool Thermal Merino.
Arc’teryx Rho LT Crew ($100): Better for High-Output
Instead of merino, the Rho LT uses a polyester-elastane blend (85% polyester, 16% elastane), making it slightly less warm but noticeably more breathable. That difference is especially apparent during high-output efforts like ski touring, where the Rho LT feels far more comfortable and dries much faster if you start to sweat. Despite the shift to synthetics, it doesn’t give up much in the way of comfort—the fleece interior is soft against the skin and holds its own against the cozier feel of the merino version. The tradeoff, of course, is odor resistance: while the Rho LT performs reasonably well for a synthetic, it can’t match the natural anti-odor properties of merino. But unless you’re prioritizing maximum warmth or simply prefer the feel of wool, the Rho LT stands out as the more versatile—and better value—option. For a deeper dive, read our Rho LT review.
The Rho Merino is a warm, exceptionally comfortable base layer that excels in cold conditions and moderate-output use. With its plush feel, strong odor resistance, and relatively durable build (compared to 100% merino base layers), it’s one of the top winter-weight options.
Warmth
Breathability
Durability
Comfort
Odor Resistance
Sustainability
The Classic Thermal is an exceptionally warm, 100% merino base layer tailor-made for deep winter conditions. It's soft, cozy, and odor resistant, although you’ll have to treat it with care if you want it to last.
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
The Arc’teryx Rho Merino Crew Neck LS is one of the highest-performing merino base layers we’ve tested. It stands out for its impressive warmth and next-to-skin comfort, and the merino-wrapped nylon core adds a welcome boost in durability over more traditional wool designs. It’s not the best fit for high-output use in mild conditions, but as a foundational layer for cold-weather pursuits—like winter hiking, skiing, and trail running—it’s a dialed, dependable option.
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