REASONS TO BUY
Fuzzy inner face perfect to capture heat
Smooth outer face enhances layerability
Can be casually worn as an outer layer
REASONS TO AVOID
Weighs more than other options
Thick fabric has poorer breathability
Pricey
Arc'teryx Rho Heavyweight Bottoms Review
A furnace-like pair of synthetic bottoms, perfect for either the coldest days outside or the laziest days in
Credit: Maddock Rigby
Our Verdict
The Arc'teryx Rho Heavyweight Bottoms are the pair of synthetic long underwear you'll want to grab to keep you as warm as possible on wintry days. They feature thick Polartec Power Stretch Pro fabric expertly sewn together to deliver excellent insulation for your bottom half. These are perfect to wear on that extra chilly snowshoe tour underneath your softshell pants or worn alone while lounging around inside, nursing a headache from too much nighttime cheer. If you like everything you've heard about wool but your skin finds it itchy, the Rho Heavyweight Bottoms are an excellent, no-compromise alternative. But you will pay for such performance — these bottoms don't come cheap.
Compare to Similar Products
 This Product
Arc'teryx Rho Heavyweight Bottoms | |||||
|---|---|---|---|---|---|
| Awards |  Best Synthetic Long Underwear for Men | Best Overall Long Underwear for Men |  Best Lightweight Long Underwear |  Best Synthetic Bang for the Buck | Our Favorite Merino Wool on a Budget |
| Price | $96.00 at Amazon Compare at 2 sellers | $109.99 at Amazon Compare at 4 sellers | $120 List | $55 List $54.95 at REI | $59 List $58.99 at Amazon |
Overall Score  |
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| Star Rating | |||||
| Bottom Line | One of the warmest synthetic bottoms we've seen, with heat retention that rivals its woolen brethren | An extra-warm pair of bottoms that will properly thermoregulate through the coldest temperatures | When sparing no expense on a lightweight pair of long underwear, these comfy and breathable leggings perform at the top of their class | More than just a budget choice, this synthetic base layer performs magnificently as a thermal bottom | Grab these if you want a pair of less expensive Merino wool long underwear that's worth their price |
| Rating Categories | Arc'teryx Rho Heavy... | Smartwool Classic T... | Artilect Boulder 125 | REI Co-op Midweight... | Merino.tech Bottoms |
| Warmth (25%) | |||||
| Breathability (20%) | |||||
| Comfort and Fit (20%) | |||||
| Durability (15%) | |||||
| Drying Speed (10%) | |||||
| Layering Ability (10%) | |||||
| Totals (100%) | |||||
| [Editor to delete grey columns before uploading] (MSRP%) | |||||
| Specifications | Arc'teryx Rho Heavy... | Smartwool Classic T... | Artilect Boulder 125 | REI Co-op Midweight... | Merino.tech Bottoms |
| Material | 53% polyester, 38% nylon, 9% elastane | 100% Merino wool | Nuyarn: 85% Merino wool, 15% nylon | 92% recycled polyester, 8% spandex | 100% Merino wool |
| Insulation Weight Class | Midweight/heavyweight | Midweight | Lightweight | Midweight | Midweight |
| Measured Weight (Size Medium) | 9.5 oz | 8.3 oz | 4.3 oz | 7.4 oz | 7.8 oz |
| Measured Inseam (Size Medium) | 32.0 in. | 31.0 in | 31.0 in | 29.5 in | 28.8 in |
| Listed Inseam Length (Size) | 32.5 in (medium), short and tall options also available | 30.0 in (medium) | None listed | 28.0 in (medium), 31.0 in tall options | 28.0 in (medium) |
| Odor Control Fabric | None | Naturally odor resistant | Naturally odor resistant | None | Naturally odor resistant |
| Fly? | No | Yes | No | Yes | Yes |
| Dryer Safe? | No | Yes | No | Yes | No |
| Listed Fabric Weight (g/m²) | Not stated | 250 g/m² | 125 g/m² | Not stated | 250 g/m² |
| 3/4 Boot-Cut Available? | Yes | Yes | Yes | No | No |
Our Analysis and Test Results
By using a soft, fuzzy fabric with a sheer outer face, the Arc'teryx Rho Heavyweight Bottoms are warmer long bottoms than most. Well-constructed, they're stitched together with flatlock seams throughout and feature a gusseted crotch, which allows you to move freely without them awkwardly clinging to areas that it shouldn't. The fabric used is also tough, perfect for those who aren't the greatest at being careful with laundering their clothes. A small zippered pocket is found on the right side for stowing small items like a key card to keep when you provocatively dip for a continental breakfast, wearing these as your outer bottom layer.
Performance Comparison
Credit: Maddock Rigby
Warmth
Care of their thick and lofty fabric, the Arc'teryx Rho Heavyweight Bottoms are total scorchers. “These are the long bottoms I grab when I want serious warmth retention into my kit for the day,” notes tester Just Simoni. They're among our lineup's warmest synthetic long underwear, matching — or even bettering — the toastiest Merino wool bottoms.
The explanation is found in fabric choice. Arc'teryx has chosen a relatively heavy Polartec Power Stretch Pro fleece for the Rho Heavyweights. This fabric traps air within its fibers, keeping it close to your body and powering its warmth. Our controlled exercise test left us borderline uncomfortable in an indoor environment — nearly too warm. We usually walk around this space in shorts, and it took many minutes after the test to get back to baseline. Outside, this is the long bottom we grabbed for when heading to the high country.
The Arc'teryx Rho Heavyweight Bottoms hugged comfortably onto our legs, warming them up quickly in our controlled environment tests.
Credit: Justin Simoni
Breathability
As one may expect, when warmth retention is high, breathability suffers. This is the case for the Arc'teryx Rho Heavyweight Bottoms. Our portable airflow meter recorded some of the lowest numbers for this pair compared to the rest of our lineup and slightly lower than a comparable pair of 100% Merino wool pants. In general, synthetics usually lag behind wool pairs in this metric, no matter the thickness of the bottom.
A very warm pair of long underwear with relatively low breathability suggests the best use cases for the Rho Heavyweight: frigid conditions would be where these will excel. “These are an excellent choice underneath ski pants for resort skiing and also worn alone when curled up with a book by a roaring fire in a comfy cabin afterward,” Simoni muses. “A poor matchup would be a brisk day and a fast run: you'll quickly overheat, and sweat will start to build up.”
The Arc'teryx Rho Heavyweight Bottoms return reasonable air flow numbers when using our portable meter, but airflow is going to be lower than most lighter pairs of long undies, as well as most wools -- even of similar warmth.
Credit: Justin Simoni
Comfort and Fit
True to size, the Rho Heavyweight gently hugs close to the body. The soft inner fleece face won't scratch your legs and easily allows the pants to move with your body rather than fighting it. “I usually wear a 32-inch waist and inseam for most of my normal pants, and the Rhos in medium fit me perfectly,” notes Justin Simoni, who tested these both inside and outside.
Flatlock seams are found throughout the Rho Heavyweight, negating any pressure points. The elastane waistband is covered in the same fleece that makes up the rest of the bottoms. And the bottom of the pant leg is finished with a simple shallow hem. One standout feature is a gusseted crotch, which removes pesky seams away from more sensitive areas of the inner thigh and above.
The gusseted crotch of the Arc'teryx Rho Heavyweight Bottoms make them more comfortable as they move pesky seams away from sensitive areas of the body while also creating a more three dimensional fit, allowing us to pay attention to more important things.
Credit: Maddock Rigby
Durability
The Arc'teryx Rho Heavyweight Bottoms show excellent durability characteristics, especially when compared to midweight wool bottoms, which have earned the reputation of being delicate. The fabric on the Rho is far more durable, making these long bottoms a good choice if you're hard on your gear. Strong, high-quality flatlock seams are found throughout the entire garment, and the fabric's four-way stretch resists tearing. A gusseted crotch further minimizes pressure points. These could be bottoms worn as an outer layer in casual settings.
Since these are fleecy, shedding is likely, mostly during washing and drying. The specific Polartec fabric is designed and marketed to shed less than conventional fleece. We couldn't easily “pluck” off the fuzz from the inside face like we can with some of our vintage fleece tops, so there may be something to it. When washing, don't turn these inside out. Keep the fuzzy side from seeing too much aggressive agitation.
A soft, fuzzy hand is found in the skin-side fabric face of the Arc'teryx Rho Heavyweight Bottoms, which traps warm air closet to your body.
Credit: Maddock Rigby
Drying Speed
The drying speed was excellent for the Arc'teryx Rho Heavyweight Bottoms — especially for their 9.5 oz (size M) weight on the scale. Once grabbed from out of the spin cycle, these bottoms weighed approximately 150% of what they weighed dry, showing fairly normal water retention. By and large, synthetics dry faster than wool layers and cotton layers. These are no exception.
Being synthetic, you may feel better about drying these in a dryer rather than resorting to line dry or flat drying as you would like for most wool bottoms. There won't be an issue with the shrinkage of the Rho Heavyweights. But hanging them to dry when you can is probably best for the longest life expectancy, especially since the shedding of fleece and fleece-like material is exasperated when drying.
In size Medium, the Arc'teryx Rho Heavyweight Bottoms come in at 9 1/2 ounces, giving us a hint at how long they take to dry.
Credit: Justin Simoni
Layering Ability
The secret weapon that makes the Arc'teryx Rho Heavyweight Bottoms excellent for layering is the smooth front jersey knit face, which resists snagging from layers above and protects the softer, loftier fleece nap underneath.
The four-way stretch keeps the fabric moving with you, so you won't need to adjust mid-ski run awkwardly, and the waistband does a reasonable job keeping the pants from slipping down.
Seen under the microscope, the smooth outer face which won't catch on upper layers is in stark contrast to the fuzzy, lofty fleece of the inside face which traps warm air .
Credit: Justin Simoni
These bottoms are also quite flexible. Wear a layer underneath them so long that it's sheer enough not to interface with the fleece face. Or, wear these as the top layer — the design is modest enough to be “meggings approved” in a casual setting. A clever side pocket hints that they're made to be worn in such a fashion.
The Arc'teryx Rho Heavyweight Bottoms works excellent as a thick, warm base layer that won't catch on layers piled on above it.
Credit: Maddock Rigby
Should You Buy the Arc'teryx Rho Heavyweight Bottoms?
The Arc'teryx Rho Heavyweight Bottoms make a great pair of long underwear for both bitter cold conditions where staying warm is a requirement for survival and durability a necessity, while also being a fun pair of tight sweats to wear back at home on the couch napping with your dog as the snow comes down outside in sheets. We enjoyed testing the Rho Heavyweight Bottoms in both. This flexibility may make them a great value to you and worth their high MSRP.
Stand tall, strong (and warm!) wearing the Arc'teryx Rho Heavyweight Bottoms.
Credit: Maddock Rigby
What Other Base Layers Should You Consider?
For cold weather conditions, we usually suggest wool. The Smartwool Classic Thermal Merino Bottom, Minus33 Kancamagus Midweight Merino Wool Bottoms, and Merino Tech Bottoms all make good frigid conditions options. You'll likely see better breathability but less durability in these pairs. Among synthetics, the REI Co-op Midweight Bottoms have excellent durability, weigh less, but won't be as warm.
—Justin Simoni
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