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Smartwool Classic Thermal Merino Bottom Review

Designed to keep you warm in the coldest conditions, this is a versatile pair of Merino wool bottoms
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Smartwool Classic Thermal Merino Bottom Review
Credit: Justin Simoni
Price:  $115 List
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Manufacturer:   Smartwool
By Aaron Rice and Justin Simoni  ⋅  Mar 21, 2025

Our Verdict

Just like our team's favorite base layer top, the Smartwool Classic Thermal Merino Bottoms are designed for work and play through the coldest months. These 100% Merino wool bottoms are plush in all the right ways. Starting with an extra thick waistband lined with Merino wool, their comfort extends down to the cuffs, finished off with flatlock seams to minimize rubbing and chafing. Smartwool incorporates an interlock knitted Merino wool into every piece in this line, which is superb when it comes to breathability and thermoregulation, making these bottoms very warm yet still quite versatile. To see how it stacks up versus the competition, check out our review of long underwear.
REASONS TO BUY
A nice balance of warmth and breathability
Super cozy
Nicely articulated design
REASONS TO AVOID
Expensive
Stretches out in between washes
Slow to dry when saturated

Compare to Similar Products

 
Awards Editors' Choice Award
Best Overall Long Underwear for Men
Editors' Choice Award
Best Synthetic Long Underwear for Men
Top Pick Award
Best Lightweight Long Underwear
Best Buy Award
Best Synthetic Bang for the Buck
Best Buy Award
Our Favorite Merino Wool on a Budget
Price $109.99 at Amazon
Compare at 4 sellers
$96.00 at Amazon
Compare at 2 sellers
$120 List$55 List
$54.95 at REI
$59 List
$58.99 at Amazon
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Bottom Line An extra-warm pair of bottoms that will properly thermoregulate through the coldest temperaturesOne of the warmest synthetic bottoms we've seen, with heat retention that rivals its woolen brethrenWhen sparing no expense on a lightweight pair of long underwear, these comfy and breathable leggings perform at the top of their classMore than just a budget choice, this synthetic base layer performs magnificently as a thermal bottomGrab these if you want a pair of less expensive Merino wool long underwear that's worth their price
Rating Categories Smartwool Classic T... Arc'teryx Rho Heavy... Artilect Boulder 125 REI Co-op Midweight... Merino.tech Bottoms
Warmth (25%)
9.0
9.0
5.0
7.0
8.0
Breathability (20%)
7.0
6.0
10.0
7.0
6.0
Comfort and Fit (20%)
8.0
9.0
8.0
7.0
6.5
Durability (15%)
7.0
8.0
6.0
9.0
5.0
Drying Speed (10%)
6.0
9.0
8.0
7.0
6.0
Layering Ability (10%)
7.0
7.0
8.0
7.0
7.0
Totals (100%)
10.0
10.0
10.0
10.0
10.0
[Editor to delete grey columns before uploading] (MSRP%)
0
0
0
0
0
Specifications Smartwool Classic T... Arc'teryx Rho Heavy... Artilect Boulder 125 REI Co-op Midweight... Merino.tech Bottoms
Material 100% Merino wool 53% polyester, 38% nylon, 9% elastane Nuyarn: 85% Merino wool, 15% nylon 92% recycled polyester, 8% spandex 100% Merino wool
Insulation Weight Class Midweight Midweight/heavyweight Lightweight Midweight Midweight
Measured Weight (Size Medium) 8.3 oz 9.5 oz 4.3 oz 7.4 oz 7.8 oz
Measured Inseam (Size Medium) 31.0 in 32.0 in. 31.0 in 29.5 in 28.8 in
Listed Inseam Length (Size) 30.0 in (medium) 32.5 in (medium), short and tall options also available None listed 28.0 in (medium), 31.0 in tall options 28.0 in (medium)
Odor Control Fabric Naturally odor resistant None Naturally odor resistant None Naturally odor resistant
Fly? Yes No No Yes Yes
Dryer Safe? Yes No No Yes No
Listed Fabric Weight (g/m²) 250 g/m² Not stated 125 g/m² Not stated 250 g/m²
3/4 Boot-Cut Available? Yes Yes Yes No No

Our Analysis and Test Results

The Smartwool Classic Thermal Merino Bottom is a 250 g/m² thick 100% Merino wool pair of full-length long underwear that we've found excels overall in warmth and comfort. Excellent thermoregulation and breathability mean you won't be sweltering in these bottoms when temps start to rise. Layer on up, starting with these snug bottoms, and know that they'll be compatible with whatever you pair them up with. The thick waistband, gusseted crotch, and flatlock seams throughout are just some of the standout features you'll find that make these some of the best thermal bottoms we've had the privilege of putting on.

Performance Comparison


smartwool classic thermal merino bottom - nearing the summit of mt hood where long underwear is an absolute...
Nearing the summit of Mt Hood where long underwear is an absolute necessity.
Credit: Roland Mott

Warmth


Constructed to excel in harsh winter conditions, the Classic Thermal Bottom boasts a fabric weight of 250 g/m², the heaviest in Smartwool's lineup. The 100% Merino wool is thick cut, making these bottoms much more plush next to the skin than comparably weighted models we tested. Lead reviewer Justin Simoni offered quick praise: “I loved how plush these pants were, making them an ideal companion for full days spent outside through the coldest months of winter.


Boasting a thick, cozy 100% Merino wool construction, these bottoms are meant to excel in harsh conditions. They do a superb job of wicking away sweat when you're working hard, and they dry quickly against your skin so that after skinning endlessly for the perfect line, you're not soaked and freezing when you finally reach the windblown summit.

smartwool classic thermal merino bottom - tough like the summit winds, the thick fabric of the smartwool...
Tough like the summit winds, the thick fabric of the Smartwool Classic Thermal is durable and impressively warm.
Credit: Roland Mott

The exterior of Merino wool fabric is naturally hydrophobic, which keeps these base layer pants warm and comfortable throughout the day, regardless of how intensely you are working up a sweat. The thick Merino wool traps just enough heat to allow this layer to dry against your skin quickly. Even through 6,000 vertical feet of skinning and climbing to reach the summit of Mt. Hood on a mid-winter ascent, the Classic Thermal Bottom proved warm and dry when we reached the top. It is important to mention that while these bottoms are warm, they are not expedition-weight warm. So, if you're planning a trip north of the Arctic Circle, buy something even thicker.

smartwool classic thermal merino bottom - even if you have to eventually get out your bag in the morning...
Even if you have to eventually get out your bag in the morning, these bottoms are a great bet to keep you warm and cozy.
Credit: Jill Rice

Breathability


What continues to impress us about these bottoms is how aptly they balance top-notch insulation with near-top-notch breathability. Functional thermoregulation is not common amongst base layers, but the Classic Thermal handles that task masterfully.


The special interlock knit used in the design of these bottoms sets them apart from the competition. The lattice-like knit adds additional loft, supplementing the natural breathability of Merino wool fibers with a microclimate that uses heat to force moist air up through the fabric, where it can evaporate on the surface. Unlike thick layers that hold onto sweat and become clammy once you stop moving, these avoid becoming sweat-soaked by harnessing the power of simple thermodynamics. The harder you work, the more powerful the temperature/pressure gradient between your skin and the outside air, and the faster these bottoms dry out.

smartwool classic thermal merino bottom - nearing the end of the skinning slog and getting excited to put on...
Nearing the end of the skinning slog and getting excited to put on crampons to summit Mt. Hood in Oregon.
Credit: Adam Zagorski

Comfort and Fit


The Classic Thermal has effectively won over the hearts of all of our testers — when it comes to pure comfort, these bottoms are heavenly. They are warm and cozy, and the 100% Merino knit is thick, soft, and stretchy. Smartwool designers seem to take this fact with pride, and this updated version thoughtfully lines the interior of the extra-wide waistband with Merino wool to increase comfort further and cut down on a common spot for irritation. The entire construction is supported by flatlock seams designed to minimize rubbing and chafing, giving you more reason to wear these bottoms directly next to your skin.


We often wear these as true long underwear, opting to comfortably layer them under our jeans during the coldest months of winter. The natural elasticity of Merino wool makes these bottoms a versatile option to prepare for those extra-cold moments, like freezing your buns off standing outside. The material is stretchy enough to pull over a lighter-weight base layer and yet somewhat snug enough to wear below a mid-layer pant without any constrictions.

smartwool classic thermal merino bottom - the fabric of the smartwool classic thermal merino bottom is soft...
The fabric of the Smartwool Classic Thermal Merino Bottom is soft and cozy, with a snug fit.
Credit: Justin Simoni

The gusseted crotch is a big win for me, as I'm riding my bike all year round — getting the seams out of the way is just way more comfortable,” reports tester Justin Simoni. A single seam runs the length of the inseam without losing out on user mobility. While we've dealt with saggy-bum issues in the past, the current version of these long johns takes the same gusseted design from the front and applies it to the back of the pants. You don't have to pull these bottoms past your belly button to secure a snug fit.

smartwool classic thermal merino bottom - the soft and thick waistband complements the rest of the comfy 100%...
The soft and thick waistband complements the rest of the comfy 100% Merino wool fabric.
Credit: Justin Simoni

Durability


While Merino wool is reputed to have durability issues, modern knitting and design techniques have improved upon concerns. The Classic Thermal is our review's thick Merino layer, easily landing them among the most durable all-natural options we tested. Still, our harsh abrasion test was able to tear holes in the fabric. These bottoms will not be as durable as a synthetic option against snags and punctures. Still, they offer breathability and wicking capabilities and resist odor build-up very well. “These Smartwool Thermals aren't designed to be worn as a top layer anyways — they have a fly — so I didn't find I needed to be super careful not to rip them, and I don't think you are likely to see them on anyone in your next yoga class,” notes Simoni.


Proper laundering is the main way you can protect your purchase and invest in the longevity of a Merino base layer. By carefully washing and, most importantly, flat drying, you will likely have the Classic Thermal Bottom as your go-to cold weather bottoms for years. While you may experience them stretching out between washings, don't feel inclined to shrink them up in the dryer — that is the surest path to an early grave for this high-quality layer. Simoni points out such requirements across all his gear, commenting, “As an outdoor athlete, my collection of technical clothes that are not meant to go in the dryer now rivals the casual clothes in my closet, so it's not hard to remember to hang these up with everything else.”

smartwool classic thermal merino bottom - while thick and well made, merino wool is still more delicate than a...
While thick and well made, Merino wool is still more delicate than a synthetic. Our abrasion testing is harsh, as evidenced by the hole we wore in the Classic Thermal.
Credit: Justin Simoni

Drying Speed


The Classic Thermal did well in our on-the-line drying tests, especially given the 250 g/m² of the 100% Merino wool fabric. Merino wool, especially at this thickness, will dry slower than synthetics, and the heavier weight of this wool means it'll never win a race with lesser-weighted materials. Still, given the other top marks these bottoms have won, we're more than happy with the performance.


The manufacturer's recommendations note that you can tumble dry the Classic Thermal Bottom on low, but we would opt to hang these up to dry, wherever that's realistic, to extend the product's life. Given our experience, these should dry out easily overnight, whether in a ski hut next to a roaring fire or just outside on a clear day.

smartwool classic thermal merino bottom - a hanging tab is a nice touch for air-drying, and keeps these...
A hanging tab is a nice touch for air-drying, and keeps these bottoms from getting stretched out in a weird way.
Credit: Jill Rice

Layering Ability


The snug but not overly tight fit of the Classic Thermal Merino Bottom makes them excel as a next-to-skin layer, where you can add a mid and outer layer. These are the thickest base bottoms this manufacturer puts out, so you most likely don't want to add another base layer-like legging underneath or over.


These bottoms are best for when temperatures drop to winter-like conditions and will most likely be too warm for summertime climes. That said, good breathability means you can stretch the usable months of these bottoms and wear them even if the weather gets warm without worrying that you'll become a personal sweat lodge. “I like that the Classic Thermal Merinos are sheer enough they don't stick to any of my other top layers — ski pants, especially,” notes our main reviewer.

smartwool classic thermal merino bottom - the snug but not compression tight fit works well to layer up with...
The snug but not compression tight fit works well to layer up with mid and outer layers. The bottom of the Thermal Merino ended just at the top of the ankle for our 32" inseam model.
Credit: Justin Simoni

Should You Buy the Smartwool Classic Thermal Merino Bottom?


These leggings offer superb warmth and thermoregulation wrapped up in an uber-comfortable package. For those who need an extra boost of warmth through the winter, these are a great investment that will prove worthwhile even after the mercury rises from sub-freezing temperatures. The quality of construction and sheer ability to balance warmth and breathability warrants the price tag. “These are investment, but it's money well-spent if you're outside in the cold often”, states Simoni.

smartwool classic thermal merino bottom - from the high alpine to your next backpacking trip, these versatile...
From the high alpine to your next backpacking trip, these versatile bottoms are sure to be a constant companion on many outdoor adventures.
Credit: Jill Rice

What Other Long Underwear Should You Consider?


If the price of the Classic Thermal is too high but you would like the warmth of a 250 g/m² bottom, look into the no-nonsense Meriwool Merino 250 Bottom, which makes a great entry-level warm Merino wool base layer. If the Classic Thermal is too warm for your needs, grab the Smartwool Classic All-Season Merino Bottom, which features a 150 g/m² weighted Merino wool fabric and a very similar design. Finally, if you'd rather go with a synthetic bottom, look no further than the REI Co-op Midweight Bottoms. These won't be as warm, but they are much more durable.

If you're looking for a top base layer to perfectly complement these bottoms, we recommend the Smartwool Classic Thermal Merino 1/4 Zip, which utilizes the same 250 g/m² fabric as the Classic Thermal bottoms.

smartwool classic thermal merino bottom - full flatlock seams keep itchiness at bay.
Full flatlock seams keep itchiness at bay.
Credit: Jill Rice

Aaron Rice and Justin Simoni