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Smartwool Intraknit Thermal Merino Bottom - Women's Review

These high-performing midweight bottoms maximize warmth through strategic ventilation for optimal moisture management during cold weather conditions
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Smartwool Intraknit Thermal Merino Bottom - Women's Review (These are our favorite pair of long underwear bottoms created specifically for cold weather performance.)
These are our favorite pair of long underwear bottoms created specifically for cold weather performance.
Credit: Trish Matheny
Price:  $130 List
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Manufacturer:   Smartwool
By Trish Matheny ⋅ Review Editor  ⋅  Apr 12, 2024
75
OVERALL
SCORE


RANKED
#9 of 22
  • Warmth - 25% 8.5
  • Breathability - 20% 8.0
  • Comfort and Fit - 20% 7.0
  • Durability - 15% 8.0
  • Drying Speed - 10% 6.0
  • Layering Ability - 10% 6.0

Our Verdict

The high-performing Smartwool Intraknit Thermal Merino Bottom offers incredible warmth thanks to its unique design. These bottoms feature gender-specific pattern articulation for increased mobility and strategically placed ventilation panels that effectively offload excess moisture. We were continually impressed by their ability to manage moisture and optimize warmth during a variety of activities and weather conditions. If you're looking for a thicker pair of long underwear that will keep you warm and dry during high-output activities, the Intraknit are our favorite bottoms for their cold-weather performance. See how it compared against other top competitors in our review of the best women's long underwear.
REASONS TO BUY
Awesome breathability and thermoregulation
Highly functional waistband
Great mid-rise fit
Articulated design increases mobility
REASONS TO AVOID
Expensive
Longer drying speed
Thicker waistband creates bulk when layered
Performance fabric lacks comfort
Editors Note: This review was updated on April 12, 2024, to include new alternative suggestions to our top performers.

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Bottom Line In addition to warmth and breathability, these bottoms feature articulated patterning for excellent range of motionThese lightweight bottoms offer top-notch breathability and unique thermoregulationWhile not recommended as a stand-alone piece due to the see-through polyester fabric, we love the superior breathability that helps keep us warm and dryThese bottoms offer a ton of warmth and substantial comfort at a more affordable priceThese lightweight tights feature a soft and stretchy fabric that is easy to layer and maintains substantial warmth when dry
Rating Categories Smartwool Intraknit... Icebreaker 200 Zone... Patagonia Capilene... Minus33 Franconia M... REI Co-op Midweight...
Warmth (25%)
8.5
8.0
8.5
9.0
7.0
Breathability (20%)
8.0
9.0
9.0
7.0
6.0
Comfort and Fit (20%)
7.0
9.0
8.0
9.0
8.0
Durability (15%)
8.0
6.5
6.5
8.0
7.0
Drying Speed (10%)
6.0
9.0
8.5
7.0
8.0
Layering Ability (10%)
6.0
9.0
7.0
6.0
7.0
Specs Smartwool Intraknit... Icebreaker 200 Zone... Patagonia Capilene... Minus33 Franconia M... REI Co-op Midweight...
Measured Weight (Size Small) 8.32 oz 5.63 oz 4.90 oz 7.52 oz 6.40 oz
Material 52% Merino wool, 45% polyester, 2% elastane, 1% polyamide 100% Merino wool 92% recycled polyester, 8% spandex 100% Merino wool 92% recycled polyester, 8% spandex
Insulation Weight Class Midweight Lightweight Midweight Midwewight Midweight
Smelly Over Time? No No Yes No Yes
Odor Control Fabric Merino yes, polyester no Naturally odor resistant HeiQ Fresh Naturally odor resistant No
Cuts Avaliable Full-length Full-length Full-length, boot-length Full-length Full-length
Listed Fabric Weight (g/m²) Not disclosed 200 g/m² 129 g/m² 235 g/m² Not disclosed

Our Analysis and Test Results

The Intraknit Thermal stands apart from the other long underwear bottoms in this review thanks to a modern design with gender-specific body mapping for enhanced warmth and ventilation. Unlike other midweight bottoms that provide warmth but retain moisture during activity, these bottoms keep you warm by offloading sweat.

Performance Comparison


smartwool intraknit thermal merino bottom for women - cold weather performance just got better with the smartwool intraknit.
Cold weather performance just got better with the Smartwool Intraknit.
Credit: Trish Matheny


Warmth


SmartWool really nailed a niche category by adding articulated patterning and ventilation panels to a midweight design. The intelligent Intraknit technology efficiently regulates body temperature by offloading excess moisture to trap and maintain the body heat generated during rigorous activity. Unlike most mid- and heavyweight options in this review, which retain moisture, these tights are highly functional at thermoregulation.


During our testing period, we hit the trails, skin track, and dry roads in temperatures ranging from 15-55 degrees Fahrenheit. We were impressed with the warmth these bottoms offered without causing us to feel overheated or cold from sweat. When temperatures dropped below 10 degrees (depending on humidity), we would have been more comfortable with another layer over the top. Still, we were surprised with how well the Intraknit can retain body heat. Likewise, the same ventilation panels successfully released warmth in temperatures above 55 degrees. These are a great option if you're looking for a cold-weather long underwear bottom that offers a ton of warmth without sacrificing moisture management.

These thicker long underwear bottoms are incredibly warm and feature one of the heaviest weights that we tested, best utilized for activities during the coldest months of the year.
Credit: Trish Matheny

Breathability


While we were initially skeptical, the Intraknit repeatedly proved us wrong. To measure the overall breathability of these bottoms, we hit the trails, sprinted on dry roads, and got moving on the skin track. Throughout each activity, with varying weather patterns, these bottoms successfully wicked moisture away from the body and released it through the fabric.


These are some of the thickest bottoms we tested, but the ventilation panels help set them apart from the rest, amplifying cold weather insulation and managing moisture. If we have any complaints, it is the thicker waistband. Although admittedly, it is hard to complain about this feature because it provides such excellent structure and holds these bottoms in place. But if sweat is going to pool anywhere, it's here. Overall, the breathability of the ventilation panels improves the design, making these bottoms perfect for high-output activities like backcountry skiing and trail running.

smartwool intraknit thermal merino bottom for women - we turned the fabric inside out so you can more easily see the...
We turned the fabric inside out so you can more easily see the ventilation detail.
Credit: Trish Matheny

Comfort and Fit


While we absolutely love the fit, especially the mid-rise 2" waistband that hugs the curves and provides some structure to the Merino sport yarn fabric, we had to dock these tights a few points for minimized comfort. The Merino sport yarn and polyester blended material add a ton of value to these tights but, unfortunately, sacrifices that next-to-skin Merino wool comfort — we found the fabric to be itchy at times.


The fit is a different story. The Intraknit truly offers extraordinary fit due to its integrated body mapping technology with minimal use of flatlock seams. The full-length fit is true to size, and perhaps our only complaint is that the articulated knees sit just above our actual knee when wearing a size small on our 5'6" frame. We love the increased range of motion we can achieve with this thoughtful fit, in addition to the ventilation panels that increase comfort by keeping us dry; we just wish we didn't have to give up that super soft next-to-skin comfort we've grown accustomed to with Merino wool.

smartwool intraknit thermal merino bottom for women - we love the high rise fit and the thicker waistband that holds these...
We love the high rise fit and the thicker waistband that holds these bottoms in place just above our hips.
Credit: Trish Matheny

Durability


We're happy to report that we've experienced no durability issues with these tights, even after taking an unexpected digger while running fast through some technical terrain in the desert. SmartWool really increased the durability of the Merino sport yarn by adding polyester, which helps give some structure to these wool tights.


If you want to extend the life of these high-quality long underwear bottoms, we recommend following the care instructions by washing them in a machine on a gentle cycle and laying them flat to dry. This will prevent any damage from the heat of the dryer or unwanted stretch from drying on a hanger.

smartwool intraknit thermal merino bottom for women - the merino wool is reinforced with polyester to increased durability...
The Merino wool is reinforced with polyester to increased durability in addition to a thick and functional waistband as well as burly flatlock seams.
Credit: Trish Matheny

Drying Speed


It was important to our testers to consider the drying speed of each layer while simulating what it would be like to wash and dry the layer during an extended trip in the backcountry. As expected, due to the thicker, heavier material, the Intraknit also required more time to dry at 65 minutes.


We found that the drying speed increased by more than double when the layer could utilize body heat generated during high output activity such as runs and skinning uphill. In those circumstances, these bottoms dried as quickly as 25 minutes on the body. It's not a stellar performance here, but considering these weigh 8.32 ounces and offer stellar warmth and thermoregulation, it's not too bad, either.

smartwool intraknit thermal merino bottom for women - the thicker merino wool fabric of the intraknit really slowed down...
The thicker Merino wool fabric of the Intraknit really slowed down the drying speed for these bottoms - they needed 65 minutes to completely dry.
Credit: Trish Matheny

Layering Ability


Unfortunately, we found these tights to be a little tricky to layer due to a very slim fit. The Merino/polyester blended fabric can get a little grabby with things like ski bibs or pants unless those layers offer a smooth fabric interface. This resulted in some friction that was noticed during our uphill skin track testing drills.


The thicker fabric of the bottoms and the wide 2" waistband can also reduce the space you might normally be accustomed to inside your ski bibs. That said, we love that the Intraknit can easily be worn as a stand-alone piece, in addition to easy layering with rain and puffy pants that feature a smooth fabric to interface with.

The SmartWool Intraknit Thermals feature a thicker fabric that adds some friction when combined with ski bibs, layers like rain and puffy pants are easier to layer due to a smoother interface between the fabrics.
Credit: Trish Matheny

Should You Buy the SmartWool Intraknit Thermal Merino?


These high-performing bottoms are worth every penny. For those looking for a versatile cold-weather bottom that amplifies thermoregulation and breathability through strategically placed ventilation panels, whether worn on their own or layered within a system, the Intraknit puts a modern twist on midweight insulation.

smartwool intraknit thermal merino bottom for women - these bottoms feature a thick and durable merino wool and polyester...
These bottoms feature a thick and durable Merino wool and polyester blend that breathes well thanks to strategically placed ventilation panels, making them our favorite bottom for winter training runs.
Credit: Trish Matheny

What Other Long Underwear Should You Consider?


If you like a versatile bottom but want a stash pocket for gels, keys, or your smartphone, check out our favorite bottoms for multi-sport performance, the Arc'teryx Rho Bottom. If you want to save a little money and don't need all the bells and whistles, the Minus33 Franconia Midweight Wool is a more affordable 100% Merino wool design that feels soft and cozy next to your skin. And finally, if you want to invest in our absolute favorite Merino bottoms, the Icebreaker 200 ZoneKnit Merino Leggings really ups the ante with body-mapping technology and exceptional performance for year-round use. And finally, if you wanna consider this option for a top-performing synthetic long underwear bottom, our testers love the Patagonia Capilene Thermal Weight Bottoms. These bottoms offer a ton of breathability within your layered system. However, we don't recommend them as stand-alone leggings.

Trish Matheny