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Smartwool Intraknit Thermal Merino 3/4 Bottom Review

Carefully engineered 3/4 length thermals with compression pants-like tightness that’ll keep you warm and super comfortable while pursuing high output activities
smartwool intraknit thermal merino 3/4 bottom long underwear review
The well ventilated side panels and gusseted crotch are some of our favorite features for high-intensity activities in the cold.
Credit: Justin Simoni
Price:  $115 List
Manufacturer:   Smartwool
By Justin Simoni ⋅ Review Editor  ⋅  Dec 2, 2022
72
OVERALL
SCORE


RANKED
#4 of 12
  • Warmth - 25% 7.0
  • Breathability - 20% 7.0
  • Comfort and Fit - 20% 7.0
  • Durability - 15% 8.0
  • Drying Speed - 10% 5.0
  • Layering Ability - 10% 9.0

Our Verdict

Warm, versatile, and techy! The Smartwool Intraknit Thermal Merino 3/4 Bottom has a seamless "Intraknit" technology that allows discrete fabrics to be blended together, fine-tuning how the product behaves and performs. More ventilated fabric is located in the lower back, outer thighs, and crotch to help with breathability, while supple and elastic fabric is located at the knees to promote movement. The rest of the fabric focuses on warmth. The shorter length helps with layering, and an ultra-thick, high waistband and tons of compression keep everything in place. These leggings aren't for everyone: the lack of a fly may be a deal breaker, as could the slim, next-to-skin fit. But if you're looking for an extremely well-thought-out layer, here it is.
REASONS TO BUY
Shorter length reduces bunching near ankles
Seamless panels provide additional articulation and ventilation only where needed
High waist and thick waistband keeps bottoms in place
REASONS TO AVOID
No-fly design could be inconvenient underneath other layers
Fit may be too tight for some body types
Pricey

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Pros Shorter length reduces bunching near ankles, seamless panels provide additional articulation and ventilation only where needed, high waist and thick waistband keeps bottoms in placeA nice balance of warmth and breathability, super cozy, nicely articulated designExtremely breathable, ultralight stretchy nylon/Merino wool blendTough synthetic fabric, four-way stretch for great range of motion, excellent performance for the priceAffordable intro to Merino, warm and cozy, wide spectrum of available sizes
Cons No-fly design could be inconvenient underneath other layers, fit may be too tight for some body types, priceyExpensive, stretches out in between washes, slow to dry when saturatedPrice may make you do a double-take, care instructions need to be followed when dryingHeavier than wool bottoms for their warmthStarts off slightly itchy, simplified construction may not be as comfy, Merino wool is delicate
Bottom Line Carefully engineered 3/4 length thermals with compression pants-like tightness that’ll keep you warm and super comfortable while pursuing high output activitiesDesigned to keep you warm in the coldest conditions, this is a versatile pair of Merino wool bottomsA seriously tuned ultralight pair of wool/nylon blend leggings built for fast and light outdoor adventuresA great synthetic thermal long underwear at an unbeatable price for the warmth and durability they provideGrab a pair of these if you are looking for an affordable and warm pair of Merino wool bottoms
Rating Categories Smartwool Intraknit... Smartwool Classic T... Artilect Boulder 125 REI Co-op Midweight... Meriwool Merino 250...
Warmth (25%)
7.0
9.0
5.0
7.0
8.0
Breathability (20%)
7.0
7.0
10.0
7.0
6.0
Comfort and Fit (20%)
7.0
8.0
8.0
7.0
7.0
Durability (15%)
8.0
7.0
6.0
9.0
6.0
Drying Speed (10%)
5.0
7.0
8.0
7.0
6.0
Layering Ability (10%)
9.0
7.0
8.0
7.0
6.0
Specs Smartwool Intraknit... Smartwool Classic T... Artilect Boulder 125 REI Co-op Midweight... Meriwool Merino 250...
Material 54% Merino wool, 43% polyester, 2% elastane, 1% polyamide 100% Merino wool 85% Nuyarn Merino wool, 15% nylon 100% polyester 100% Merino wool
Fabric Weight ~200 g/m² 250 g/m² 125 g/m² Not stated 250 g/m²
Weight (size medium) 9.2 oz 8.3 oz 4.3 oz 7.4 oz 7.9 oz
Insulation Weight Lightweight Midweight Lightweight Midweight Midweight
3/4 Boot-Cut Available? Yes Yes Yes No No
Fly? No Yes No Yes Yes
Dryer Safe? No No No Yes No

Our Analysis and Test Results

Distancing itself from more conventional designs, the Intraknit 3/4 Bottom throws a ton of technology toward keeping you warm and comfortable while running, cycling, skiing, riding — anything self-powered. The proprietary Intraknit seamless paneling and Merino wool/synthetic blend work to wrap you in a fabric that performs differently in key areas. For example, better ventilation is found in the back, crotch, and outer side of the leg. Warmer panels are found on the inner thigh, and more flexible material is used on the knee. Is the price of entry worth it, or is this all just marketing smoke? In our opinion, the Intranknit paneling and fabric blend really sets these bottoms apart from almost every other pair of thermal bottoms we've tested.

Performance Comparison


smartwool intraknit thermal merino 3/4 bottom long underwear review - wintry hikes with a lot of elevation gain are a perfect time to wear...
Wintry hikes with a lot of elevation gain are a perfect time to wear these warm 3/4 bottoms underneath your outer layers.
Credit: Justin Simoni

Warmth


The Intraknit Thermal presents a few different personalities at once, given the nature of the seamless panels and the characteristics of each. The warmest of panels, coming close to a 200-250 g/m² effective 100% Merino wool weight, are found in the interior surfaces of the thighs and lower legs. The knees, back, crotch, and outer thighs have panels that are not quite as warm but have additional characteristics, like better ventilation and articulation. What this means to you is that these bottoms will keep you warm without overheating.


We noticed this fine balance of warmth without overheating when performing high-output activities or sweating profusely. Our main tester is known to do both and found these bottoms warm for his always-in-motion lifestyle and something he could keep on all day without needing to do an awkward trailside wardrobe change. We also enjoyed the thick, high-set waistband, which helped prevent drafts from venturing too far down our legs during jacket adjustments. The 3/4 length of the bottoms requires at least a crew-length sock for maximum warmth.

smartwool intraknit thermal merino 3/4 bottom long underwear review - these bottoms are some of the heavier in weight we've tested, and...
These bottoms are some of the heavier in weight we've tested, and also one of the warmest, despite them being only 3/4 in length.
Credit: Justin Simoni

Breathability


We awarded great marks to the Intraknit Thermal for breathability. In general, both wool and synthetic polyester fabrics have good breathability traits for their warmth, which is why they're used in all the base layers we've tested. The seamless Intraknit panels of this layer attempt to target specific areas to be more breathable where it's most useful. Ventilation panels help with allowing you to wick excess moisture from your lower back and crotch area, as keeping dry is a key to staying warm and comfortable. The 3/4 length may also be preferred so you don't have to double your thermal base layer over socks.


Wearing these bottoms in a variety of situations, including trail running, mountaineering, backpacking, and while sleeping snug in our bivvies, we were happy with the breathability performance and overall experience. It makes us believe the hype around the Intraknit tech, especially when compared to how we've felt with other thick, 100% Merino wool bottoms. We suggest these bottoms for when the temperature dips down, but your activity level stays high: things like cross-country skiing, winter bike rides, and snowy peak bagging.

smartwool intraknit thermal merino 3/4 bottom long underwear review - breathability is enhanced by the panels on the outside of the...
Breathability is enhanced by the panels on the outside of the thighs, lower back, and crotch that allow more ventilation than the rest of the fabric panels.
Credit: Justin Simoni

Comfort and Fit


The fit of the Intraknit Thermal Bottom is unique. Expect them to feel much different than other full-length wool long underwear, whose fit may not be as snug. If you prefer to be hugged tightly by your thermal bottom, these are for you. A size medium was true-to-size for our main tester, whose cross-the-country cycling legs define the slim, athletic fit the manufacturer advertises the cut for. In fact, from a distance, these bottoms could be mistaken for cycling tights or compression leggings. The thick, 2" waistband creates a higher-than-normal-for-men waistline, keeping them from sagging, and the ¾ length helps avoid bunching. A crew-length sock will just about reach the bottom of the Intraknit Thermal. There's no additional elastane at the bottom of these, as they'll rest right in the middle of your calf.


The characteristics of the Merino wool blend vary, depending on the placement of the differing seamless panels. For example, the gusseted crotch, back, and side are more breathable, less warm, and feel slightly scaly as you run your hand over the ventilation holes. The knee panels are ever so slightly ribbed and more elastic to allow articulation for bending and moving. The inner thighs are composed of a much softer and warmer fabric. Taken all together, the Intraknit Thermal feels a little less soft than a pure, 100% Merino wool bottom, given that almost 50% of the materials used are synthetics.

smartwool intraknit thermal merino 3/4 bottom long underwear review - seamless paneling on the knees of the intraknit are made of a more...
Seamless paneling on the knees of the Intraknit are made of a more flexible fabric, helping with articulation. The 3/4 length ends at about the same place a crew-length sock will meet up.
Credit: Justin Simoni

Durability


Surprising to us, the Intraknit Thermal stands out as being a more durable pair of bottoms than every other wool and wool blend we've tested — though they still can't really hang toe to toe with the synthetics. The softer and warmer panels are located on the inner thigh area, away from most abrasive surfaces you're likely to encounter. This durability can like be attributed to the percentage of Merino wool used: just over 50%, with the rest being synthetics. This isn't to say you should bring these out to wear when surrounded by abrasive rock and shrubbery, but wearing these alone without a top layer in more casual settings gets our thumbs up.


The high waistband is the thickest we've tested at a full two inches and should perform and last a long time. Flatlock seams are found exclusively throughout these bottoms. The complex Intraknit fabric also seems to be woven in a particular way where two layers are almost sandwiched together, meaning that a tear on the fabric's exterior won't create a hole that goes through entirely.

smartwool intraknit thermal merino 3/4 bottom long underwear review - the thick, 2" high-rise waistband is bombproof. the dual layer...
The thick, 2" high-rise waistband is bombproof. The dual layer nature of the fabric doesn't resist tears entirely, but tears may only go through the exterior layer and may then be more repairable with a simple patch.
Credit: Justin Simoni

Drying Speed


Our on-the-line drying lab tests reveal that this isn't the strongest suit for the Intraknit Thermal. We chalk this up to the unique sandwiched fabric, which has great wicking qualities and thus good breathability, but the extra material can't purge all that moisture as quickly. The very thick and comfortable 2" waistband also soaks up moisture and holds onto it even after the rest of the bottom has dried.


Smartwool recommends not using a commercial dryer for the Intraknit and to always hang dry instead. Along with the wool used in the fabric blend, which can shrink and lose its shape, the fabric includes a small amount of elastane that will degrade with repeated tumble drying. Back-to-back days of wearing these bottoms didn't reveal any out-of-place stink, so you can probably stretch out washings and just air dry overnight to extend the life of these bottoms.

smartwool intraknit thermal merino 3/4 bottom long underwear review - hang drying may take a little longer for the smartwool intraknit...
Hang drying may take a little longer for the Smartwool Intraknit than other Merino wool bottoms.
Credit: Justin Simoni

Layering Ability


Pile 'em on! The fit of the Smartwool Intraknit is almost compression-tight, moving along with your body as you move through your environment. Certain panels of fabric, such as on the knees, are designed to enhance ease of movement by being more flexible. Putting on an array of different additional layers (from running tights to hiking pants to mountaineering shells) didn't bring up any problematic points, as the Merino wool/synthetic blend fabric slid under most anything with ease. Only the most scratchy of mid-layers, like a grid fleece, may prove a troublesome pairing.


The 3/4 length version we tested means that socks and these bottoms don't have to fight over who gets to cover your lower legs. This can reduce bunching in an otherwise problematic area if you're wearing high winter boots. What may make the Intraknit Thermal most unique is the thick, durable fabric and no-fly front — meaning you could potentially wear this piece alone in more casual situations. The no-fly may be a dealbreaker for some, so be aware.

smartwool intraknit thermal merino 3/4 bottom long underwear review - the smartwool intraknit thermal merino 3/4 bottom layer well...
The Smartwool Intraknit Thermal Merino 3/4 Bottom layer well, integrating with other thick layers like a good warm wool sock.
Credit: Justin Simoni

Should You Buy the Smartwool Intraknit Thermal 3/4 Bottom?


As we've mentioned, these bottoms are tight and compression-like. Our main tester comes from a cycling and trail running background where such bottoms are typically used, so the Intraknit Thermal felt like second nature. If you're not used to such a slim, athletic fit, it may take some getting used to. We think it's worth it, as such a tight fit makes layering easier. We weren't initially sure about the 3/4 length (a full-length model is also available), but now that we've tried them out, we can see the appeal for high-output activities like trail running. If high exertion also means high perspiration for you, these may be the perfect pair, as they pack both good warmth and breathability without overheating.

smartwool intraknit thermal merino 3/4 bottom long underwear review - the 3/4 length helps prevent bunching up at the bottom of the leg...
The 3/4 length helps prevent bunching up at the bottom of the leg where thermal bottoms, socks, and boots meet.
Credit: Justin Simoni

What Other Long Underwear Should You Consider?


If you think the Intraknit Thermal will be too warm, consider the Ortovox 185 Rock'N'Wool Short Pant, which also features a 3/4 length, but comes in at a 185 g/m² 100% Merino wool thickness instead of 200-250 g/m². If more ventilated panels are what interests you, but you would like a full-length bottom, look into the Icebreaker BodyfitZone Merino 260 Zone Thermal Leggings, which have more breathable panels in the gusseted crotch, back, and all around the knees.

Finally, if you're looking for a base layer top to complement these bottoms, we also loved the Smartwool Intraknit Thermal Merino, which has similar seamless panel technology. The fabric on this top is softer than on these bottoms, which may make them warmer for their weight, but not as durable. The fit for the top is not compression-tight but much more relaxed and conventional.

Justin Simoni
 
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