Smartwool Intraknit Thermal Merino 3/4 Bottom Review

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

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.

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.

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.

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.

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.

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.

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.

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.