Smartwool Intraknit Thermal Merino Base Layer Review

Bottom Line

The Smartwool Intraknit Thermal base layer is similar to Smartwool’s other thermals but takes things up a notch to improve its performance during aerobic activities. It’s lightweight, dries quickly, has excellent thermoregulation, and provides superior mobility. We thoroughly tested this layer by running, backpacking, and working up a sweat in the cold Rocky Mountains. We were not disappointed.

This layer is intended to keep the body dry and comfortable with mesh ventilation zones seamlessly integrated into gender-specific areas. Additionally, these zones are surrounded by a unique knit pattern that enables the fabric to stretch and compress. Despite its slim fit, this is one of the least constricting base layers that we tested.

The high-tech design of Smartwool’s Intraknit line will be appreciated by runners, climbers, and anyone needing tip-top performance. However, it comes at a higher price. If you decide to buy one of these base layers, be sure to take good care of it because the uniquely knit wool fabric is quite delicate.

Comfort

The Smartwool Intraknit Thermal Merino Base Layer offers comfort through its mobility, thermoregulation, and breathability. The wool fabric is slightly itchy, but the seams and integrated mesh zones are completely unnoticeable.

The top shoulder seams aren’t rolled forward like other base layers, but they still don’t cause any unwanted rubbing when worn with backpack straps.

Because the Intraknit is designed as an active layer, it has a snug, athletic fit that some may find runs a bit small. However, 3D mapping and engineered construction improve mobility in key areas.

Smartwool uses unique knit structures in the elbows, wrists, forearms, shoulders, and waist to increase freedom of movement while reducing weight. This creates a slimmer fit without feeling constrictive or bunching up under layers. The layer also has plenty of tail to tuck in, helping to keep the top securely in place.

Warmth

The Intraknit Thermal achieves some of the highest warmth-to-weight ratios of any layer we tested. The merino wool has natural thermoregulation and moisture-wicking properties, keeping the wearer dry and comfortable.

The slim fit helps trap heat, and although the integrated mesh zones are thinner and more breathable, they are not placed in key heat-retaining areas. The Intraknit provides enough warmth on its own for brisk fall walks or winter trail running, but the snug fit is sleek enough to layer smoothly under additional mid and insulating layers.

Overall, the Intraknit Thermal is a cold-weather base layer. Despite its athletic and breathable look, it’s best for high-intensity activities when temperatures drop. Luckily, the Intraknit Thermal is just one product in the Intraknit line, and Smartwool offers this high-tech layer in a more moderate-weather-friendly option.

Breathability

Breathability is the cornerstone feature of the Smartwool Intraknit thermal, and ultimately the reason for its high price. Smartwool has incorporated mesh zones and looser knit structures in important, gender-specific areas. 

For men, these zones are on the shoulders, elbows, and down the middle of the back. The women’s version has similar zones but adds a crisscross pattern along the midriff and a back design that mimics a sports bra. 

Through the mesh zones and the natural qualities of merino wool, the Intraknit is one of the warmest and most breathable base layers we tested. It easily compares to the most breathable, lightweight synthetic layer in our drying test.

While most layers aim to balance warmth and breathability globally, the Intraknit makes no compromise, tailoring heat retention and breathability only where it’s most needed. It’s quite nice to feel the breeze penetrate only in the areas where you are sweating the most, while the rest of your body stays warm.

Versatility

Although the Intraknit Thermal is highly breathable, we wouldn’t necessarily consider it a year-round base layer. The breathable mesh zones help keep the body dry and let in more breeze, but it remains very warm.

This base layer is ideal for moderate to high-intensity activities in cold weather. It works great as a standalone layer for winter trail running since it dries quickly, and it’s also perfect for snowshoeing, cross-country skiing, and winter camping. It could even serve as a good camp shirt for spring to fall backpacking. 

Overall, the Intraknit is excellent at what it’s designed for, but it’s not a all-in-one base layer.

Style

The Smartwool Intraknit Thermal probably isn’t our first choice for a stylish outer layer. It features an active fit that hugs the body, a long tail, and technical-looking knit patterns. It also lacks aesthetic touches like a neck zipper or hood.

The simple crew design and slim fit are intended to fit close to the skin and keep you warm rather than make a fashion statement. You might get more casual use out of it by sizing up, but you’ll lose some of the layer’s practical qualities. 

The Intraknit Thermal is available in six different color options for both men and women. Although some of the colors are quite similar, most styles are well represented. If you’re on a budget and need a versatile base layer that can also be worn casually, the Intraknit Thermal might not be the best choice for you.

Should You Buy the Smartwool Intraknit Thermal Merino?

The Smartwool Intraknit Thermal is perfect for active adventurers needing a base layer for high-intensity winter activities. This includes everyone from mountaineers to trail runners, skiers, and ice climbers. Of course, it will keep you warm in winter regardless of the activity, but for the price, it’s overkill for the casual lodge-goer. 

The Intraknit Thermal really stands out for its breathability and moisture-wicking capabilities. Other layers might be just as warm but won’t dry as quickly. In certain situations, a wet base layer could be be a life-or-death matter. 

The mix of merino wool, polyester, and elastane provides many advantages. Wool gives the layer natural odor resistance and moisture-wicking properties, polyester adds warmth and durability, and elastane enhances mobility. 

The Intraknit Thermal is definitely not a budget choice, and the costly technical features are less essential for casual or moderate activities.

What Other Base Layers Should You Consider?

If the Intraknit Thermal isn’t what you’re looking for, check out our full guide of best base layers.

REI Midweight Half Zip Review: The REI Midweight Half Zip is a warm base layer that costs nearly half as much as the Intraknit Thermal. It doesn’t have the same level of breathability, but it features a neck zipper for releasing excess heat and convenient thumb loops. 

Smartwool Classic Thermal Review: The Classic Thermal is similar to the Intraknit Thermal but lacks the extra technical features. It features a simpler design, is just as warm, and is a bit more affordable. 

Smartwool Classic All-Season Review: The Classic All-Season is a great choice if you’re seeking more versatility. Likewise, it features a minimalist crew cut design and merino wool blend, but functions more as a four-season base layer.