Smartwool Classic Thermal Merino Base Layers Review

Whether you’re new to merino wool or you’re a devoted gear-head with a dialed-in pack, I’m revisiting a staple of Smartwool’s repertoire: the Classic Thermal Merino Base Layers. You may know these popular picks by their previous name, the Merino 250 Base Layers. Although the name has changed, the idea hasn’t: a fantastically versatile layer for cool to cold weather. But is merino wool worth the hype, and do the base layers really stand up to thorough testing on trail? Read on to find out.

smartwool classic thermal merino

Smartwool Classic Thermal Merino Base Layers At-a-Glance

Classic Thermal Merino Bottom

  • MSRP: $110
  • Weight: 7.34 ounces
  • Material: 100% merino wool

Classic Thermal Merino Crew

  • MSRP: $110
  • Weight: 7.69 ounces
  • Material: 100% merino wool

Circumstances of Review

I rolled with these base layers on the 350-mile Appalachian High Route from August through late September in the southern Appalachians. These layers were the only set of clothes I brought besides my daily hiking gear and, besides my down jacket and booties, were the only cold weather gear I had in my kit. Over the course of the hike, I experienced hot, sunny road walks and characteristically stormy weather in the Smoky Mountains that left me blue-lipped and nearly shivering when I arrived at camp.

Merino 150 vs 250: What’s the Deal?

If you’ve kept up with Smartwool over the last few years, you’ll be as surprised as I was to learn that things have changed in the world of merino wool naming conventions. Originally this review was titled 250 Merino Wool Base Layers – so what’s the difference now that they’re “Classic Thermal?” The short answer: nothing at all. 

Smartwool is moving away from the 150 and 250 labels, which indicated the weight of merino in the garment in grams, i.e., 150g or 250g. Instead, the new names emphasize the base layers’ intended use. All Smartwool 250 products are now labeled “Classic Thermal,” while the lighter 150 products have been rechristened “Classic All-Season Merino.”  To learn more about Smartwool’s fabric construction, check out this link.

READ NEXT – Gear Wars: Merino Wool vs. Synthetic Base Layers


100% Merino Wool – Excellent next-to-skin feel with an interlock knit for enhanced comfort, breathability, thermoregulation, and odor resistance. 

Flat Seam Construction – Designed to minimize chafing.

Raglan Sleeves (Top), Mid-Rise (Bottom) – Increased mobility. 

Merino-Covered Elastic Waistband (Bottom) – Comfortable next-to-skin fit.


These base layers were excellent as camp clothes. They kept me warm and comfortable in my 20-degree bag and even motivated me to brave the trail early on particularly chilly mornings. As someone who gets chilled by wind on a summer day, the effectiveness of these layers at camp was paramount, and they certainly delivered. Though they didn’t last long once I started moving, on colder days, I would feel comfortable hiking in these layers as long as was necessary before generating enough heat to don my hiking clothes.

Their versatility is notable, as evidenced by the fact that even after three weeks of living in them, I opted to wear them on the plane ride home (because let’s be real, airplanes can be ridiculously cold). Though I wore these layers next-to-skin, you can also add them to your layering repertoire for ultimate warmth through the seasons.

Piece by Piece Performance

smartwool classic thermal merino

Classic Thermal Merino Crew 

Overall, I had a fantastic experience with the Thermal Crew Base Layer. It kept me warm when I needed it and had a next-to-skin feel I found myself craving as a reward after a long day of hiking. The raglan sleeves left space enough that I didn’t feel restricted and allowed for plenty of mobility at camp and on trail. At nearly eight ounces, I didn’t add this top to my kit lightly, but I can confidently say it’s been worth its weight several times over for the use I got out of it on trail. 

Also, the “Blueberry Hill Heather” was a fantastic color choice.

smartwool classic thermal merino

Classic Thermal Merino Bottoms 

Though they kept me satisfyingly warm (an accomplishment for any piece of gear added to my kit), I had a similar issue with the Thermal Bottoms that has plagued my search for the ultimate base layer leggings for years: stretching over time. After spending 2-3 evenings in the bottoms at camp, I found slack in the bottom, waist, and thighs that would’ve made hiking in them difficult if I’d gotten the whim.

Thankfully, a trip through the washer is all this layer needs to reset the fit to like-new standards. I found myself looking forward to laundry day to reset the tighter fit of the bottoms almost as much as I desired to no longer smell like I’d been out in the woods for a week (but truly, who doesn’t grow to love their own adventure funk?)

Despite the issue, I’ve never found a base layer bottom that has held up my skin-tight preference throughout a thru-hike, so take this judgment with a grain of salt. (Smartwool IntraKnit base layers feature a merino/polyester/elastane blend that may hold its shape better for active wear.)

Minus the slipping issue, these leggings were fantastic on trail. They were as comfortable as the Classic Thermal Crew and kept me warm when I most needed them, so even with my religiously ultralight kit, I was always happy to toss them in my clothes bag. 

Smartwool Classic Thermal Merino Base Layer Pros 

  • Excellent thermoregulation
  • Overall fit
  • Comfortable and soft next-to-skin feel 
  • Versatile
  • Quick drying 

Smartwool Classic Thermal Merino Base Layer Cons

  • Price
  • Stretching with use may cause loose fit until laundry day (bottoms)

Final Thoughts

All things considered, these base layers are a fantastic option for your next cool-to-cold weather adventure. If the price or weight are driving you off, consider that these may be the clothes you live in every spare moment you’re not actively hiking on your next adventure – like me, you may find yourself looking forward to one more reward at the end of the day with these fantastic layers in your pack. 

Shop the Classic Thermal Merino Crew – Women’s Shop the Classic Thermal Merino Crew – Men’s

Shop the Classic Thermal Merino Bottoms – Women’s Shop the Classic Thermal Merino Bottoms – Men’s

Comparable Base Layers

REI Co-op Merino 185 Long-Sleeve Base Layer Top and Bottom

  • MSRP: $79.95 (each)
  • Weight: “Lightweight” (actual weight not provided)
  • Fabric: 100% merino wool

Icebreaker Sonebula Base Layer High Neck Top and Bottom

  • MSRP: $120 (each)
  • Weight: 6.3 – 8.6 oz, size and article dependent
  • Fabric: 100% merino wool

Patagonia Capilene Midweight Base Layer Crew and Bottom

  • MSRP: $79 (each)
  • Weight: 5.5 oz (each)
  • Fabric: 100% recycled polyester

These Smartwool Classic Thermal Merino Base Layers were donated for purpose of review.

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