Smartwool Classic Thermal Merino Bottom Review

The Smartwool Classic Thermal Merino Bottom is a 250 g/m² thick 100% Merino wool pair of full-length long underwear that we've found excels overall in warmth and comfort. Excellent thermoregulation and breathability mean you won't be sweltering in these bottoms when temps start to rise. Layer on up starting with these snug bottoms, and know that they'll be compatible with whatever you pair them up with. The thick waistband, gusseted crotch, and flatlock seams throughout are just some of the standout features you'll find that make these some of the best thermal bottoms we've had the privilege of putting on.

Performance Comparison

Warmth

Constructed to excel in harsh winter conditions, the Classic Thermal Bottom boasts a fabric weight of 250 g/m², the heaviest in Smartwool's lineup. The 100% Merino wool is thick cut, making these bottoms much more plush feeling next-to-skin than comparably weighted models we tested. These pants are plush, an ideal companion for full days spent outside through the coldest months of winter.

Boasting 250 g/m² of thick and cozy 100% Merino wool, these bottoms are constructed to excel in harsh conditions. They do a superb job of wicking away sweat when you're working hard, and they dry quickly against your skin so that after skinning endlessly for the perfect line, you're not soaked and freezing when you finally reach the windblown and frozen summit.

The exterior of Merino wool fabric is naturally hydrophobic, which keeps these base layer pants warm and comfortable throughout the day, regardless of how intensely you are working up a sweat. The thick Merino wool traps just enough heat to allow this layer to dry against your skin quickly. Even through 6,000 vertical feet of skinning and climbing to reach the summit of Mt. Hood on a mid-winter ascent, the Classic Thermal Bottom was still warm and dry by the time we reached the summit. It is important to mention that while these bottoms are warm, they are not expedition-weight warm. So if you're planning a trip north of the Arctic Circle, buy something even thicker.

Breathability

What continues to impress us about these bottoms is how aptly they balance top-notch insulation with near-top-notch breathability. Functional thermoregulation is not a common trait amongst base layers, but the Classic Thermal handles that task masterfully.

The special interlock knit used in the design of these bottoms sets them apart from the competition. The lattice-like knit adds additional loft, supplementing the natural breathability of Merino wool fibers with a microclimate that uses heat to force moist air up through the fabric, where it can evaporate on the surface. Unlike other thick layers that hold onto sweat and become clammy once you stop moving, these avoid becoming sweat-soaked by harnessing the power of simple thermodynamics. The harder you work, the more powerful the temperature/pressure gradient between your skin and the outside air, and the faster these bottoms dry out.

Comfort and Fit

The Classic Thermal has effectively won over the hearts of all of our testers — when it comes to pure comfort, these bottoms are heavenly. Not only are they warm and cozy, but the 100% Merino knit is thick, soft, and stretchy. Smartwool designers seem to take this fact with pride, and this updated version thoughtfully lines the interior of the extra-wide waistband with Merino wool to increase comfort further and cut down on a common spot for irritation. The entire construction is supported by flatlock seams that are specifically designed to minimize rubbing and chafing, giving you even more reason to wear these bottoms directly next to skin.

We often wear these as true long underwear, opting to comfortably layer them under our jeans during the coldest months of winter. The natural elasticity of Merino wool makes these bottoms a versatile option to prepare for those extra-cold moments, like freezing your buns off standing outside. The material is stretchy enough to pull over a lighter-weight base layer and yet somehow snug-enough fitting to wear below a mid-layer pant without any constrictions.

A minimalist design incorporates a gusseted crotch, allowing for a single seam to run the length of the inseam without losing out on flexibility. While we've dealt with saggy-bum issues in the past, the current version of these long johns takes the same gusseted design from the front and applies it to the back of the pants. Now you don't have to pull these bottoms up past your belly button to secure a snug fit.

Durability

While Merino wool is reputed to have durability issues, modern knitting and design techniques have improved upon concerns. The Classic Thermal is the thickest Merino layer in our review, easily landing them among the most durable all-natural options we tested. Still, our harsh abrasion test was able to tear holes in the fabric. For snags and punctures, these bottoms will simply not be as durable as a synthetic option. Still, they offer breathability, wicking capabilities, and resist odor build-up very well.

The main way you can protect your purchase and invest in the longevity of a Merino base layer is through proper laundering. By carefully washing, and most importantly, flat drying, you will likely have the Classic Thermal Bottom as your go-to cold weather bottoms for years. While you may experience them stretching out between washings, don't feel inclined to shrink them up in the dryer — that is the surest path to an early grave for this high-quality layer.

Drying Speed

The Classic Thermal did well in our on-the-line drying tests, especially given the 250 g/m² of the 100% Merino wool fabric. Merino wool — especially at this thickness — will dry slower than a synthetic, and the heavier weight of this wool means it'll never win a race with lesser-weighted materials. Still, we're more than happy with the performance, given all the other top marks these bottoms have won.

The manufacturer's recommendations do note that you can tumble dry the Classic Thermal Bottom on low, but we would opt to hang these up to dry, wherever that's realistic, to extend the life of the product. Given our experience, these should dry out easily overnight, no matter if you're in a ski hut next to a roaring fire or just outside on a clear day.

Layering Ability

The snug but not overly tight fit of the Classic Thermal Merino Bottom makes them excel as a next-to-skin layer, where you can then add a mid and outer layer. These are the thickest base layer this manufacturer puts out, so you most likely don't want to add another base layer-like bottom underneath or over.

These bottoms are best for when the temps drop to winter-like conditions and will most likely be too warm for summertime climes. That said, good breathability means you can stretch the usable months of these bottoms and wear them even if the weather gets warm without worrying that you'll become a personal sweat lodge.

Should You Buy the Smartwool Classic Thermal Merino Bottom?

These leggings offer superb warmth and thermoregulation, wrapped up in an uber-comfortable package. For those that need an extra boost of warmth to get them through the winter, these are a great investment that will prove worthwhile even after the mercury rises from sub-freezing temperatures. The quality of construction and sheer ability to balance warmth and breathability warrants the price tag.

What Other Long Underwear Should You Consider?

If the price of the Classic Thermal is too high but you would like the warmth of a 250 g/m² bottom, look into the no-nonsense Meriwool Merino 250 Bottom, which makes a great entry-level warm Merino wool base layer. If the Classic Thermal is too warm for your needs, grab the Smartwool Classic All-Season Merino Bottom, which features a 150 g/m² weighted Merino wool fabric and a very similar design. Finally, if you'd rather go with a synthetic bottom, look no further than the REI Co-op Midweight Bottoms. These won't be as warm, but they are much more durable.

If you're looking for a top base layer to perfectly complement these bottoms, we recommend the Smartwool Classic Thermal Merino 1/4 Zip, which utilizes the same 250 g/m² fabric as the Classic Thermal bottoms.