These Expert-Recommended Merino Wool Base Layers Will Keep You Warm During Cold Runs

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When it comes to base layers, the main choice is whether you want a synthetic (usually polyester) layer, or a natural fiber like Merino. Historically, Merino has been heavier and held moisture longer than synthetics, but each year the performance gap in those categories grows smaller. Merino tops are more durable than ever, and with so many different merino-poly blends to choose from, there’s an option for every runner and every anticipated condition.

Merino is an excellent option for all temperatures, but especially for cold-weather runs. This natural fiber is soft enough to wear comfortably next to skin and is often blended with other technical materials for added wicking and durability. The material functions much like it does on the sheep it’s sourced from: regulating temperature, pulling moisture away from the skin, and retaining warmth even when it’s wet.

It’s also naturally odor-resistant, so there’s no need for a synthetic odor treatment. Depending on the fabric weight and fit, Merino wool makes for a high-quality base or midlayer.

Merino wool base layers are an obvious choice for running in the cold, as well as cross-training outdoors, camping, or wearing around town. Most high-percentage Merino items will take longer to dry than synthetic fibers, so take a peek at the material blends and your anticipated output before committing to a certain item.

Best Merino Wool Base Layers

• Best Lightweight Layer: Tracksmith Brighton Base Layer

• Best Midweight Layer: Smartwool Classic Thermal Merino Base Layer Crew

• Most Versatile: Icebreaker Merino 200 Oasis Long-Sleeve Half-Zip

• Budget-Friendly: Meriwool Merino Wool Base Layer

• Maximum Mobility: Smartwool Intraknit Thermal Merino Base Layer Quarter-Zip

What to Look For

Materials: Although you’ll probably see Merino listed in product names, many Merino wool layers are actually wool blended with polyester, nylon, or other materials. This combination increases durability, sheds weight, and generally speaking, reduces cost. There are plenty of 100-percent Merino wool options, too. These typically feel softer to the touch and will have more odor-resistance and warmth.

Fabric Weight: By virtue of its temperature-regulating property, Merino wool apparel is incredibly versatile. Of course, I don’t recommend wearing a heavy long-sleeve shirt on a blazing summer day, but a short-sleeve Merino-blend shirt will wick sweat as well as a synthetic option.

Pay attention to fabric weight, sometimes reported in grams per square meter (GSM), which can help you select the right level of insulation. Choose a 150-gsm Merino top or lightweight poly-blend shirt for brisk days as a next-to-skin active layer. For below-freezing days or for warming up post-run, wear a heavier weight running top with 200- to 250-gsm Merino wool. Merino tops at or above 200-gsm are often marketed as “mid weight” or “heavyweight.”

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Style and Fit: A true base layer fits snugly as you wear it next to your skin. For most people, Merino wool base layers feel soft enough to wear without anything underneath, and they are warm enough to wear alone on 40- to 50-degree days. A midlayer is usually heavier with a looser fit, perfect for wearing over a next-to-skin layer. These are great for colder temperatures and can be paired with a jacket or vest on lower intensity cross-training days.

How We Evaluated

I’ve run and hiked for years in Merino wool base layers. During winter and shoulder season runs, my go-to is Tracksmith’s Brighton Base Layer. And on a 200-mile hike, I wore this Isobaa Hoodie. I tested all the Merino wool layers below, evaluating the breathability, comfort, value, and seasonal use of each in the process. Whether you want a natural next-to-skin top, a Merino-blend midlayer, or a do-it-all base layer, there’s something on this list for you.

Brighton Base Layer

The Brighton features Tracksmith’s Merino mesh, which has a more open weave around the core to increase airflow and a tighter knit on the sleeves to keep extremities warm. The specific combination of wool, nylon, and polyester is designed for optimal wicking, odor-resistance, and performance during higher output activities.

This is a go-to active layer for cooler weather, designed to vent where you need it without sacrificing too much warmth. Wear the true-to-size top as part of a layering system or on its own.

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Classic Thermal Merino Base Layer Crew

This recently updated piece has flatlock seams for chafe-free wear under a running pack and is specifically designed for sweat-inducing activities. This is one of the heavier-weight models in Smartwool’s base layer lineup, designed for getting in your miles in colder temperatures.

The 250-gsm fully Merino wool construction drives up the cost, but this is a dependable and warm option for below-freezing days. The men’s and women’s versions have gender-specific body-mapped panels of lighter fabric to increase breathability and comfort.

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Merino 200 Oasis Long-Sleeve Half-Zip

The 100-percent Merino wool Oasis is equally comfortable as a base or midlayer, as well as during sweaty workouts or while sitting around a fire pit. The tight knit Merino jersey fabric is quite soft and resists pilling and holes, so my Oasis has last season after season. It’s also one of the most stink-resistant tops I’ve ever worn.

The hem features a drop tail for added coverage, and the flat seams and underarm gussets are comfortable during repetitive motion (ahem, your running stride).

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Merino 190 Long-Sleeve Base Layer

Merino and other natural fibers have a higher baseline price point compared to many synthetics, but this Meriwool long-sleeve is a reasonable price for a 100-percent Merino wool base layer.

The lightweight layer, which is good for moderate output on cool days, has a soft-knit construction for a comfortable next-to-skin feel and a crewneck design that won’t get in the way of other layers. Because it has more of a performance fit, I recommend sizing up if you’re looking for a looser cut.

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Intraknit Thermal Merino Base Layer Quarter-Zip

Smartwool designs its Intraknit line to maximize movement, with a 3D-mapped fit based on the kind of freedom bodies in motion need to not feel constricted. This means fewer seams, an articulated construction, and more of a performance cut than you’ll get with some of the brand’s other layers.

This top is excellent for running and hiking but also holds its own with the big arm movements necessary for Nordic skiing. The men’s and women’s material construction is slightly different for a custom-feeling fit, and the quarter-length zipper allows for easy on-and-off, but still has a fairly low-profile neckline that won’t get in the way of a shell.

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LIFA Merino Midweight Crew

This uniquely designed base layer acts as a next-to-skin top while incorporating two types of fiber: a lightweight merino knit and Helly Hansen’s LIFA, a highly specific single-filament design with dedicated wicking technology and a soft feel.

This is a more insulating top than others on this list, ideal for cold-weather runs where you might still work up a sweat. LIFA’s moisture-management combined with merino’s natural quick-dry attributes make this a warm-yet-wicking piece we reach for on cold-weather, high-output runs. Be aware that the sizing can feel large, and some runners looking for a true next-to-skin fit might want to size down.

Capilene Cool Merino Shirt

This ultra-breathable, smooth-as-silk merino top is made with 65-percent sustainably sourced, RWS Merino, and the remaining polyester components are fully recycled fibers. While this shirt is more than half wool, it still feels light and silky, wicking sweat and staying cool next to skin.

The construction was made for movement, with set-in sleeves that won’t pull during long runs with repetitive arm motions, and it has a drop hem that doesn’t shift or ride up. This is a welcome addition to Patagonia’s Capilene line, ideal for runners looking to incorporate natural fibers in their apparel for enhanced thermoregulation and higher odor resistance.

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Boulder 125 Crew Base Layer Top

With remarkable stretch and freedom of movement thanks to the Nuyarn technology, this crew-neck top is a go-to for layering under a wind shell, as a camp base layer, or wearing on its own in moderate temperatures for everything from tempo runs to longer trail outings.

This shirt retains its shape well after many uses and washes, and the Nuyarn gives it an extra heft for relative lightness of a 125-gsm weight. The flatlock seams won’t chafe under a pack or along the sides, and the fit is optimized as a base layer that won’t impede freedom of movement if you layer on a vest or jacket.

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Expert Maggie Slepian on When She Wears and Washes Her Merino Wool Base Layers

What’s your preferred base layer style and weight?

My go-to Merino base layer is a 200-weight long-sleeve crewneck. If I’m wearing a Merino base layer, it means the weather is fairly cold, so I want something that feels substantial but isn’t overwhelming. A 200-gram Merino top can be worn on its own or layered with a wind shell or insulated vest for more protection, so it’s a versatile weight for lots of conditions.

I typically opt for a crewneck style so I don’t feel constricted. My midlayers are often quarter-zip for draft protection, but I like my base layers to be more low profile.

For running, what do you consider to be base layer weather?

For me, base layer running weather is in the low 40s. This might call for a lighter Merino layer, but if I’m leaving the house and feeling chilly, I do want something warm and long-sleeve that will still breathe and wick sweat.

Like I said above, my favorite base layer for running is a 200-gram Merino crewneck, and that usually starts to make an appearance when the mercury hits the 30s. I’m still wearing a Merino base as it gets even colder and supplementing my layer with a wind shell or vest.

How often do you wash your Merino wool performance apparel?

I’ll go a week of runs and hikes in my Merino layers without washing them. Merino is naturally odor-resistant, dries fast, and wicks sweat. When I take off a Merino layer at home and hang it back up, it feels as fresh the next day as it did the day before.

My synthetic layers get washed after every run or hike, but I’ll get out four or five times in the same Merino piece before I feel the need to wash it. No judgment, right?

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