When A Merino Wool Base Layer Isn't the Answer
Nov 20, 2024
Table of contents
I love activities that get my heart pumping; in the winter I get my ya-ya’s out by cross-country skiing, ski touring, and running. Where I live in North-Central Washington, where winter temps are often around 20 degrees Fahrenheit, I’ll usually wear a base layer and a windbreaker jacket or lightly insulated jacket (like the Arc’teryx Atom Lightweight) for my daily winter workout.
Now, merino wool is said to be very moisture-wicking, which sounds like a great quality for sweat management. And it’s true: It has hydrophilic properties that cause it to readily absorb sweat from the skin. However, once merino absorbs moisture, it is very slow to dry. So the process goes like this: You sweat, your base layer gets wet, your base layer stays wet. This can result in chafing and discomfort, but that’s not the worst part. The real kicker is that merino actually heats up when wet, thanks to a chemical reaction known as sorption.
Now, for start-stop activities like ski touring, this is a miracle of a process. You sweat your shirt out on the skin track, and now it’s providing extra insulation for you as you transition to the downhill. Great news. But for sustained activities like an hour-long run or ski marathon, it’s a downward spiral of sweat, overheat, sweat, overheat, sweat, overheat—you get the picture. And if it’s happened to you, you know there’s something special about the experience of overheating in a merino base layer: Things can quickly go from warm to too warm, and suddenly you feel like you need to crawl out of your skin. The word “claustrophobic” comes to mind.
Suffice it to say, I don’t like merino for activities where I know I’ll be building and sustaining heat, even in super cold conditions. In contrast, synthetic materials also wick sweat away from the skin, but they’re hydrophobic, meaning they don’t hang onto the moisture (and certainly don’t grow warmer when wet). Thus, you can avoid the downward warming spiral of merino, stay dry, and maintain a more consistent level of warmth throughout your activity.
The same goes for mild temperatures. No matter its weight, merino is instantly warming when you put it on, and it’s much warmer than a synthetic material of similar weight. I welcome this warmth in sub-freezing temperatures, but it can quickly become too much when temperatures creep up. In mild-to-warm temperatures (around the range of 50 degrees Fahrenheit or above, depending on the weight of your base layer), a merino base layer might simply be too warm.
And there’s a second level here: If you do choose to wear a merino base layer in above-freezing temperatures, you might find yourself stripping off your outer layers to dump heat (this is especially true if you partake in prolonged rigorous activities). Some merino base layers can pass as standalone pieces, especially those with relaxed fits and added structure from nylon or polyester. Other merino base layers, on the other hand, look and feel like underwear (they don’t call it “long underwear” for nothing), with skin-tight sizing and thin materials. Not only is this not an ideal look, but it’s not a great long-term plan for the merino, which will have a much longer lifespan if hidden beneath a mid or outer layer.
After wearing countless base layers during high-output activity or mild weather, I have found one key exception to my merino wool rule: hybrids. Hybrid merino base layers blend merino wool with another base layer material (usually polyester or nylon) to offer the best of both worlds. You get the coziness and odor resistance of merino tempered by the synthetic materials’ more moderate warmth and hydrophobic nature.
The Smartwool Intraknit Thermal is a great example. Its fabric is composed of 53% merino, 45% polyester, and 2% elastane. It has a skin-tight fit and mesh vents throughout, which help to expedite the flow of heat and sweat away from the body. After swearing off merino base layers for cross-country skiing, I was convinced to try the Intraknit Thermal and was blown away. It doesn’t overheat or get bogged down with sweat and is now one of the base layers I reach for most for high-output days in mid-winter. (For more, read our full Intraknit review.)
A few other notable hybrids include the Ortovox 120 Comp Light, which combines 70% merino with 30% nylon and, like the Intraknit, features body-mapped mesh. The Black Diamond Solution 150, made with Nuyarn, is another standout. Nuyarn is an innovative merino blend that claims to have a five-times faster dry time than 100% merino. I’ve worn the Solution 150 and can confirm that it’s a great layer to sweat in, although I wish the Solution had integrated venting (for more, read our in-depth review of the Black Diamond Solution 150). Nuyarn is also available in a range of other products, including the Artilect Flatiron 185 and Le Bent Feathertop Ultralight.
Despite my aversion to pure merino base layers for high-intensity activities and mild conditions, the fabric is still my number one choice in many settings. These settings include:
Low-output activities in cool-to-cold temperatures
Resort skiers, this one’s for you. If you’re out all day in winter weather and staying warm is your top priority, it doesn’t get much better than a merino base layer. Merino is cozy next to the skin, can absorb about a third of its weight before it feels wet, and doesn’t retain odors. What’s more, it’s significantly warmer than synthetic materials. You won’t notice any of its shortcomings if you keep your heart rate down, and you can hedge your bets by matching your base layer weight to the outside temperature (i.e. in spring conditions, go with a thinner fabric).
Multi-day trips
Merino truly shines on multi-day trips, whether you’re ski traversing, backpacking, or traveling internationally without access to laundry. It simply does not retain odors, thanks to its antibacterial and antifungal properties. I’ve tested this to its full extent, recently wearing a merino shirt nonstop during a week-long backpacking trip followed by two weeks of traveling. Where a synthetic base layer would have gathered all sorts of foul smells, my merino still smelled fresh after three weeks of constant wear. As long as you’re not stringing together multiple days’ worth of high-intensity activity (during which you’d just be sweaty and uncomfortably hot), merino is the best choice.
Start-stop activities
I mentioned that merino is best suited for low-output activities that don’t generate too much sweat, but it’s also great for keeping a steady body temperature during start-stop activities in cold temps. Ski touring is the best example of this: You go ham on the skin track and build heat, which a well-matched merino base layer can handle until a certain point (as we learned, merino can absorb about a third of its body weight before it feels wet). Then, as you transition to the downhill, your body heat dissipates, but the merino continues to provide warmth. If you keep your output moderate or stop occasionally to cool off, merino can provide balanced warmth.
Warmth: Merino
Breathability: Synthetics, merino/synthetic blends
Durability: Synthetics, merino/synthetic blends
Comfort: Merino
Odor Resistance: Merino, merino/synthetic blends
Quick-drying: Synthetics
If you’re getting out in cold temperatures, a base layer is an essential tool to have in your kit. That said, it’s important to match your base layer wisely with your activity level and weather conditions. In our guide to the best base layers, we’ve assembled our favorite base layers and ranked them based on warmth, breathability, durability, odor resistance, comfort, and sustainability.
stay warm, manage sweat
Gear Guide
Best Base Layers of 2025
Education
How to Layer for Skiing
Gear Guide
Best Down Jackets of 2025
