Merino Wool vs. Synthetic Base Layers
Nov 20, 2024
Table of contents
Resort skiing: Sub-freezing temperatures warrant the added warmth of merino, and with resort skiing, you don’t have to be too concerned about generating excessive internal heat. Plus, the odor resistance is a great bonus for lunch and après at the lodge.
Backcountry skiing: Start-stop activities like backcountry skiing highlight the temperature range of merino. Merino’s great breathability and serviceable moisture-wicking should be able to handle the demands of most uphill climbs, and its warmth is a welcome addition when pausing to transition and skiing downhill. And remember: Because of sorption, if the merino layer does grow wet with sweat, it still stays warm.
Low-output winter activities: For activities like snowshoeing or winter walking, where you don’t plan to generate much of a sweat, merino is a warm and cozy next-to-skin layer.
Backpacking (camp clothes): If you aren’t a crazy ultralighter and can afford the extra ounces for dry and fresh bedtime attire, we love bringing merino base layers to the backcountry as dedicated sleep clothes. They offer excellent warmth and stay fresh day after day.
Other multi-day trips: On extended trips where you don’t have access to laundry, merino’s odor resistance is a massive value (for both you and your trip partners).
Everyday winter warmth: In the winter months, you can throw a merino base layer on under a sweater for an extra dose of warmth.
Sustained, high-output activities in cold weather: Synthetics are more breathable, less insulative, and quicker to dry, which is a great combination for any activity that generates a sweat. We prefer them for activities like winter running and cross-country skiing.
Warmer temperatures: When you’re more concerned with moisture-wicking than warmth, synthetics make more sense.
Merino wool is a popular material for base layers, socks, and underwear, and for good reason. You’ll know a base layer is merino the moment you put it on—it’s just that much softer than any synthetic material, and immediately insulating, too. We’ve been blown away by how much warmth this all-natural material provides, even the thinner varieties. Merino wool also breathes decently well and wicks moisture away from the skin (although not as well as synthetic materials), which is exactly what you want from a base layer. Finally, it is miraculously odor resistant. Merino base layers simply don’t get stinky with body odor (we’ve tested this to its full extent), which is an incredible feature to have in a next-to-skin layer.
However, merino wool is not a super strong fiber and can wear down or form holes when subjected to rubbing, sharp objects, or heavy use. Lightweight merino is the worst culprit, but even heavyweight constructions have this issue. What’s more, merino is not a great match for high-output use: It quickly becomes too hot, and it hangs onto sweat rather than sluffing it away (this can become a real issue in cold temperatures). We love merino for slower-paced activities like resort skiing and winter camping, but opt against it for rigorous pursuits like winter running, nordic skiing, and ski touring.
Many base layers are made out of synthetic materials like nylon and polyester. Synthetics are generally more affordable than merino, breathe better, don’t absorb as much moisture, and dry out very quickly. For this reason, they’re our top recommendation for sweat-inducing activities, mild conditions, or those who run warm. Finally, synthetics are quite durable and can handle abrasion and heavy use, making them a great fit for those who wear their base layers without a mid or outer layer overtop.
That said, there are a few notable downsides to synthetics. Nylon and polyester are nowhere near as cozy or warm as merino, and they also tend to absorb odors and not let them go—even after a thorough washing, your synthetic base layer might smell like you’ve been sweating in it for a week. Relatively new technologies like Polygiene, HeiQ Fresh, and ActiveFresh have gone a long way to improve the odor resistance of synthetic materials, but they still can’t match the resistance of merino.
Warmth is one of the primary considerations in choosing a base layer, and merino wool is warmer than synthetics without a doubt. The wool fibers do an excellent job trapping heat and retaining it— even when wet—which is a key reason merino has grown so popular. For this reason, we love merino for most cold-weather activities when warmth is at a premium and aerobic output is relatively low.
Base layers that excel in breathability allow the hot air generated by your body to pass through small holes in their material rather than trapping it against the skin. Both merino and synthetic base layers are porous, with thinner varieties being more breathable than thicker designs (there’s simply less material for hot air to hang around in). But given that synthetics as a whole are less insulative than merino, they tend to breathe slightly better. For this reason, we reach for synthetics for activities that get our heart pumping, whether it’s running, hiking, cross-country skiing, or mountain biking.
You may come across claims of merino’s exceptional cooling capabilities and temperature regulation in warm conditions, which is largely due to its ability to breathe (see above) and wick sweat away from the skin. This excellent wicking power is due to merino’s hydrophilic (i.e. water-loving) nature—it pulls moisture in and hangs onto it. On the other hand, synthetics are hydrophobic (water-hating), meaning they aren’t as efficient at absorbing sweat from the skin.
But merino’s hydrophilic nature becomes an issue when it comes to dry time. Whereas synthetic materials keep moisture on their surface, where it quickly evaporates, merino sops it up and is slow to dry. Not only that, but due to a reaction known as sorption, merino actually gets warmer when wet, which is of course a worst-case scenario for high-output activities. Thus, when we anticipate prolonged, sweat-inducing pursuits, we reach for synthetics. For more on this topic, see our article on When A Merino Wool Base Layer Isn't the Answer.
Perhaps merino wool’s biggest selling point is that it doesn’t retain stink, while synthetics like polyester do. In practice, we have found this to be undoubtedly true. You can wear a merino base layer for long periods—even while exerting yourself and sweating into it—yet after it dries, the base layer will still smell relatively fresh. In contrast, synthetic materials will hold onto stink, even after washings. Merino’s odor resistance makes it hard to beat for multi-day backcountry trips, ski weekends, or international travel when you’ll go for days without doing laundry. (While this article pertains to base layers, merino is especially great for socks, the stinkiest gear item of all. Virtually all of our socks for outdoor recreation are made primarily from merino wool.)
Most base layers have come a long way in the comfort department, and some synthetic base layers are pretty darn cozy these days. But merino wool does get the nod in next-to-skin comfort. Merino base layers are super soft and pleasant to wear for extended periods, which is a big factor in their popularity. Synthetics vary based on the build and quality of the product, but it’s tough to beat the natural softness of merino wool.
Base layers aren’t nearly as durable as burlier outerwear like ski jackets, but they get the job done for next-to-skin use. In general, synthetics fare much better than merino wool, which is less durable in thinner varieties and when not blended with a small dose of nylon or polyester. Our merino base layers often gather holes in high-use areas (like the cuffs and stomach) and see early thinning at the shoulders, especially if worn with a backpack. Merino also requires more attentive laundering—you’ll need to wash it in cold water and line dry it. If you really want merino but are concerned about durability, we recommend trying a merino blend, which offers all the performance of merino with the added toughness of synthetics.
In general, merino is noticeably more expensive than synthetics. The cost component is understandable: Merino comes from a specific alpine breed of sheep known for fine, soft wool, while synthetic base layers use polyester, which is much easier and cheaper to produce. For a point of reference, REI’s leading synthetic base layer, the “Lightweight,” is a mix of 92% polyester and 8% spandex and costs $40, while the REI Merino 185 is $80. Smartwool’s uber-popular Merino Quarter-Zip Top, which has a premium look and feel, is a whopping $120. But if you put your base layer to good use, particularly for longer trips when the anti-stink characteristic becomes increasingly valuable, merino can be well worth it.
Sustainability is a key part of what we do at Better Trail. In our base layers gear guide, we’ve provided a product-by-product analysis with how each base layer stacks up. This includes sustainability factors at the product level (sustainably sourced wool, fair trade certification, bluesign-approved materials, etc.) and at the brand level (repair services, recycled or reduced packaging, annual impact report, etc.). This information and our sustainability ratings should be helpful in determining how sustainable a certain base layer is, particularly wool against wool and synthetic against synthetic.
At a macro level, it’s a bit challenging to categorically compare merino and synthetics. Merino is derived from sheep, so as with any animal product is accompanied by much debate, including factors like feed, land impact, ethical treatment of animals, and more. That said, recent studies suggest that grazing sheep leads to carbon sequestration, and merino biodegrades relatively quickly as a natural fiber. On the other hand, synthetic materials are just another form of plastic, and their production can result in greenhouse gas emissions and potentially unsafe work environments. The microfibers of nylon and polyester have also been found all over the wastewater system and do not degrade on their own.
The basis for this article is comparing merino wool to synthetic base layers, and most products fall squarely into one camp or the other. But we have had some great luck with blended designs that mix merino with polyester or nylon to give you the best of both worlds: The warmth, softness, and odor resistance of merino alongside the quick dry time and durability of synthetics.
One of our favorite hybrids is the Smartwool Intraknit Thermal, which contains a mix of merino and synthetic materials (53% merino wool, 45% polyester, 2% elastane) alongside mesh ventilation. In use, this base layer successfully threaded the needle for high-output activities in cold weather; it’s one of the only merino base layers we can wear without overheating or getting bogged down with sweat. What's more, it doesn't hold onto stink. For a complete breakdown, see our Smartwool Intraknit Thermal review. Other offerings in this hybrid category include the Black Diamond Solution 150 (78% merino, 22% polyamide) and Ortovox 120 Comp Light (70% merino with 30% nylon).
Looking for more deets on merino and synthetic market offerings? We’ve spent countless hours testing various base layers across both categories (and hybrids, too) and have compiled all our findings into the best base layers. As you research, use our handy sort and filter function and peruse our detailed sustainability boxes for each product.
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