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Jul 31, 2023

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An in-depth review of the Smartwool Classic All-Season Merino Base Layer Long

An in-depth review of the Smartwool Classic All-Season Merino Base Layer Long Sleeve.

The Smartwool Classic All-Season Merino Base Layer Long Sleeve ($85) is a lightweight long-sleeve layer made to go the distance in a variety of conditions. From chilly to warm and from drizzly to dry, this shirt will keep you comfortable and looking stylish too.

This long-sleeve base layer's merino and nylon blend keeps it light and durable with a slim fit that even Goldilocks would have to agree isn't too tight or too loose but is just right. The shirt comes in men's and women's versions, with a men's size range of small to XXL and a women's of XS to XL. Whether you’re wearing it as a single layer on a chilly morning run, a base layer for cold-weather adventures, or as a casual top on a long travel day, it will work its merino magic to keep your temperature regulated and your stoke high.

Shop the Women's Smartwool Classic All-Season Merino Base Layer Long SleeveShop the Men's Smartwool Classic All-Season Merino Base Layer Long Sleeve

The Smartwool Classic All-Season Merino Base Layer 1/4 Zip, which is the same as the Smartwool Classic All-Season Merino Base Layer Long Sleeve except for its zip. Photo: iRunFar/Eszter Horanyi

The Smartwool Classic All-Season Merino Base Layer Long Sleev e is made of merino wool with a nylon core that helps with durability. The natural wool fiber from merino sheep is thinner, softer, and less scratchy than regular wool and is much more comfortable against your skin. The fibers are inherently good at temperature and moisture regulation and are also odor resistant. The natural bends in merino fibers trap air and provide insulation in cold temperatures. The merino fibers are also porous, allowing sweat to escape as a vapor and eliminating that wet, clammy feeling some synthetic materials can have in cooler weather. In short, the material that helps merino sheep thrive in extreme conditions does the same for us! For more details about merino wool, check out our best wool running apparel guide, where this shirt was our top lightweight long-sleeve pick.

In addition to merino's technical capability, it is also environmentally friendly and biodegradable. Smartwool's merino comes from New Zealand growers who are dedicated to protecting the well-being of the animals, producing ethically sourced wool, and improving sustainability and environmental impact.

Many merino activewear pieces are a blend of merino plus other materials to enhance the item's performance. This shirt is made from 87% merino wool and 13% nylon. The merino fibers are spun around a thin nylon core to create a fabric that is stronger and more durable than merino would be alone. The merino fibers are still next to the skin, allowing the shirt to keep its valuable moisture and temperature-regulating properties. This shirt's fabric is light, soft, and very comfortable when worn both casually and for hours of trail running.

A front view of the Smartwool Classic All-Season Merino Base Layer Long Sleeve. All photos iRunFar/Carly Eisley unless otherwise noted.

The Smartwool Classic All-Season Merino Base Layer Long Sleeve comes in both a crew neck and a quarter zip fit. It is a slim-fit design that is ideal for layering. I found it to fit close to the body with no sagging or bunching, but it was not overly tight or constrictive. It is comfortable both as a standalone or light top layer in warm to cool conditions and as a base layer in cold temperatures. It's roomy enough to fit over a tank top or short sleeve shirt but snug enough not to bunch up under another layer. I am typically a size small in most manufacturers and found the shirt to run true to size for me.

The sleeves have a raglan design with seams angled more to the front of the shirt rather than right over the shoulder, eliminating a potential friction point under a pack. The seams wrap toward the front of the shirt, giving it a unique look. Smartwool claims this improves the fit, and I have to say that I agree. The length of the hem and the sleeves were just right.

A back view of the Smartwool Classic All-Season Merino Base Layer Long Sleeve.

As mentioned above, merino wool's moisture- and temperature-regulation properties are second to none, especially in colder weather. I have worn the Smartwool Classic All-Season Merino Base Layer Long Sleeve shirt on its own in everything from a sunny 60-degree Fahrenheit day to a rainy and cold slog, and the shirt has stayed comfortable without feeling muggy or clammy. I ran a 100-kilometer trail race in the hills of Pennsylvania in 50-degree F temperatures and on-and-off rain with this shirt over a merino tank top and stayed comfortable and warm all day, even despite my poor choice of a non-waterproof jacket. That race really solidified my love affair with all things merino!

The shirt also works well as a base layer, and I have found it to be very comfortable when layered under a thicker merino top, vest, fleece, or sweater for casual wear. It feels warm yet light against the skin and is soft with no itchiness. I had no issues with any rubbing or chafing from the seams, even when the shirt was layered or worn alone under a hydration pack.

The odor-resistant properties of merino wool make it a fantastic choice for endurance events, stage races, and travel. I have to say that I am inherently skeptical of all manufacturers’ claims about their products. While merino's odor-resistant properties don't belong to any one company, I was still dubious as to whether they would be as good as promised. After giving my shirts a solid sniff test after some very long races and travel days, I can attest to the fact that this shirt has stayed impressively stink-free. It has become my go-to for travel days and has stayed dry and odor-free for multiple very long flights and airport sprints.

A close-up view of the Smartwool Classic All-Season Merino Base Layer Long Sleeve.

The Smartwool Classic All-Season Merino Base Layer Long Sleeve has quickly moved its way to the top of my long-sleeve layer pile. It is the perfect weight for those cool days when I need a little extra warmth but light enough that it stays comfortable if the temperatures rise. It works well on its own as a light top layer or a next-to-skin base layer for colder days. It is fairly thin, so it will probably not work for most people as a standalone shirt in cold weather.

The shirt comes in both a crew-neck and quarter-zip style and has a number of color choices that will please everyone from the earth-tone lovers to the Rainbow Brites of the endurance world. This is especially great for those of us who want good visibility on road runs or on the trails during hunting season but also want a neutral color option for casual wear. There are also plant-based dye options in the crew neck for those who prefer a more eco-conscious option.

The temperature- and moisture-regulating ability of this shirt is impressive, and the odor resistance makes it a no-brainer when laundry options are few and far between. If you are looking for a light, versatile layer that can be layered, dressed up, and get you through multiple wearings before a wash, this shirt is a great option. To learn more, read our best wool running apparel guide, where this shirt was named our top lightweight long-sleeve pick.

Shop the Women's Smartwool Classic All-Season Merino Base Layer Long SleeveShop the Men's Smartwool Classic All-Season Merino Base Layer Long Sleeve

[Editor's Note: If you’re affiliated (i.e., an employee, ambassador, etc.) with a brand, please share your relation in each of your comments on this article. Thanks!]

The Smartwool Classic All-Season Merino Base Layer Long Sleeve.

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