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The term "thermal shirt" leaves a lot of room for interpretation. Usually when I categorize apparel, parameters include material, weight, and style, but here we're just talking about the broad categorization of thermal shirts, which means a base layer that helps retain heat during colder outings.
Thermal shirts help trap heat against your skin while (ideally) wicking sweat and helping prevent you from getting cold and clammy when you stop moving. Think of this like your first insulating layer, without the actual insulation of a coat. Depending on the weather, I wear this layer on its own, under a shell, or under an insulated jacket.
Thermal shirt material--synthetic, natural, or a blend--is ultimately up to your own preferences, intended use, and budget. I opt for merino or merino blend base layers, which are soft against my skin, wick sweat, and I can wear them multiple days in a row without much odor retention. Natural fibers are naturally wicking and odor resistant, but they take longer to dry than synthetic fibers and they're often more expensive.
A thermal shirt made from synthetic fiber won't be quite as warm as a natural fiber, and it will start to stink faster. However, they're often less expensive and they dry faster from sweat or precipitation than a natural fiber. For me, the best shirts are a blend of synthetic and natural, which results in a shirt that won't stretch out, stays relatively stink-free, and wicks sweat effectively. These are my top picks for thermal shirts this season, including natural, synthetic, and blended fibers.
1. Best Hooded Thermal Layer
Give me a go-anywhere full-zip hoodie and I'll never take it off. This easy-to-wear merino hoodie has a long hemline, a silhouette that manages to be comfortable without being sloppy, and has multiple zippered pockets for stashing small items (phone, keys, snacks) for hikes or fall scouting missions where you want easy-access items securely stowed. This layer is fully merino, so it lacks some of the structure of a blend, and you're going to pay a premium for the top-notch merino (certified to the Responsible Wool Standard), but if you want a layer to throw on over a tank, that can layer under a shell, and look good at the brewery post-hike, this is it. Men's model can be found here.
2. Best Lightweight Thermal Shirt
For a more streamlined thermal shirt, Artilect's Boulder 125 is an easy pick. This is an 85% merino top with 15% polyester, a blend that optimizes structure and dry time. This is a fitted next-to-skin top for hikes, runs, bike rides, or fall scouting, and will take you through the later season with a down vest and a shell. Artilect clothes fit true-to-size, and are made from a tight knit that lasts wash after wash. They build their merino clothes with a Nuyarn merino fabric technology that is lighter and loftier than other knits and dries faster than other similar shirts. Check out the women's version here.
3. Most Versatile Thermal Shirt
Smartwool has the market cornered on a wide range of versatile weights and patterns. From heavier hoodies to lightweight merino tank tops, there's something in their line for every type of weather and activity. The All-Season Merino Quarter-Zip is one of their classic tops, a mid-weight merino with a standard fit that makes for easy layering and a quarter-zip that allows for venting (unzipped) and draft protection (fully zipped). This is my go-to for mid-fall hikes. I wear it as a base layer and throw a lightweight vest over it if I need extra wind protection. This has a similar merino / nylon ratio as the Artilect layer, which is a solid combo for durability and technical wicking properties. Find the women's version here.
4. Best Budget-Friendly Option
While I'm a big fan of natural fibers, there's a time and a place for synthetics. This affordable thermal top from Under Armour was made for high-output activities, or outdoor activities where you'll be hitting it hard and then cooling off. This shirt wicks sweat during the high-energy bursts then dries quickly so you don't get clammy. I opt for a synthetic shirt during fall runs where I won't be stopping to hike the uphills, as synthetics are more comfortable for me during periods of higher exertion. This shirt is listed as a half-zip, but it's more of a classic quarter-zip with a high collar for draft protection. A comparable women's top can be found here.
5. Best Midweight Thermal Shirt
REI's house-brand line continually impresses me with their streamlined designs, quality construction, and affordability. This midweight synthetic base layer is as simple as it gets, with a standard fit (yay for versatile layering options!) and reinforced cuffs, collar, and hem on the crewneck. A crewneck is a good option when you want a low-profile base layer, and works easily with a buff or scarf for added draft protection. Crewnecks are my go-to for campsite base layers, as I'm also usually wearing an insulation layer that has a high collar or a hood, and I'm not a fan of too much fabric around my neck. These still aren't the least expensive thermal shirts on the market, but you're getting a comparable top to a name-brand outdoors company for a really good deal, and the fabric is soft to the touch, with 8% spandex for structure and elasticity. Check out the men's here.
6. Best Trail-to-Town Thermal Shirt
This might be close to what you have in mind when you think of a classic thermal shirt. While I wouldn't necessarily recommend 1800's long johns or cotton waffle-weave tops, this stylish Henley from prAna is a great update to this classic look. PrAna does an excellent job bridging the gap between technical outdoorsy clothing and items you feel good wearing to town, and this cotton / poly blend thermal shirt slots nicely into that category. This is ideal for people on a local trail or bike ride that takes them right to a hot cider or stroll to a local brewery. I love pairing this style with a down vest, and while this isn't the top pick for a high-output activity, it's super soft, made with organic cotton, and a classic layer to take you through the middle of fall.
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