Why do you need to buy base layers? When you get warm, your body begins to sweat. Sweat is great—it’s your body’s way of keeping you cool—but if that sweat stays trapped next to your skin for long enough, it can lead to a range of miserable and even dangerous happenings like chafing, overheating and hypothermia. Wool and synthetic base layers help control your body temperature and comfort levels by keeping you warm while also wicking moisture. You can buy base layer tops and bottoms in many configurations, including crewnecks, quarter-zips, T-shirts, tank tops, leggings, long underwear and shorts. For this guide, we tested mainly crewneck and quarter-zip tops and long underwear bottoms, but we note when other options are available.
To pick the best base layers, we started by looking at customer reviews on REI.com. We read through dozens of submissions, and then spoke with several outdoors experts to get a sense for what matters most in a good set of base layer tops and bottoms. Then, we narrowed our list down to seven base layer tops and six base layer bottoms, which we tested in both women’s and men’s designs. We wore the base layers on snowy days in the city, took them on ski trips up into the mountains, layered them under snow pants and rain pants, and put them under fleece and down jackets. After several wears, we did a smell test, then we washed and dried each product to check for durability. Finally, we decided that the following base layers are the best you can buy at REI:
The Best Wool Base Layer Top for Most Activities
- Fabric: Merino wool
- Weight: Midweight
- MSRP: $100
The Smartwool Merino 250 base layer top is the best wool base layer top for hiking, skiing, snowboarding, snowshoeing, backpacking and so much more. It comes in a variety of configurations and sizes for women and men (including plus sizes), and we think its midweight makeup offers just the right mix of breathability and warmth for most outdoor activities during the winter, fall and spring seasons. We tested the quarter-zip variation, but this base layer top also comes in crew cut or a half-zip with a hood.
Smartwool’s Merino 250 line is made with merino wool, a high-quality wool that comes from merino sheep, which are primarily farmed in New Zealand. Merino wool is soft, breathable, moisture-wicking and odor-resistant, and those properties are evident in Smartwool’s 250 line. These base layers are soft against the skin, which is important for items that get up-close and personal, and they repel odor famously: We wore these base layer tops for several days of skiing and hiking and noticed not even a whiff of smell. The Smartwool Merino 250 also emerged from the washing machine unscathed, with no pilling, shrinking or rigidity.
The best thing about the Smartwool Merino 250 line is its breathability-to-warmth ratio. We wore the quarter-zip version on a winter run through the city and stayed warm but not overheated. When we got home, there wasn’t much moisture left in the top, either. We also liked the Smartwool Merino 250 for winter activities like snowshoeing and skiing because the layers kept us warm and blocked the wind but didn’t cause us to overheat. The Smartwool Merino 250 top fits true to size, too: The profile is narrow but not too tight, so you can easily layer it under a fleece, down jacket or vest.
The Smartwool 250 is a midweight base layer option, which we thought gave us the right balance of warmth and breathability even on the coldest winter days. However, if you prefer something a bit lighter for fall, spring or summer, check out Smartwool’s 150 line.
“I’m spending this winter working outside in upper BC and Alberta,” said one reviewer. “[Wearing] Smartwool from head to toe has been the key to staying warm while being less bulky. A whole set fits easy rolled up in a pack. You can hand wash a set after days of use and move into the 2nd set while drying. A must for any long period of cold weather exposure.”
The Best Wool Base Layer Bottoms for Most Activities
- Fabric: Merino wool
- Weight: Midweight
- MSRP: $95-$100
Like the Smartwool Merino 250 top, the Smartwool 250 base layer bottoms are in a league all their own when it comes to keeping you warm without excessive sweat. The bottoms come in many sizes, colors and patterns, and they fit fairly true to size (our petite woman tester found them to be a few inches too long, but our 6’3” male tester thought they were the perfect length).
The Merino 250 bottoms have a wide, soft waistband that fits snuggly around the hips to prevent sliding. Our male tester also noted that he “had appropriate space around my man parts.” Because of these pants’ thicker profile around the crotch and knees, it can be hard to fit them under tighter-fitting pants; however, they’ll work just fine under rain, hiking or snow pants.
Like the Merino 250 top, the bottoms are made with high-quality merino wool that quickly wicks sweat and odor. The fabric is comfortable against the skin and the ankle cuffs are cozy, especially with a pair of thick socks. Compared to the other merino wool base layers we tested, these were the warmest and lightest option with the best durability. We had one of the most comfortable ski days in recent memory with these base layers and we brought them along for several winter runs and hikes, too, with similarly stellar results. As with the Smartwool tops, these bottoms are available in a lightweight 150 version for people who run hot or need base layers for warmer climates.
“These work great for me,” said one customer. “They are also very soft on the skin and do not get itchy or too warm, along with having no sweat smell after wearing them all day. Another feature I like is how they do not slip down my waist ever and they go all the way down to cover half of my foot and I am 6'1" (bought size XL). I am a forest preserve worker in Illinois and I wear these outdoors 8 hours a day with thin pants over them and I am very warm and comfortable.”
The Best Wool Base Layer Top for Under $60
- Fabric: Merino wool
- Weight: Midweight
- MSRP: $79.50
The REI Co-op Midweight base layer top (it also comes in women’s and men’s half-zip styles) is almost as good as the Smartwool Merino 250 and it’s only half the price! We loved how comfortable the fabric was against our skin and found the fit to be near perfect for a hiking or skiing excursion. The only downside was slightly reduced durability, but for the price, we think this purchase can’t be beat.
The REI Co-op Midweight base layer top is made with high-quality merino wool, which means it will keep you warm and dry, in addition to controlling odor like a champ. The top fit both our testers true-to-size, with a slightly longer overall length and sleeve length. (We think this longer length is a bonus because it allowed for overlap between our tops and bottoms and didn’t let cold air in.) The top is offered in a number of sizes, colors and configurations, including a short-sleeve version in men's and men's tall sizes.
We loved how comfortable the REI Co-op base layer top was: it was easy to throw on, perfect for layering and not itchy. We did notice some pilling (more than the Smartwool 250) and felt as though this top felt a bit rigid after coming out of the washing machine, which can happen sometimes with merino wool products. But if you avoid the drier and wash this top with wool soap only rarely, it’s a great purchase that’ll last a while.
“I bought this top to wear as a base layer or as a single layer during a hiking trip in New Zealand,” said one customer. “I'm very fair skinned and this top was perfect to deal with the changing weather from sea level up to the Southern Alps' glaciers – super breathable during the hotter locations and provided plenty of warmth while hiking on the glacier.”
The Best Synthetic Base Layer Top
- Fabric: Polyester
- Weight: Midweight
- MSRP: $49.95–$54.95
If you run hot or find yourself in wet environments often, or if wool makes you feel itchy, a synthetic base layer top is a good choice. Our favorite synthetic, crew-neck base layer top was the REI Co-op Midweight base layer, which comes in a range of colors, patterns, sizes and cuts, for both women and men. It’s also available in a half-zip configuration in both men’s and women’s cuts.
Even though the REI Co-op Midweight base layer is made with 100 percent polyester, it feels like cotton against the skin. It’s incredibly breathable, wicks sweat quickly and dries in a flash. While synthetic base layers aren’t quite as warm as wool products, we found that the REI Co-op Midweight top still kept us quite warm on a ski day in the North Cascade mountains, where the temperatures ran in the low 20s. This base layer is also a great choice for late fall, winter and early spring runs, when you need to stay warm but you’ll also be sweaty. Our male tester loved this top so much that he bought several more and started wearing them to work.
The REI Co-op Midweight base layer fits true to size, with a narrow profile that’s easy to layer over but has no extra tightness around the shoulders or armpits. The price is also right for this base layer, which sells for $54.95. After wearing this base layer for a few days, we did notice a bit of a smell, which is common for synthetic base layers (they don’t wick odor quite as quickly as wool base layers). But after a quick trip through the washer and dryer, the top smelled great and didn’t look any worse for the wear. If you prefer a lightweight version of this top, REI offers that, too, in women’s and men’s.
“Bought this over the more expensive options as I've done very good so far putting together a lightweight extended trip gear list,” said one customer. “This is the heaviest base layer I carry, along with two lightweight tops. I use this in the evening when we're hanging out by the fire, to sleep in, and to start hikes in the morning. I'm a very hot hiker and a warm sleeper, and this fits exactly the niche it was meant for. I have not hiked in it long enough to comment on its smell containing capabilities, but it does wick sweat well, and dries fast. Overall, for the price, it’s a great top.”
The Best Synthetic Base Layer Bottoms
- Fabric: 97% polyester, 3% spandex
- Weight: Midweight
- MSRP: $59
Patagonia’s Capilene Midweight long underwear bottoms were our favorite synthetic option. If you run hot, don’t like wool products or plan to spend lots of time in rainy climates, a synthetic base layer is the right choice for you. We thought the Patagonia Capilene Midweight long underwear bottoms offered the best blend of warmth, fit, breathability, moisture wicking and odor control, compared to the other synthetic bottoms we tested.
The Patagonia Capilene Midweight long underwear bottoms are made with 97 percent polyester and 3 percent spandex. The spandex lends a legging-like quality to the pants, which we liked because the pants stayed firmly in place all day long, were easy to layer over and allowed for quick movements. The polyester material is what helps these pants dry quickly during sweaty excursions. The polyester materials in these pants are also recycled, according to Patagonia, and the sewing on the Capilenes is Fair Trade Certified™.
Both our male and female testers found these pants to be comfortable on the slopes. They have a wide waistband that’s not the softest but works just fine, and padded ankle cuffs. Note that these base layer pants are long (our petite female tester had to roll hers up to accommodate her ski boots). Thus, you may want to head into the store to try them on before buying. The men’s version of these pants is also built to allow for “plenty of room around the important parts,” according to our male tester.
While synthetic pants simply aren’t as warm as their merino wool counterparts, the best thing about these pants is how fast they dry. Whether you’re on a long run or working hard during a cross-country ski adventure, these pants will keep you dry and warm all day long. After a few days of use, they didn’t smell bad either, which is a major win for synthetic products.
“Never really got that saying "slept like a baby", but wowee did I sleep well despite the bitter cold of winter in Big Sur,” said one customer. “And in the morning, they were the perfect base layer for a morning hike. Definitely recommend to anyone for any circumstance, extremely comfortable and affordable.”
The Best Women’s Base Layer Top for City Winters
- Fabric: Merino wool
- Weight: Lightweight
- MSRP: $75
The Kari Traa Tikse base layer top is a soft, comfortable, breathable base layer that works well for layering under your work clothes on a cold day in the city, although it doesn’t perform as well as some of the other base layer tops we tested in frigid conditions. Still, after trying this base layer top once, we found ourselves reaching for it again and again. The fabric is so thin that it layers seamlessly under even the tightest clothes, and the material is surprisingly warm for its weight. In addition to the long-sleeved crew style, it also comes in a T-shirt and tank top.
The Tikse top is made with ultrafine merino wool, which is soft against the skin and quite flexible. The top’s design is stylish, with seams running down the inner arms and side body, and offers some interesting color variations. We ended up wearing this top for a run through the city, and then for a yoga class on a cold day—and it performed admirably on both occasions, keeping us warm and dry. When we took these base layers up to the mountains, however, we found them to be lacking in warmth compared to the other base layer tops we tested. The Tikse top fits a little small, so plan to try it on at the store before buying. It comes in a range of sizes and three color options.
“Love the design and weight of this top,” said one customer, noting that she went up a size because of her strong shoulders.
The Best Women’s Base Layer Bottoms for City Winters
- Fabric: Merino wool
- Weight: Lightweight
- MSRP: $75.00
Like their counterpart top, the Kari Traa Tikse base layer Bottoms are comfortable, flexible and easy to layer under tight-fitting clothes. They’re made with merino wool and offered in four color options, as well as a range of sizes.
The Tikse bottoms fit a bit small so we’d suggest going up a size, or trying them on at the store before you buy. On our 5’3” tester, the size small reached from the hips to the ankles and stayed firmly in place all day. The waist of these bottoms is a bit lower than the others we tested, although we didn’t find it to be a huge problem, and a few testers complained about the thinner waistband.
While these pants aren’t as warm as some of the other pants we tested (likely due to the ultrafine merino wool), we thought the Tikse bottoms were perfect for layering under jeans on cold days in the city. In fact, we ended up reaching for these almost every day during a seven-day Seattle snowstorm. They performed less well up in the mountains, where we felt we needed a bit more thickness. As with the top, these bottoms wicked moisture quickly and didn’t retain odor.
“These pants are a great base layer – not too thick but still warm,” said one tester, noting that there is no vanity sizing with this brand. She went up one size and was happy with the fit and style, also noting that the pants had a “kind of spiderman vibe.”
What is a base layer and why do I need one?
Base layers are clothes that sit next to your skin, and they typically include tops and bottoms in a variety of configurations (T-shirts, long-sleeves, quarter-zips, tank tops, long bottoms, and more). When you sweat, a cotton garment is likely to soak up that sweat and then hold it next to your body, leading to chafing and even hypothermia. Synthetic and wool base layers solve for this problem, as they wick moisture away from your body while keeping you warm.
What is a layering system?
According to Tim Brown, senior product manager at REI, a layering system usually involves wearing a base layer, mid layer (often a fleece) and top layer (often a down jacket) with a weatherproof shell, as needed.
“The largest benefit [of layering] is warmth without overheating,” says Jason Knode, an assistant category manager for men’s outerwear and men’s base layers at REI. “Being able to remove layers as you start to exert more energy is paramount to performance and safety. Preventing overheating and sweating in cold conditions can prevent hypothermia when you stop your chosen activity.”
What base layer weight should I choose?
We tested mostly midweight base layers for this guide because they’re best for moderate-intensity activities during the fall and early spring, when mornings may be chilly but afternoons are warmer, as well as for winter activities when you’re working hard (like skiing, snowshoeing, snowboarding and more).
If you plan to spend time outside during the summer or you run very warm, you might consider lightweight base layers. It might seem counterintuitive to wear base layers in warm weather, but base layers wick sweat, which can prevent chafing and blisters. Some brands also offer heavyweight base layers for extra cold days.
“A base layer is essential during the winter months. For the warmer months I recommend something that is fast drying,” says Knode. “Lightweight items with cooling properties are my personal go to in the warmer months.”
What is merino wool?
Merino wool is high-quality wool that comes from merino sheep, which are primarily found in New Zealand. Soft, breathable, moisture-wicking and naturally odor-resistant, it’s an ideal fabric for active base layers.
- Moisture wicking: When you sweat, merino wool fibers absorb the sweat vapor away from your skin through their hollow centers and wick moisture out through small openings within the fabric. The result: The surface of the wool remains dry to the touch.
- Temperature regulation: Merino wool’s moisture wicking properties help our bodies regulate temperature more effectively, too. Sweat is a natural human reaction that cools your body down when you’re working hard. However, when excess moisture gets trapped in your base layers, you can experience problems such as hypothermia when it’s cold out or chafing when it’s warm. To solve for this, merino wool fibers absorb the sweat vapor, pulling it away from your skin so your body can regulate temperature naturally in a variety of climates, without trapped sweat or excess heat.
- Odor control: The fibers in merino wool remove sweat and moisture away from the body and out of the wool fabric quickly, which means bacteria cannot thrive. Thus, the smells caused by bacteria develop more slowly in merino wool than they would otherwise. Typically, you can wear merino wool garments several times (and sometimes for weeks!), depending on your individual level of stink.
When should I choose synthetic base layers over wool base layers?
It’s really up to you. Brown says synthetic materials are popular for people who run hot and sweat a lot, as well as for people who plan to spend a lot of time in wet climates. This is because synthetic fibers dry more quickly than wool fibers. Synthetic products also tend to be a bit cheaper than wool products, but they tend to pick up bad smells a bit more quickly.
“Merino is good because of its ability to pull moisture away from the body and still maintain its warming properties,” says Knode. “It also handles odor better than most fabrics. Synthetic is also very good as it tends to be more form-fitting. These products typically have similar properties to merino wool but aren’t as effective at moisture transfer and odor control. More aerobic activities tend to call for more form-fitting silhouettes.”
Care and maintenance:
In general, you should wash your merino base layers as infrequently as possible. (Most people can get three to four wears out of a merino wool garment before needing to wash it because of the odor, but this will vary depending on the person and the garment.) When you do wash your merino gear, consider handwashing it with a wool-specific soap; avoid products containing bleach or fabric softeners, as these can damage and weaken the wool fibers, causing them to warp when they dry.
Synthetics are a bit more sturdy and can be washed using a washing machine, although you’ll still want to turn your garment inside out to keep it from pilling. Turn the washing machine to a gentle or delicate cycle on warm. Hang your garments to dry unless otherwise directed by the tags on your gear.