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We put men's long underwear to the test from brands like Smartwool, REI, Icebreaker, Patagonia, and others to find the best and warmest options on the market
Need new long underwear bottoms? We've been testing them for over a decade and showcase 12 of the best options in this review. Our team of experts spent months, in every season, taking these layers on river trips, ski slopes, cold winter trail runs, long multi-pitch climbs, and multi-day backpacking trips. We sought out different climates to determine which pairs are best for warmer seasons and which are best to wear through the coldest depths of winter. We backed up our field research with objective, laboratory-style tests designed to analyze specific features and offer this comprehensive, unbiased review to help introduce you to your next pair of long johns.
We test both men's and women's versions of most apparel categories. Explore our in-depth testing by female reviewers in our women's specific long underwear review. Then, check out our writeup on the best base layer tops to see what to pair with the bottoms reviewed here. Finally, pick out some top-rated cozy socks, and you'll be well on your way to adventure-ready.
Editor's Note: We updated this review on April 23, 2024, to offer alternative recommendations against our current award winners.
Tough synthetic fabric, four-way stretch for great range of motion, excellent performance for the price
Shorter length reduces bunching near ankles, seamless panels provide additional articulation and ventilation only where needed, high waist and thick waistband keeps bottoms in place
100% recycled polyester, light fabric weight, impressive loft and warmth
Cons
Expensive, stretches out in between washes, slow to dry when saturated
Price may make you do a double-take, care instructions need to be followed when drying
Heavier than wool bottoms for their warmth
No-fly design could be inconvenient underneath other layers, fit may be too tight for some body types, pricey
Questionable durability, saggy bottom
Bottom Line
Designed to keep you warm in the coldest conditions, this is a versatile pair of Merino wool bottoms
A seriously tuned ultralight pair of wool/nylon blend leggings built for fast and light outdoor adventures
A great synthetic thermal long underwear at an unbeatable price for the warmth and durability they provide
Carefully engineered 3/4 length thermals with compression pants-like tightness that’ll keep you warm and super comfortable while pursuing high output activities
Warmer than most and lighter than the rest, this pair of long underwear redefines what is possible from a synthetic base layer
Materials: 100% Merino wool | Weight: 7.6 oz (size Medium)
REASONS TO BUY
Cozy and comfortable
Terrific thermoregulation
Merino wool-lined waistband
REASONS TO AVOID
Stretches out between washings
Expensive
We adore the Smartwool Classic Thermal Merino Bottom and have bestowed upon it our top honors. This combined warmth and comfort of this long john are excellent when the temps really dip down. The special interlock knit provides exceptional breathability, especially considering the thick, 250 g/m² cut of the 100% Merino wool. You'll find rear gussets to match the front, enhancing comfort and fit. The extra-wide, wool-lined waistband and full-flatlock seams cut down on any common complaints of itchiness. While these are designed specifically for cold conditions, their balance of warmth and breathability makes them versatile even when temperatures begin to warm up a bit.
Despite the thick construction — and indeed, these bottoms are one of the most durable all-natural layers we tested — all Merino layers require a certain level of care that goes above and beyond most articles of clothing. With regular use between washings, the Classic Thermal tends to stretch out, so you may find it tempting to throw them in the dryer to shrink them back up. But don't! Careful laundering, and mainly flat or line air-drying, will help improve the longevity of this and all Merino base layers. We know the laundering process can be annoying, but the excellent thermoregulation capability makes this pair well worth the extra effort, especially for those playing or working in frigid climates. If carefully laundering wool sounds like too much of a hassle, a synthetic option like the REI Co-op Midweight Bottoms may suit you better.
Materials: 100% polyester | Weight: 7.4 oz (size Medium)
REASONS TO BUY
Withstands harsh use
Low-cost, high performance
Cozy four-way stretch fabric
REASONS TO AVOID
Not as warm for the weight as wool
Back from a total redesign, we're now even more impressed by the REI Co-op Midweight Bottoms as a value-driven, super tough, and no-nonsense pair of synthetic long undies. Our last critiques must have been heard at REI World Headquarters, as these long johns feature a very much improved-upon four-way stretch 100% polyester fabric that we absolutely adore. The bombproof durability and warmth of the Midweight remain, as does the overall understated cut and design — which is snug but not athletically tight — a fit that we think works great for most people partaking in most activities. This is the warmest synthetic mid-layer we've tested, giving even some of the 200 g/m² thick 100% Merino wool long underwear a run for their money.
Since REI updated the Midweight Bottoms, there are even fewer attributes we can point to as weaknesses. This no-nonsense piece of gear gets the job done well enough and isn't looking to collect accolades. The performance is really hard to beat, considering the price. If you're in the market for a warmer layer, and maybe a softer and comfier one at that, it would be time to reach for a wool option like the Smartwool Classic Thermal Merino Bottom and leave these for a milder day.
Materials: 100% Merino Wool | Weight: 7.9 oz (size Medium)
REASONS TO BUY
Good performance at a great value
Super warm 100% Merino wool
Wide range of available sizes
REASONS TO AVOID
Not as comfortable as more paneled construction
Run slightly small
Making 100% Merino Wool apparel approachable for the masses, enter the Meriwool Merino 250 Bottom. We appreciate the journeyman, no-nonsense construction, and super plush, cozy 100% Merino wool fabric of these affordable leggings. High-end features like an extra-comfortable waistband and well-thought-out ankle cuffs will make you double-take on the extremely fair price. These are excellent thermal bottoms to wear when the temperatures really start to dip down.
Aside from the overall warmth and the value of getting a 100% Merino wool bottom at a steal of a price, there is not a whole lot about these bottoms that makes them stand out among the competition. But that's their charm: they're working behind the scenes (or underneath your pants) to ensure you stay comfortable in the cold without needing any more fanfare. We should note there may be a wash-in period to get the ultra-fine Merino wool to soften up a smidge. So if you find the Merino 250 Bottom is somewhat itchy when you first wear them, wash them before bestowing your final judgment. If you'd rather go with a comfier option, we like the feel of the Artilect Boulder 125, which blends wool and stretchy nylon.
The Meriwool Merino 250 Bottoms fit comfortably - not too tight or loose - and are a great way to add major warmth to your layering system without breaking the bank.
Imperative to follow the washing/drying instructions closely
Tuned like a street-legal race car, the Artilect Boulder 125 leggings use advanced composite materials, like the comfy and warm 125 g/m² Nuyarn: an 85% Merino/15% nylon blend. These bottoms also have a complex panel pattern with literal speed holes/perforations built-in to release excess heat and moisture. All this will keep you perfectly thermoregulated on your next light and fast chilly mission. And even though you'll most likely wear these over a pair of pants or tights, they do look pretty darn good all on their own when you're lounging around your house or in the back of your van.
Just like its speed-over-everything analogy, the Boulder 125 isn't an all-rounder. Wear them when the weather is fairly mild, and swap them for something more substantial on those colder winter missions. The price for such a luxe pair of long underwear is also predictably higher than other options, but you get what you pay for. These are the long underwear we choose to pack along to make sure our next outdoor adventure is the coziest and lightest we can possibly have. For colder conditions, reach for the Smartwool Classic Thermal Merino Bottom instead.
Seamless panels provide better warmth, ventilation, and articulation
Nice thick waistband
3/4 length helps with keeping your wearables tidy by your ankles
REASONS TO AVOID
No fly may be a deal-breaker
Athletic fit may not be for everyone
We're not shy about our love affair with Smartwool's Intraknit technology, and it's a big reason why the Smartwool Intraknit Thermal Merino 3/4 Bottom landed in the winner's circle. The seamless panels found throughout blend different fabrics together to clothe you in a bottom that's warm yet well-ventilated in exactly the right places. For example, the gusseted crotch fabric lets more perspiration leave your body and the fabric on the knees promotes articulation. All this in a tight, athletic fit with a high waist held up by a chunky waistband.
What's not to like? If an almost compression level of tightness isn't your thing, you won't fall in love with the Intraknit Thermal as we have. The lack of a fly also dissuades some consumers from purchasing these bottoms. As does the price: the tech puts a heavy dent in the wallet. Also, Intraknit's polymath of fabrics makes them a little heavier than other pairs in our roundup. If ultimate warmth performance is of the utmost importance to you, then know there are better choices out there (and these are available in full-length, too), but we love these for many applications. However, if you prefer a snug-fitting layer over a compression tight, the REI Co-op Midweight Bottoms offer a comfortable four-way stretch at an extremely reasonable price.
We don't regularly get to try out washing our warm long underwear underneath a raging waterfall, but we couldn't help giving a try with the Smartwool Intraknit Thermal Merino 3/4 Bottoms. This pair probably benefits from hand washing, and always being hung out to dry.
Credit: Justin Simoni
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River trips down the Colorado River are a great testing ground for long underwear bottoms.
Credit: AJ Wojtalik
Why Trust GearLab
We take a two-pronged approach while testing men's long underwear. One side was in lab settings where we performed specific, repeatable tests to directly compare each bottom with all the others. Metrics like durability involve our harsh abrasion test and repeated washing and drying. Warmth and breathability tests are done in climate-controlled rooms so we can compare all bottoms in a more standard and equal setting. On the other side, we take these bottoms with us on our favorite outdoor pursuits and test them in their natural environments. We're looking for how they perform, how comfortable they feel, how they fit, and how well they survive our abuse. These bottoms found themselves accompanying us on trips to mountain summits, off-route fastpacking, and every day on our daily bike commutes and trail runs in the hills.
We took advantage of the spin bikes at our local gym to don our toasty selection of long underwear — often at the bemusement of our gym buddies — to gauge characteristics like warmth and breathability in a controlled setting.
Credit: Justin Simoni
Our testing of long underwear is divided across six rating metrics:
Warmth (25% of total score weighting)
Breathability (20% weighting)
Comfort and Fit (20% weighting)
Durability (15% weighting)
Drying Speed (10% weighting)
Layering Ability (10% weighting)
Our expert on cold weather-layering systems is Justin Simoni, a Boulder, CO-based athlete, adventurer, and backpacking guide. He specializes in ultra-long distance, self-powered, and self-supported challenges in the mountains of the American West. Many of those challenges see him gain and lose thousands of feet of elevation, so his layering systems have become finely tuned. He has worked closely with outdoor gear companies for over a decade, bringing his experience and expertise to the table.
Justin is joined by Aaron Rice. Pursuing snowy adventure as a way of life, Aaron works as a ski patroller, avalanche educator, and snow scientist — ask him about stellar dendrites or the finer aspects of back-door cold fronts, and he'll gladly talk your ear off. In his free time, you might chance across him in the seldom-traveled high mountains of New Mexico, while out trail running, backpacking, alpine climbing, and ski touring.
Field testing the Smartwool Merino 250 bottoms, now called the Classic Thermal.
Side-by-side testing is our bread-and-butter. Here, we compare the fat waistband of the Ortovox 185 Rock'N'Wool with the thinner waistband of the Outdoor Research Alpine Onset bottoms.
Long underwear bottoms are a versatile addition to your outdoor collection.
Our team of intrepid adventurers took out these pairs of long underwear on their travels to test their mettle and shake out which ones are the very best for you.
Analysis and Test Results
Long underwear (also commonly referred to as base layer bottoms) keeps you warm and regulates your body temperature while you're out playing in colder conditions. Base layer bottoms often only get pulled out when the cold of winter sets in, but even if they aren't used as frequently as tops, they can still be quite versatile. You can pack them as pajamas on a backpacking trip, have them pull double-duty as cold-weather running tights, or just layer them under your favorite pair of jeans for a night around a bonfire. Obviously, for cold-weather activities like ice climbing and skiing, these are more of a necessity. But regardless of the situation, these bottoms work to keep you dry and comfortable by wicking sweat away from your body, improving thermoregulation, and ultimately improving your outdoor experiences.
Grab a bottom base layer once the flurries start to fly!
Credit: Justin Simoni
We design our field and laboratory testing to be both comprehensive and mutually exclusive. We determine scores for each metric, compile those numbers and assign a total score for each pair. It is important to note that the assigned scores are determined relative to the other products in the review. We specifically test only the best layers on the market, so a low score does not mean that a product is not worth its weight-in-wool — it simply means that it does not perform as well as other products when compared side-by-side. By testing and rating each of these layers relative to one another, we can highlight specifics to inform your next purchase.
Value
Though it may come as a surprise, some of these bottoms are expensive. If you're a mountaineer, go on multi-day ski tours, love winter camping, or just spend a lot of time recreating in cold conditions, investing in a high-end, performance base layer is easier to justify. However, if you're just looking for a pair to pull on to shovel the sidewalk, you likely have some more wiggle room and can still get your hands on a decent-quality product for a lower cost.
These Ortovox woolies are exceptional, but may just be too much long underwear for what you need.
Credit: Justin Simoni
Merino wool models, in general, will cost more because the material is more expensive to produce. However, you help offset external costs by opting for an all-natural product. Synthetic models are almost always less expensive and often carry quick-drying capacity, which can be very important when the weather begins to warm up. Despite the environmental costs of production, synthetics tend to be more durable than Merino, lasting through many wash cycles, so their longevity adds to their value.
For those needing a warm pair of bottoms at a great price, consider the REI Co-op Midweight. For the sustainably-minded, the Patagonia Capilene Midweight Bottoms are a synthetic option made from 100% recycled polyester. For wool, the Meriwool Merino 250 Bottom bucks the trend of wool always being super expensive and are a great introductory pair to the world of wool.
Some bottoms spell it out for you, like these Capilene Midweights, a base layer that is clearly designed for “cool to cold” conditions - most times we're not so lucky with up-front information on the tag.
Credit: Jill Rice
Warmth
Warmth is one of the most important factors to consider when shopping for long underwear. The ability of a layer to keep you warm is rooted in its ability to wick sweat away from your body, keeping you dry and comfortable while active. While we often notice a correlation between warmth and the thickness of a material, we also have to consider the type of fabric and fit.
Base layer bottoms are versatile, but warmth is a dynamic metric because we don't all run at the same temperature. When considering a pair of bottoms, first consider how you plan to use them. We've worn these layers while working up a sweat skinning steep terrain, walking the dog in freezing temperatures, backpacking in the fall, and sleeping in the back of a truck or out in a tent through the long, dark, cold nights. Do you want versatility across all four seasons or just mid-winter warmth? Are you layering them under a single, technical hardshell pair of climbing pants or under a thick pair of ski pants that already offer a level of warmth and insulation? It is important to be aware that the warmest bottoms aren't always the most practical.
The Smartwool Intraknit Thermal Merino 3/4 Bottoms were great on wintry hikes - the 3/4 length works well when paired with thick warm socks, preventing too much overlap.
Credit: Justin Simoni
If you need bottoms for activities in the bitter cold, we recommend the Smartwool Classic Thermal Merino or the Meriwool Merino 250 based on overall warmth. If you're looking for a more versatile pair of long underwear, the REI Co-op Midweight Bottoms and Ortovox 185 Rock'N'Wool Short Pant are midweight options that tend to blend the seasons. If warmth isn't as pressing for you, a lightweight option like the Outdoor Research Alpine Onset Bottoms and Patagonia Capilene Midweight may be the ticket for summer backpacking, cross-country skiing, or winter climbing trips to the desert. Similarly, if you're looking for a lightweight layer for your SkiMo training, the Artilect Boulder 125 rely upon moisture-wicking capability more than warmth by insulation all alone.
A good pair of long underwear will keep you warm enough while cooking so that you don't have to huddle around the campfire.
Credit: Aaron RIce
Breathability
Breathability is a quality that can make or break the effectiveness of a base layer when it comes to regulating body temperature. A garment's ability to breathe lives in the fibers of the fabric. Breathability is the fiber's capacity to move heat-induced moisture (i.e., sweat) from the inside of the garment to the outside, allowing it to evaporate from the surface of the layer. This process keeps you from overheating and enables the bottoms to continuously dry out even when you're working up a sweat. Improved breathability keeps you from becoming a soggy mess, ultimately keeping you warmer in cold temperatures.
Our body temperature doesn't fluctuate as intensely on our bottom half as it does on our top half. But it does house some of our major working muscles and thus is an area that is good at producing consistent heat. Having a pair of long underwear that will dump heat and stay dry is important when participating in highly aerobic, start/stop activities like backcountry skiing. The most breathable layers we tested, not so shockingly, are on the lighter side in terms of fabric weight.
There's snow somewhere around here, we just know it! Putting breathability to the test on a much too warm and dry day to be hiking in ski bottoms.
Credit: Kricket Servis
Almost every Merino wool layer we tested offers superior moisture-wicking ability. This is a natural quality of Merino wool, allowing these layers to breathe while you're on the move. If you are seeking a high level of breathability from a full-Merino pair of long underwear, the Smartwool Classic Thermal and Ortovox 185 Rock'N'Wool are great choices. While all-natural fibers dominate in terms of wicking ability, synthetics tend to dry faster on the body. The Patagonia Capilene Midweight and REI Co-op Lightweight Bottoms are both full synthetics that do a solid job of wicking away moisture.
Feel wild and free when your bottoms feature great breathability,
Credit: Justin Simoni
The front runner in this metric is the Artilect Boulder 125, which uses an 85% Merino wool and 15% nylon blend to produce a thin 125 g/m² fabric that's super breathable while staying relatively durable. The Ortovox 185 Rock'N'Wool Short Pant uses a super soft and lofty 185 g/m² to perform well in both breathability and warmth.
The Artilect has great breathability, perfect for active wear.
Credit: Justin Simoni
The Mons Royale Cascade Merino Flex Legging also scores admirably in this metric. It seems like the Merino-blended fabric is thinner than its advertised 180 GSM, resulting in a lighter weight pair of bottoms than we expected. It also has an athletic fit — hugging tightly around your legs — which helps with transferring moisture away from your skin.
The Mons Royale Cascade Merino leggins are thin, very stretchy, highly breathable, and feature a thick, comfortable waistband.
Credit: Justin Simoni
Comfort and Fit
Comfort is a metric that is reasonably uniform across different users — we all want to wrap our legs in plush, soft-spun wool or silk-like polyester. But the fit is very much a subjective matter, as the perfect fit understandably varies just as much as we vary in body type. Through analysis of articulation, seam placement, cut, and design features, we do our best to identify any areas of concern that may apply to every user, not just those that match our bodies. But with that being said, we always suggest trying clothing on in person so you can decide for yourself if it fits well.
We consider several factors to determine comfort and fit:
Feel of the fabric next to the skin
How tight or loose they fit in particular zones, such as the knees and cuffs
The stretch of fabric and freedom of mobility
Width and comfort of the waistband
Length of the legs and fit of the cuffs
Depth of the crotch, factoring in any bottom-sag
The type of seam stitch, articulation, and the associated comfort of the type of stitch used
Having our collection of different long underwear to try and compare in succession really helps reveal how each one feels and fits differently.
Credit: Justin Simoni
We only test high-quality products here, so comfort is fairly uniform. So when it comes to ranking pairs of long underwear side-by-side, the fit sets them apart. If you are looking for that ultra-soft pair, perfect for getting after it or just hanging out and relaxing, check out the Smartwool Classic Thermal Merino and its lighter weight brethren, the Classic All-Season.
The Artilect Boulder 125s are made of Nuyarn: a Merino wool spun on a nylon core. This makes them as comfortable as Merino but as stretchy as a synthetic layer
Credit: Justin Simoni
Many bottoms mix synthetic fibers with their wool to add long-lasting stretch to hug the body. Find this material in the uber comfy Smartwool Intraknit and Artilect Boulder 125. We adore the extra wide waistband of the Mons Royale Cascade Merino Flex Legging, which we found distributes the pressure of the band around a wide area.
The Intraknit tech in the Smartwool Intraknit Thermal Merino 3/4 Bottom certainly did impress on when it come to the stretch and movement of the panels found on areas of the bottoms, such as the knee.
Credit: Justin Simoni
Durability
Durability has never been a particularly strong suit for base layers — remember, these bottoms are specifically designed to be worn at the base of your kit. When these layers can pull double-duty as a pair of running tights, we like to think of it as more of a bonus than a requirement.
Merino wool is historically susceptible to rips, tears, and punctures, but brands have made serious strides in recent years to enhance the durability of this all-natural fiber. For example, Smartwool uses a process that wraps the merino wool around a nylon core to increase durability, which makes the Intraknit Thermal Merino particularly durable for its weight. The Artilect Boulder 125 does a similar thing, using NuYarn, instead of a proprietary blend. The Outdoor Research Alpine Onset are a polyester-wool blend that also tease the benefits of both materials into one.
Durability is key for pair of long underwear to last on multi-day trips.
Credit: Jill Rice
While all-natural options are improving, the most durable pairs of long underwear we tested are still synthetic. The REI Co-op Midweight scored extremely well in durability, thanks to its hardened yet comfy four-way stretch fabric. Another synthetic bottom, the Patagonia Capilene Midweight, is almost as durable when you consider the thinner fabric utilized.
Durability of the REI Midweight Bottoms is excellent as the synthethic fabric resists abrasions. The snug fit and four way stretch of the fabric survived our repeated washings.
Credit: Justin Simoni
Drying Speed
We test and report on drying speed for a few important reasons. First and foremost: many of the materials used in these base layer bottoms can't be thrown in the dryer with your everyday clothes. Wool tends to shrink and lose shape as well as shed fibers, making this the quickest way for it to meet an early grave. Even 100% Merino wool bottoms and most synthetics utilize elastane in their waistband, not to mention the fabric itself, which also degrades with drying. We suggest drying your long underwear on the line or laying the underwear flat to dry. Secondly, on-the-line drying speed can be a good indicator of how fast the bottoms will dry out when you're wearing them, which also correlates with breathability.
Lighter-weight fabrics of all threads will dry faster on the line than heavier fabrics, but we're always surprised at how fast some of the heavier bottoms dry out. The Icebreaker Merino 200 Oasis Bottoms had excellent drying speed performance for their weight, as did the 3/4 length Ortovox 185 Rock'N'Wool Short Pant. Consolation prizes go to the Patagonia Capilene Midweight, our fastest synthetic dryer, as well as the Artilect Boulder 125 and Mons Royale Cascade Merino Flex Legging which dried out quickly due to their overall svelteness on the scale.
The Icebreaker Oasis 200 dries impressively fast, especially considering their weight.
Credit: Justin Simoni
Layering Ability
What's the point of a good pair of bottoms if they don't pair well with anything else you own? This metric judges how well these long underwear work within your clothing system. We look at a variety of factors: does the manufacturer use features like low-profile flatlock seams to reduce chafing? Does the waistband keep the bottoms up, and are the leg cuffs adequate to keep them down? A front fly can add convenience, but a lack of fly may mean you can wear these bottoms au naturel - a nice extra benefit. A tighter fit can also help outer layers slide easily over the top.
The Smartwool Intraknit Thermal features a tight, compression-like fit, and the 3/4 length we tested helps with preventing the bunching up of your socks above the ankles. The Outdoor Research Alpine Onset Bottoms also fit close-to-skin, with fabric that easily slides over mid and outer layers.
Grab a versatile pair of long underwear bottoms, like the Outdoor Research Alpine Onset bottoms, and set out for your next adventure!
Credit: Jill Rice
Conclusion
For work and play, long underwear bottoms are a great way to extend your seasons and help keep you outside through the coldest months of the year. To find your next perfect pair, it is important to consider your temperature disposition, as well as how and when you plan to use them. From all-day ski tours to desert backpacking trips to shoveling the driveway and hanging on the couch, these versatile bottoms are sure to keep you warm and dry no matter the activity or occasion.
Where will your next pair of long undies take you?
