Thermal Underwear: Buying Guide and Best Choices

Thermal underwear is a must-have if you are doing any cold weather activity or live in a northern climate. The big question is: Which brand or specific underwear is best?

We know cold weather, so we did a head-to-head test with our 5 favorite lines of thermal underwear: Filson, Smartwool, Patagonia, L.L. Bean, and Carhartt. We also took a look at a couple favorite pieces from Duofold, who is owned by Hanes. In this list, we uncovered some clear winners.

Understanding the world of thermal underwear is a little more complicated than it seems, though. You definitely get what you pay for, and there are many choices to consider. It is not as simple as going to Walmart and grabbing the first product you find. You need to think about what activity you need the underwear for, how much warmth is a priority versus things like breathability, and, of course, your budget.

Here is an overview of thermal underwear, along with our favorite brands and products.

The Basics of Thermal Underwear

How Thermal Underwear Works

Thermal Underwear helps keep you warm by using your own body heat. By keeping your body heat within a few millimeters of your skin, that heat can produce a warming effect on your body rather than escaping into the air. That is why a loose-fitting garment typically feels cooler than a tight-fitting garment. The snugness of thermal underwear is important for this process of convection. Think of it like a piece of foil placed snugly over a plate of hot food — instead of the food cooling quickly from the air, the foil helps the heat stay trapped allowing the food to stay warm.

Materials Used in Thermal Underwear

Wool

Wool thermal underwear is perhaps our favorite option on the market today, but it needs to be high-quality wool. Lower-quality technically works well, but it can itch. High-quality such as merino wool does not itch and has incredible thermal qualities for underwear, socks, you name it. It is amazing that one of the best thermal fabrics that exists today has been around for thousands of years.

Of course, wool does a great job of keeping you warm, but what really sets it apart is its ability to dry quickly and not get wet. Because wool can amazingly absorb up to 30% of its water weight and still feel dry to the touch, you are able to use it while being quite active. It will not get cold and clammy like some other fabrics. We also like that wool is so abundantly available — many manufacturers have it in their lineup.

Synthetic

Synthetic thermal underwear is usually made of a blend of man-made materials, and engineered to be durable and moisture-wicking. Most of the time a synthetic will have some blend of nylon or polyester, and perhaps elastane, but each company will adjust their specific blend. Some manufacturers will even blend wool into the synthetics to get a hybrid of natural and man-made fibers.

Silk

Silk thermal underwear is exactly what you might expect — smooth, exceptionally comfortable against the skin, and a great base layer. The problem, if there is one, is that “silk” and “thermal” are two words that don’t always go together. Most silk thermal underwear options we see requires a pretty tight and heavy knit of silk in order to provide the warmth you need. Once you get that knit, it can be pretty effective, but costly at the same time.

The good news is that what silk might give up on the thermal side, it makes up for in terms of comfort and moisture-wicking qualities. Still, if your task is to stay warm in extreme cold, you may want to stick with wool or synthetics.

Avoid Cotton

Cotton thermal underwear is readily available in nearly any retail outlet, and can usually be found for reasonable prices. The problem is that it is not a very good insulator when it becomes wet. As soon as you exert yourself and begin to sweat, cotton becomes compromised as a thermal layer.

It can definitely be tempting to spend a little less and go with a cotton option because it is so darn comfortable, but trust us; cotton does not perform well from a thermal standpoint. If you are at all active, you will find that cotton really doesn’t do the job.

Our other issue with cotton is that it tends to retain odor once it has taken any on.

We know that cotton feels nice on the skin, which is why so many people like to use it, but when conditions get really cold or nature throws some bad weather at you, it tends to fail.

Best Thermal Underwear Brands and Products

Patagonia Thermal Underwear

Patagonia is an American clothing company based in Ventura, California. They’ve been around for over 45 years specializing in the outdoor clothing department, and are the brainchild of a group of outdoor junkies and die-hard climbers. They make their products with an eye on both durability and sustainability. You can find almost everything related to outdoor clothing through their brand, including wetsuits, boardshorts, waders, and much more. Since we’re talking about thermal underwear, let’s take a look at the best ones Patagonia has to offer.

Patagonia’s Capilene Thermal Underwear

Capilene by far is the best line of thermal underwear you can buy from Patagonia. Here are some key features to take note of:

• Great wicking
  • Their underwear is made of a recycled polyester and spandex blend which is a great synthetic for wicking away moisture, especially sweat.
• Comfortable
  • With the spandex blend, you can easily move your arms and legs without feeling any restriction. The superior breathability provides your body with enough air so that you won’t feel like you’re trapped in a bunch of clothing. The Capilene line of bottoms also have an elastic waistband which make it comfortable for anyone to wear.
• Warmth and odor control
  • The fabric has an open-grid pattern that presses next to the skin to provide maximum warmth. Each piece of underwear is also made with HeiQ Fresh and Polygiene to control and eliminate odor.

Find the Patagonia Capilene Thermal lineup here.

L.L. Bean Thermal Underwear

L.L. Bean is another American company that has been around for decades. They’re based way up in Freeport, Maine. For years, they specialized in making outdoor gear and base layers. In the past 20 years, they have expanded to try to have a larger mass appeal. While purists don’t like this move, we think they still make some of the best thermal layers (and socks for that matter) out there.

The line that we like from L.L. Bean is the top-of-the-line Cresta Wool lineup.  Cresta Wool is a very comfortable-against-the-skin type of Merino wool, on part with the higher-end lines elsewhere in this piece.  It is moisture-wicking and does a surprisingly good job of regulating temperature given its fit, which seems a touch loose at first.  Very quickly, you realize this thermal layer is going to perform at a very high level.  As with most lineups here, there are different weights to choose from.  We tested based on the Midweight (250) base layer, but the Ultralight (150) version is known to provide outstanding comfort in situations where thermal performance can be a bit more relaxed.

L.L. Bean Cresta wool base layer / underwear

The Cresta lineup is a great choice for thermal underwear because of the following key features:

• Breathability and wicking
  • This lineup is off-the-charts comfortable for general use. Highly-breathable, we have not had any trouble with moisture forming inside the layer due to perspiration. Like some others on this list, you will actually forget that you have them on. Going with the Ultralight version will give you a base layer that pretty much becomes unnoticeable after a minute or two.
• Warmth
  • We love the warmth provided by all L.L. Bean garments. In fact, we think it is where the company generally excels — on its “next-to-skin” layers. If you need to prioritize warmth and thermal qualities, you will want to go with the Midweight (250) version, which provides better-than-expected warmth.  Because it is nice and snug, but doesn’t hug the skin like compression tights would, it is comfortable to move around in as well.
• Relaxed fit
  • The Cresta lineup’s construction is a little more relaxed than some. Not as skin-hugging as a few of the other models, it does a great job of providing a little more relaxed fit while still keeping the heat next to your body. Due to the relaxed fit in places like the crotch, you might find it just a tad bulky for something like a Nordic ski race or something else where form-fitting clothing is helpful. For the typical hike or day working outside in subzero temps, though, this is an outstanding piece of underwear to have under loose-fitting pants or bibs. You will probably want to lounge in them when you come indoors.

Find the Bean Cresta 250 lineup here.

Smartwool Thermal Underwear 

Smartwool is based in Steamboat Springs, Colorado. The company has been in business for over 25 years, and as the name suggests, they specialize in wool clothing and accessories. While they are probably most famous for their socks, you can find everything from wool tops, bottoms, jackets, and more. Smartwool’s lines of thermal underwear are excellent — let’s talk about the 150 Base Layer line.

Smartwool 150 Base Layer

If you’re looking for casual thermal underwear that you don’t want to wear underneath other undergarments such as base layers, the 150 Base Layer line is an excellent choice. We have had some issues with our Smartwool long johns riding up from the ankles, so the ankle cuffs end up around our knees which is frustrating, but we have had other garments from them where it is not an issue at all.  One of the hallmarks of Smartwool is the absolute comfort about your skin — you will forget that you are even wearing a base layer.

We find Smartwool’s long-sleeve tops to be very comfortable all-around base layers.  They are a bit more form-fitting, so order them large if you like your garments loose. We prefer that they be just a bit form-fitting, though, because it tends to keep the heat in.

• Style
  • Most items in the 150 Base Layer line look very casual. You can wear them as t-shirts, long-sleeves, and pants. However, there are some pieces that still look like underwear, so you can still put on another layer of clothing if you choose. This may be wise during very cold days.
• Comfortability and quality
  • Unlike most thermal underwear, 150 Base Layers are made of Merino Wool and Nylon Core to maximize durability while keeping a soft and comfortable touch next to the skin. The unique design allows you to easily move around your body without feeling restricted. Their clothing is also very light so you won’t feel heavy and bulky.
• Moisture and temperature control
  • This line uses Core Spun technology which allows the thermal underwear to retain and regulate your body temperature while wicking away moisture such as sweat.

Find the Smartwool 150 lineup here.  If you need something heavier — for temps consistently below zero degree Fahrenheit, strongly consider the 250 lineup instead.

Filson Thermal Underwear

In many circles, Filson is synonymous with quality. They’ve been in business for an astonishingly long time of over 120 years. Filson is based in Seattle, Washington and the company specializes in timeless outdoor clothing, accessories, and luggage. Although they have thousands of products to choose from — many of which have been around and virtually unchanged for decades —  the line of thermal underwear you should take a look at is the 280G lineup.

Filson 280G

Anyone who follows this website knows that we are big Filson fans. The 280G line is a premium line of clothing for those who want one of the best pieces of thermal underwear on the market. Not for the budget buyer, our experience with Filson garments is that, over time, you will need to actually buy less of them because they last so long.

The Filson thermal underwear top is the warmest and most all-purpose base layer top we have ever found.  It passes our Below Zero Test with flying colors, and is super comfortable. It tends to fit just a little loose, so don’t over-order when it comes to sizing.

These key features are what make the 280G one that we are willing to splurge on:

• Functionality
  • Like the line suggests, each piece of clothing is 280G in weight. Although this is heavier than other thermal underwear on the market, the functionality is superior. Each piece of clothing can absorb up to 84 grams of water weight without feeling damp. They also provide insulation even if the clothing gets wet, and this lets you stay very warm throughout cold days.
• Quality and comfortability
  • The Filson 280G line is made of premium merino wool that’s been spun ultrafine. This makes the wool soft to touch against the skin, unlike regular or cheap wool, and it naturally wicks away sweat and other moisture. The clothing also naturally resists odors so you don’t have to worry about the clothes getting smelly.
• Convenience
  • Because of how functional the 280G line is, it makes the clothing convenient for not only colder periods, but for everyday use as well. If you plan on doing outdoor activities such as playing sports, fishing, or hunting, the 280G thermal underwear is fitting.

Find Filson 280g thermal layers here.

Carhartt Thermal Underwear

Carhartt is known for making tough garments at reasonable prices. Their gear is not going to win any style awards, but for many of us it is more about how the layer performs than how it looks. We like Carhartt, the family-owned, USA-based company that still keeps it headquarters in Michigan.  We like their workwear, such as the great value Carhartt jacket, and they make some thermal underwear that is good and doesn’t break the bank.

Carhartt Force Classic Thermal Line

Carhartt makes the Force line of thermal underwear, and we like the midweight version which typically is one of the least-expensive options you will consider, but one that should last a long time. It doesn’t quite have the higher-end construction of the Filson, and you may find that the fit and seams are not quite as smooth, but for most of us it does the job. If you are looking for underwear for an active sport like Nordic skiing, you probably want something with more flex. For work or general use, though, these are great.

• Durability
  • Carhartt products usually are made with an emphasis on durability, so this will be a piece of thermal underwear that should last for a long time. No worries about the seams coming undone after a couple washings.
• Soft
  • Despite the durable construction, the Carhartt Force underwear is soft and gentle against the skin. While the Smartwool underwear probably gets the nod as the softest, these are not far behind. Impressive comfort.
• Affordable
  • Despite the rise in popularity of their label in recent years, Carhartt still makes their stuff so that everyone can buy it. We appreciate that.

Find the Carhartt thermal lineup here on Amazon

Best Thermal Underwear For 3 Situations

Sometimes it is not just about choosing the best thermal underwear out there, but matching it to your situation and use. Here are our favorites for 3 common cold-weather situations.

Best Thermal Underwear for Skiing

Smartwool or LL Bean.  When it comes to skiing, you need something that is intended for exertion, with some level of breathability and flex. This rules out those which are design for lower-impact activities, like the Carhartt. You are going to want something designed for activity, and that is the Smartwool.  It is stretchy, comfortable, and does the best job of managing moisture.

The same rules generally apply to other active winter sports like snowshoeing.

Another option is LL Bean’s Midweight Thermal Underwear. It is incredibly comfortbextable, and actually keeps it form and elasticity a little longer than the Smartwool.  It feels a more like a smooth pair of tights than old-fashioned long johns, and works incredibly well.  Find it here.

Best Thermal Underwear for Hunting

Filson.  When it comes to hunting, we like to find thermal layers that are a little heavier. This allows us to use them in a different way, almost like a fleece, when layering-up or down, and really count on them if the weather turns colder. We insist that they be wool, because when you are in the wild there is no substitute for the moisture-wicking property of wool.

If you are doing something extremely active like pheasant hunting, you may need a more exertion-based thermal layer like the Smartwool. But we are assuming lower-impact hunting here, like deer or duck. Our favorite for hunting is definitely the Filson. Plus, it is heavier and looks good, so it can double as your shirt when back at camp. It doesn’t really look like underwear.

Best Thermal Underwear for Work

Carhartt or Duluth Trading. When it comes to work, most people need something that is rugged and will be comfortable all day. Having a super heavy garment, or one that can handle a high degree of activity or exertion, may be less important.  (Before we get a bunch of emails and comments noting that people at work exert themselves, when we talk about “exertion,” we mean in the way you exert when you have your heart rate at 160 bpm for an hour straight.)  For work purposes, we would go with the Carhartt Force just because it doesn’t break the bank, but the Duluth Trading Underwear is so close that it is a virtual tie.

Best Budget Base Layer

While it did not make our honor roll, a product from Duofold (made by Hanes) has been one that we have been impressed with over the past couple years, especially for those on a budget. While it might not have the same comfort over a long day as the ones listed above, it is constructed well, does a great job of keeping the heat in, and has proven to be durable through many washings. We have been pleasantly surprised — even impressed — by the Duofold long john base layer. If you need a base layer at a budget price, check it out on Amazon. What will you forgo?  The thoughtful fit and feel that you get from some of the higher-end brands.

Thermal Underwear FAQs

Is thermal underwear the same as a “base layer”?

No. Base layer is meant to go next to your skin. Thermal underwear can also go next to your skin, but is really meant to provide warmth, and may go over a base layer for additional warmth. During very chilly or cold days, it’s a good idea to wear a base layer, thermal underwear, and then a sweater or jacket to keep yourself comfortable throughout the day.

What is your “below zero test?”

Every year, we round-up the long underwear (and for that matter, gloves, boots, and hats) we like and put it to a real-life test. We find a day when it will be below zero, preferably with a windchill between -10 and -20 Fahrenheit.

Then the test begins. Trying a bunch of different combinations, with the same control variables (same activity, same temp, same duration of exposure), we document which garments keep us warmest — noting which are also the most comfortable.

There is a good chance you will not need to use your base layers in -20 windchill, but there is always a chance that you will. Note, many of these garments come in different weights, and we take that into account. A midweight line from a brand might not keep you warm at -20, but a heavyweight line might.

What is High-Tech thermal underwear and how is it different?

High-tech thermal underwear is usually made of synthetic fabrics such as polyester, nylon, or a blend incorporating merino wool to maximize the ability for the underwear to wick away moisture and retain body temperature, all while being comfortable. It’s different because high-tech thermal underwear doesn’t come with extra bulk, and it also doesn’t restrict your range of motion. More and more cold weather gear, from face masks to winter gloves, are incorporating high-tech construction to improve performance while reducing weight.

The world of high-tech underwear is constantly evolving, as is the quality of other winter gear like winter gloves or boots.  That is a good thing for consumers, and usually means better gear that weighs less and is more functional.

Is thermal underwear supposed to be tight?

Yes, you want thermal underwear to be snug. It needs to be relatively flat against your skin in order to keep the heat in. The whole definition of “thermal” when used in the context of clothing is that it should trap heat in a very thin airspace, using your own body heat to warm you. It stands to reason that the more air there is in that space, the more heat it will take to warm that air.

At the same time, it is possible for your underwear to be too tight, especially when it comes to the pressure points like your waistline and ankles. And obviously if the underwear restricts movement or rips when you bend, it is too tight for you.

Why does some thermal underwear itch?

Some thermal underwear will itch because it’s made of lower-quality or incorrectly graded fabric such as basic wool. There are many different grades of wool, and the shorter the fibers and coarser the wool, the more it will itch.  Think of the stubble on a man’s face — it is much scratchier than a beard that has grown out an inch or two. Typically, the finer and longer the wool fibers (such as in Merino), the more expensive it will be. You may also experience itchiness if your thermal underwear fails to dry or wick away sweat.

How much should good thermal underwear cost?

Good thermal underwear can range anywhere from $25 to $110 depending on the size, quality, and brand. Think about the longevity of the garment and reputation of the brand, not just the initial purchase price. It’s always best to check customer reviews to view other peoples’ experiences with the underwear.

What is the best thermal underwear for kids?

You can find some of the best thermal underwear for kids from the same brands we listed above such as Patagonia and Smartwool. Since you already have a good idea of what their thermal underwear is like, all you’ll have to do is look for the right size. We know that people like to save a little money on children’s thermal underwear, because they will likely grow out of it somewhat quickly.

With kids, it is really important that the underwear not be itchy. Adults can tolerate a minor itch if the underwear is going to perform well, but kids will have none of it. Focus on comfort.

How do you wash thermal underwear?

You should be able to throw most thermal underwear straight into a washing machine. However, depending on the material, you may have to do a little more maintenance work. For cotton or wool, you should wash the underwear in cold water and air-dry it afterward. For silk, hand-wash the underwear or throw it in the washer using the delicate option. For synthetic fabrics, you can put it in the washer or hand-wash them with warm water. Then lay them out on a towel to dry. As with most clothing, it is not the washing that can cause damage, but rather the heat needed for drying.

Is thermal underwear necessary?

It depends on the weather, but yes, thermal underwear can be a necessary layer in some conditions. Thermal underwear can be like a second skin, keep your body warm, keep moisture out, and help you regulate your body temperature.

How does thermal underwear work?

Thermal underwear works by trapping a layer of warm air close to your skin, serving as insulation against cold temperatures. It is typically made from materials like merino wool or synthetic fabrics designed to retain heat while wicking moisture away from your body. By creating a warm and dry environment next to your skin, thermal underwear helps you stay comfortably warm in cold weather conditions, especially when used as a base layer.

When should you use thermal underwear?

You should use thermal underwear when you need extra warmth in cold or chilly conditions. It’s ideal for activities like skiing, snowboarding, hiking, camping, and other outdoor winter sports where temperatures are low. Additionally, thermal underwear can be worn in everyday situations during the colder months to provide an extra layer of insulation, such as when going for a walk, commuting in cold weather, or simply staying warm in a cold indoor environment.