How to Choose the Best Base Layer (Thermal Underwear)

The key to hiking comfort is layering your clothes. For this to work, you need the best base layer as your foundation.

Your long underwear layer needs to provide good temperature regulation, whilst not restricting your movement. It does this by moving sweat and moisture away from your skin so you don’t get cold once you stand still.

This is not a layer you’ll be taking off, so it needs to remain comfortable throughout the day.

At a Glance:

*Below, you’ll find our detailed reviews and a buyer’s guide, but you can also click the links above to see current prices or read customer reviews on Amazon.

Base Layer Reviews

We took a look at a pile of long underwear and narrowed the list down these great options:

Minus33 Merino Wool Isolation Mid-weight 1/4 Zip

Minus33 Merino Wool 714 Isolation Men's Midweight 1/4 Zip Black XS

If you’re heading out into cold weather and you want a good insulating base layer then nothing beats Merino wool.

This top from Minus33, with it’s 18.5 Micron wool is really soft and feels smooth against the skin.

The seams are flatlock, so there’s no chafing. Wear it under a jacket and you’ll feel warm in temperatures between 40 and 50 degrees.

Minus33 Merino Wool 714 Isolation Men's Midweight 1/4 Zip Black XS

If it gets colder, you’ll need to add a middle layer. If it gets a little warmer then the ¼ zip allows for some venting.

The fabric wicks and breathes well, so it’ll take a lot of activity before you feel even a little bit sweaty. The sizing runs true, although the fit is quite loose.

Being a looser fit, the Minus33 can be worn over another tight layer on colder days. As with most wool garments, you’ll want to avoid wearing it under abrasive layers, and wash sparingly to avoid durability issues.

  • Fabric: 100% Merino Wool
  • Weight: 11.85 oz / 336 g​

What We Like

  • Mock neck provides added insulation without being bulky
  • ¼ zipper for venting prevents you over-heating during strenuous activity
  • Flat lock seams prevent chafing
  • Machine washable and dryable
  • UPF rating of 50+ gives good protection from harsh sun rays.

What We Don’t Like

  • Loose fit is not to everyone’s taste.
  • Durability could be an issue if climbing or hiking in thick bush.
  • Sleeves could be a little short

Smartwool NTS Mid 250 Zip T

Smartwool Men's NTS Mid 250 Zip T, Charcoal Heather, Medium

SmartWool claim that the special way that they treat their wool makes it itch-free and shrink-resistant.

They also claim superior moisture wicking, anti-microbial and anti-odor performance.

If you’re looking for a thermal underwear layer that will perform in a range of temperatures, this is an exceptional choice.

This mid-weight top is warm, but the zipper and great breathability ensure you stay comfortable even during more intense activities.

The clever use of panels means they’ve avoided top shoulder seams, making it more comfortable when wearing a pack.

SmartWool products are a little pricey but they’re worth every penny.

  • Fabric: 100% Merino Wool
  • Weight: 12.5 oz / 354 g​

What We Like

  • Flat lock seams prevent chafing
  • No top shoulder seams making it very comfortable to wear with a backpack
  • Front zipper for venting during more strenuous activity.
  • Anti-odor means you won’t need to worry about unwanted smells

What We Don’t Like

  • Price

RAB Merino+ 120 LS Zip Top

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This slim-fit base layer top provides excellent temperature regulation. Ideal for multi-day use in a variety of conditions.

It’s warm enough for moderately cold days, but when you work up a sweat, that’s when it excels.

The blended fabric breathes well, wicks moisture quickly and dries fast.

It’s an ideal choice for trail running or high-activity hiking. The superior temperature regulation is achieved by using something called 37.5 Technology.

Active micro-particles are permanently embedded at a fiber level of the fabric. These particles use your body temperature to speed up the movement of moisture vapor from your skin to the surface of the garment.

We haven’t seen anything else breathe or dry like this shirt. The synthetic component does a great job of reducing the weight, but the wool means you sacrifice some durability.

  • Fabric: 65% merino wool 35% polyester
  • Weight: 13.4 oz / 380 g​

What We Like

  • Deep venting chest zipper so you won’t overheat
  • Close fitting collar when zipper is done up so heat can’t escape from your neck
  • Flatlock seams to prevent chafing
  • Excellent breathability even during intense activity
  • Dries really fast

What We Don’t Like

  • Price

Arcteryx RHO AR Zip Neck

Arc'teryx Rho AR Zip Neck Men's (Pilot, Large)

If you’re alternating between working up a sweat followed by periods of inactivity – in cold conditions – then this is a great choice.

Getting rid of the sweat on your skin is key to staying warm. The synthetic fabric has excellent wicking and shifts that sweat really fast.

It dries quickly too. Being 100% Polyester makes it lightweight but surprisingly warm.

If your skin is too sensitive for wool then you’ll love this shirt. The micro-fleece feels soft and comfortable when worn against your skin.

The sleeves are nice and long so they don’t ride up when you lift your arms. It’s a snug fit, but the fabric has good stretch and allows for a full range of motion.

  • Fabric: 100% Polyester
  • Weight: 11.2 oz / 318 g​

What We Like

  • Flat lock seams prevent chafing when skiing or carrying a backpack
  • Tall collar provides good insulation, no heat loss around your neck
  • Deep chest zipper allows good venting when you get a bit warmer
  • Zippered chest pocket handy for ski pass or small items
  • Excellent moisture-wicking performance so that you won’t get soaked in sweat

What We Don’t Like

  • Price – a bit steep for synthetic
  • Fit can run a little small

ColdPruf Platinum Dual Layer Crew Neck Top

ColdPruf Men's Platinum Crew (S, Heather Grey 2-Pack)

If you’re heading out into very chilly conditions then this dual layer top is an excellent choice for a base layer.

The idea is that the two layers of fabric trap air between them to keep warmth in.

Blending Merino wool and synthetic fibers results in a soft, warm top with excellent anti-odor performance.

If you up your activity level it does a fairly good job of getting rid of sweat and not allowing you to overheat. We wouldn’t recommend this for high-energy activity though.

The extended tail tucks into your pants to keep any sneaky breezes from getting up your back.

The longer back does mean that it looks a little funny if you wear it untucked.

What We Like

  • Flatlocked seams and tagless for comfort whatever your activity
  • Extended tail which is great when climbing as it won’t ride up
  • Good anti-odor performance
  • Very warm

What We Don’t Like

  • Not suitable for intense activity – could be more breathable
  • Durability is not great

Outdoor Research Sequence Crew Shirt

Outdoor Research Men's Sequence L/S Crew Shirt, Redwood/Charcoal, Large

This lightweight base works well for everyday hiking in cool to warm weather.

The Drirelease fabric has individual fibers that are both hydrophobic and hydrophilic.

This clever combination means it breathes well, wicks the sweat quickly and dries fast too.

Some synthetics have a tendency to get smelly after a few hours. This shirt has been treated with Fresh Guard which means you’ll go days before needing to give it a wash.

(Fresh Guard is an odor neutralizer that stops your body oils, and odor-causing bacteria, from attaching to the fibers.)

If you’re the kind of guy that normally needs to hike with a can of deodorant in your pack then you’ll love this!

  • Fabric: 88% Polyester, 12% Merino Wool
  • Weight: 7.8 oz / 221 g​

What We Like

  • Very lightweight so it won’t add bulk to your packing
  • Good breathability and moisture wicking, keeping you dry whatever your acitivity
  • Wrap around side-panels and flatlocked seams for added comfort
  • Fresh Guard anti-odor treated to keep your friends happy

What We Don’t Like

  • Runs a little bit snug, so get a larger size if you want a bit more room!

Best Merino Wool Base Layer

If you’re looking for a great 100% Merino wool base layer, then our top pick would be the midweight NTS 250 from SmartWool.

Some people shy away from wool because they worry it’ll be itchy as grandma’s blanket; but this is some of the softest and most comfortable we reviewed.

It’s not cheap, but the comfort factor, performance, and the 50+ UPF rating make it the best Merino wool base layer for the money.

Best Base Layer for Cold Weather

If you’re heading into some icy cold weather and don’t plan on being too active then we’d suggest the Arcteryx Rho AR.

It’s 100% synthetic so it doesn’t breathe as well as wool, but has excellent wicking performance and is super-warm.

The soft inner material and snug fit will keep you cosy in cold climes. If you suffer from wool allergies then this one is a top choice.

Best Base Layer for Summer Hiking

For summer hiking you’ll need a lightweight base. The best summer hiking base layer we reviewed is the Rab Merino+ 120.

It breathes and wicks better than most, providing excellent temperature regulation in hot weather. While it’s not going to be warm enough for winter outings, it’ll be fine for shoulder season outings.

How to Choose a Base Layer for Hiking

Choosing from different base layers really comes down to understanding why you need one in the first place.

You want it to soak up moisture (sweat) from your skin, wick it away through the fabric and then get rid of as much of it as possible (by evaporation).

So your first choice comes down to fabric type, synthetic and wool being the two you’ll need to choose from. Read more about merino wool vs synthetic fabrics.

You’ll also need to choose between different weights. The weight, or thickness, of the garment has a significant impact on how warm it’s going to keep you.

What fabric weight you need will depend on what you’ll be doing, and in what temperatures you’ll be be doing it.

There are a few other features that might have you choosing one underwear top over another. We go into detail on all these below. But first let’s understand why we need layers.

Layering Theory

No single layer of clothing is going to give you the protection you need for the variety of conditions you’ll experience.

When you’re hiking you need to stay dry when it rains, keep warm when it gets cold and block out the wind. All while keeping your skin warm and dry when you start to sweat.

To do all of this effectively we need 4 layers:

Each of these layers address one or more of the 4 main ways we get cold: Convection, Radiation, Conduction and Evaporation.


When air moves it moves heat around with it. You may have built up some body heat and the air temperature may be fairly mild but once the wind gets up you’ll have firsthand experience of what “wind chill” is. A good windproof layer addresses this issue.


If you’ve ever seen an infrared image of a person then you’ll know that the human body radiates, or gives off, heat. Heat energy naturally flows from higher temperature areas to lower temperature areas. If you wear an insulating layer it keeps you warm by trapping the heat radiated by your body and keeping it close to your skin.


When you touch a cold surface you’ve experienced how heat is conducted from a warm surface to a cold one. A good insulating layer provides a barrier between cold outer layers and your warm inner layer and skin.


When moisture evaporates it causes a drop in temperature. This is why if we get wet and the wind begins to blow, we get cold. When it starts to rain we instinctively grab our hard shell layer but people often forget the most common source of moisture when outdoors.

It’s the moisture generated by your body that is the main culprit. You get warm, build up a sweat and then, when the sun sets or that cold breeze kicks in, you quickly begin to get cold.

Your base layer keeps your skin dry so that evaporation, and consequently heat loss, is kept to a minimum.

Staying safe when hiking in extreme conditions may mean you wear all of these layers to stay warm and dry; while in milder conditions you might get away with wearing just one or two of them.

The need to keep your skin dry is essential and your base layer choice will determine how well the other layers perform.

Check what the legendary climber, Ed Viesturs has to say about layering:

How Should Your Long Underwear Fit?

If your base layer is going to do a good job of wicking sweat and shifting moist air then it needs to actually be in contact with your skin or have a small gap between skin and fabric.

You want to aim for a relaxed form fit without any part of it hanging loosely from your body. Too loose, you’ll lose temperature regulation and the fabric will bunch under your other layers.

If it’s too tight then you’re going to feel constricted, uncomfortable, and you won’t achieve effective temperature regulation.

What Is Wicking?

“Wicking”, also known as capillary action, is the ability of your base layer fabric to take moisture from the inside of the fabric (skin side) to the outside of the fabric, away from your skin.

A base layer that wicks well draws moisture away from the skin and spreads it out over the outer surface of the fabric for it to evaporate.

Your skin is kept dry, and the evaporation of the moisture happens on the side away from your skin, so you stay warmer. Some fabrics, like Merino wool, are naturally great at wicking.

Synthetic materials (nylon, polyester) are less effective wicking fabrics because they repel water. These need to be treated to be able to wick moisture. The outer side of the synthetic fabric is coated with a hydrophilic treatment so that the moisture on the inside is drawn to the outside.

Breathability Vs Warmth

One of the key ways that a base layer regulates temperature is by allowing warm moist air to move away from the body. The more breathable the fabric is the better it will be at shifting the air between your skin and the inner layer.

This is great for when you’re active and begin to get a little warm. As the breathability of a fabric increases, its ability to insulate decreases. This means that while it will keep your skin dry it will do little to prevent radiated heat from escaping or cold air getting in.

Ideally you want to be wearing a highly breathable base layer and carry an additional insulating layer or two to put on when the temperature drops.

If you want to reduce the amount of layers you’re packing then you may want to compromise a little on the breathability to get better insulation and warmth from your underwear layer.


Your base is going to be in direct contact with your skin so you want it to be soft with no scratching from seams or itchiness from the fabric itself.

A base layer with flat lock seams will reduce chafing.

Comfort doesn’t just come down to the feel of the fabric.

Thicker fabrics will insulate better in cold weather but they won’t breathe so well and they’ll also be heavier.

Your comfort is dependent on your underwear layer being able to breathe well.

Sweat trapped against your skin not only makes you feel uncomfortable but it’ll make you pretty cold too if it doesn’t dry.


Closely related to the comfort of the base layer is how it fits. Choose a size that allows for a fit that is neither too tight nor too loose.

Whether you intend wearing this base layer against your skin or over another layer will also determine whether you need a snug or slightly looser fit. For maximum mobility, choose a fabric with good stretch.

A synthetic fabric incorporating Elastene or Spandex will have the best stretch. If you want good mobility but don’t want a synthetic fabric, then get a base layer with gusseted underarms. This also avoids the fabric bunching up under your armpits.

Make sure that the sleeves and waist don’t ride up when you lift your arms. Some tops come with extended tail to help keep the shirt from riding up.


Man-made fabrics will be the most durable, but you’ll need to weigh this up against the drawbacks that these fabrics have.

Wool feels great and performs well but eventually, after a few washes, it is going to pill.

The odor-resistant property that wool has means you can wash it less often, prolonging it’s life.

Synthetic fabrics will usually need to be washed after every use, but they’re tough enough to survive this. It’s best to first choose the fabric you want based on how its properties match your expected hiking conditions.

Then, if you want more durability, simply choose a base layer with thicker fabric.​

Drying Speed

At some point during your hike, you will likely get wet from either rain or perspiration.

Once the fabric absorbs water its drying speed is important too. A base layer that doesn’t dry quickly will leave you feeling pretty cold once you stop moving.

Thicker fabrics insulate better, but once you work up a sweat you’ll find they don’t dry as quickly as thinner fabrics.

Wool has good resistance to water absorption and takes quite a lot of moisture before it actually feels wet. Once wet, it doesn’t dry as fast as synthetics though.

Fabric Weights – which one do I need?

The weight, or thickness, of the base layer material varies from ultralight all the way to heavyweight.

The thicker the base layer the more insulation it will offer. Your choice really comes down to the temperatures you’re expecting, as well as the level of activity you plan on. If you’re going to be doing aerobic activity such as skiing or climbing then you’ll want a lighter weight base layer to shift that moisture without making you too hot.

If you’re going to be walking in really cold conditions then a heavier weight is a better option. As a rule of thumb:

  • Ultralight – For high intensity aerobic activities like running or climbing.
  • Lightweight – For warmer weather and medium to high level activities where you may want to wear a single layer.
  • Midweight – For medium level activities in moderate cold.
  • Heavy/Expedition Weight – For really cold conditions and often worn over another lighter base layer.

Besides the outdoor temperatures you also need to consider your own metabolism. Do you get hot really quickly or do you tend to stay cold even while exercising?

Just remember that a thin base layer coupled with a warm insulating layer will give you the best wicking and warmth combination in moderate to cool temperatures.

Layering for Different Activities

Your choice of layering, and specifically your base layer, will have a lot to do with your activity. Here are some broad guidelines:

Extreme/High Intensity

You’re planning on working up a serious sweat and generating a lot of heat. Go for a light or ultralight base layer.

After a day or two of high intensity activity that sweat can make your base layer smell pretty funky. Go for a wool fabric to take advantage of its natural odor resistance.

Snow Sports

The intensity level of snow sports combined with the winter conditions mean you’ll be swinging between hot and cold throughout the day. A mid weight base layer with one or two additional layers that you can add or remove easily is the way to go.

High Mobility Activities

If you need greater upper body mobility for activities like climbing then opt for a base layer with a slightly looser fit. You’ll have less chance of chafing, too.


Carrying a backpack is going to reduce the wicking ability of your base layer. This means that the breathability of the fabric is going to make less of a difference. Opt for comfort and a fabric weight that matches the temperatures you’re expecting.

Light Hiking

A snug-fitting mid weight layer should offer enough insulation to be worn on its own in moderately cool weather. Just remember to bring an extra insulating layer and possibly a hard shell if there’s a chance of rain.

Types of Fabrics


Merino wool baselayers
Image ©slovegrove –

The higher-end wool base layers are normally made from Merino.

Merino wool has really fine fibers. This means it’s super soft which makes these long underwear layers extremely comfortable to wear.

If you have very sensitive skin then ‘ordinary’ wool can feel a little itchy – but you should be fine with Merino as it’s so fine.

Merino wool is great at keeping you warm but also breathes really well. With good water resistance properties and even when it does get a little soaked it dries pretty fast.

When you’re going to be working up a sweat, you’ll be happy that Merino wool has a natural ability to resist odors so your hiking buddy won’t mind being downwind from you. It seems like there’s no downside to Merino wool!

That is, until you look at the price. Base layers made from Merino are pretty expensive and they’re not as durable as synthetics.


Synthetic fabrics are normally made from polyester blended with Elastene or Spandex to give the fabric the stretch it needs for good mobility.

thermal underwear
Image © artursfoto1 –

Synthetic fibers are great at wicking sweat but they don’t breathe as well as Merino.

They also don’t have the anti-odor properties wool has, so you’re going to smell them after a few hours of sweating.

Synthetics may be cheaper than wool but they are much tougher. They’re not great in really cold or hot weather though, because they don’t insulate or regulate the skin temperature as well as wool does.

If you’re looking for a lightweight, comfortable fabric but you want to save some money then synthetic is the way to go.


Some manufacturers use a blend of wool and synthetic fibers to produce base layers that combine the benefits of both fabrics into one.

These can still be pretty pricey, but you get a comfortable, durable garment that breathes well and is odor resistant. These fabrics also combine the lightweight nature of synthetic fabrics with the warmth you get from wool.

Sometimes this works well but other times you end up with an expensive base layer that isn’t particularly good in any of the characteristics you’re looking for.

Care and Maintenance

Regardless of the fabric, you’re probably going to need to wash your base layer after each use. Synthetic fabrics have particularly poor odor resistance and will need to be washed more often than wool.

Be sure to check the washing instructions to see if machine washing is possible. Synthetic base layers are usually fine to machine wash while fabrics like wool and silk normally need to be hand-washed.

Polyester and other materials that are treated with special coatings to improve their wicking ability should be washed with mild detergents. Harsh detergents will damage the coatings and treatments and will reduce the wicking ability or even damage the fabric.

Wool base layers should preferably be hand washed in cold water to prevent shrinking and to retain their shape. Mild detergents are best and bleach and fabric softeners should be avoided.

Both wool and man-mades should be hang-dried. Wool is especially prone to shrink if it’s dried in a clothes dryer.

Additional Features

Once you’ve settled on the fabric, fit and weight of your base layer you may still be left with a few options to choose from. Here are a few features to consider:


Most base layers won’t have a hood but if you’re traveling light the added versatility of a base layer hoodie is great. Sometimes you want a little extra warmth around your neck and head without having to pull out your insulating layer. Hoods can tend to bunch up under your other layers though.


Having a zipper incorporated into your base layer makes it easy to dump heat in a hurry without having to take anything off. Zipper options are normally either quarter or half length.

Quarter Zip – Convenient way to get rid of some heat. May not be enough when you get really hot.

Half Zip – Greater temperature variation options but may have comfort issues with longer zipper against your skin.

Crew Neck vs Zipper Neck

A crew cut neck gives you the best comfort with a smooth surface to layer over, but you don’t get the venting that a zipper neck offers. Zipper necks tend to rise up higher than crew cuts and offer greater coverage of your neck area.

Thumb Loops

Base layers sleeves can creep, especially when running or lifting your arms above your head. Thumb loops built into the end of the sleeves keep the sleeves where you want them and offer better coverage of your wrists.

UV Protection

If you’re wearing your base layer without any other layers then UV resistant material is a nice feature to. We’re normally pretty careful about putting suncream on our exposed skin but UV rays can also penetrate lighter fabrics.

Other Considerations


Of course price is always an issue. You don’t want to blow your outdoor gear budget on your base layer, but you don’t want to skimp on this either.

If you can afford it, then go for a good wool option with some of the features we listed above. If you’re on a tighter budget then there are some well-priced synthetic options that will do just as well.

Sourcing Ethics

Ethical and sustainable clothing production is a hot topic, and with good reason. You want to be able to wear your gear outdoors knowing that it had as little impact on the environment or the lives of people or animals. There aren’t any perfect options but you can make more responsible choices if you do a little research.

Synthetic materials such as polyester require oil to produce the fibers, but some base layer manufacturers like The North Face and Patagonia use at least some recycled polyester in their products.

Wool production has its own carbon footprint but the treatment of the sheep should be your main concern.

Some producers practice Mulesing which involves ripping wool-bearing skin from the backside of the sheep without anesthetic to prevent flystrike.

Producers like Rab, Icebreaker and SmartWool claim to use only non-mulesed wool in their garments.


Whatever your choice, probably the biggest positive impact you can have is to buy a good quality garment and then wear it for a long time instead of replacing it every year or two.

Each one of the shirts we’ve reviewed above are great products. Whether they’re right for you depends on the conditions you expect and the activities that you’ll be engaging in.

Understanding the differences between Merino wool and synthetic is key to choosing the right long underwear. After that, consider whether warmth or breathability and temperature regulation are more important.

Once you’ve taken these factors into account then make sure that you choose the best base layer that your budget allows for.

*Product Images credits: ©

Last update on 2021-12-03 / Affiliate links / Images from Amazon Product Advertising API

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