Best Base Layer & Thermal Underwear 2018

The key to maximum hiking comfort is to wear the best base layer under your jacket or other outer layers. Think of your upper body long underwear as the foundation of your hiking apparel.



Top quality Merino, pricey

Best in class synthetic, pricey

​You won't go wrong with this one

Excellent blend, odor-free!

Solid Merino baselayer

Great for very cold conditions

*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.

Next-to-skin long underwear needs to provide good temperature regulation, breathe well and not restrict your movement. It also needs to do a good job of 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 do all of this while remaining comfortable throughout the day.

We’ve reviewed some of the best base layers and put together a buying guide so you’ll know what to look out for before making your purchase.


Your base layer 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 flatlock seams will ensure that you don’t feel any chafing from the seams when moving around.

best base layer

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

Your base layer needs to keep you warm too. Wool is your best bet for warmth but some of the newer synthetic fabrics have good insulation properties.

Thicker fabrics will insulate better in cold weather but they won’t breathe as well as thinner fabrics 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 you need a snug or slightly looser fit. If you need maximum mobility then choose a fabric with good stretch.

These will normally be a synthetic fabric incorporating Elastene or Spandex. If you want good mobility but don’t want a synthetic fabric then get a base layer with Gusseted underarms. This will also avoid the fabric bunching up under your armpits.

Your base layer needs to fit well throughout the full range of motions you go through.

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


best long underwear for hiking

Synthetic fabrics will have the best durability 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 and prolong the life of the garment.

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.​

Breathability, Wicking and Drying Speed

At some point during your hike, you will likely get wet from either rain or perspiration. In the case of perspiration it’s important to choose a fabric with good wicking and breathability.

A breathable layer will carry the moist air away from your skin to the outer surface of the fabric. Fabric with good wicking properties soaks up the sweat from your skin and carries it to the other side of the fabric where it can dry.

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 about.

Thicker fabrics may insulate better but if you’re going to work up a sweat you’ll find that they don’t dry as quickly as thinner fabrics do.

Wool has good resistance to water absorption and takes quite a lot of moisture before it actually feels wet. It doesn’t dry as fast as synthetic fabrics though.​

Types of Base Layer Fabrics


Merino wool baselayers

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

Merino wool has really fine fibers and it’s considered softer and finer than any other sheep wool.

This means it’s super soft which makes these long underwear layers extremely comfortable to wear.

If you have very sensitive skin then wool can sometimes 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.

Wool has good water resistance properties and even when it does get a little soaked it dries pretty fast. When you’re engaging in high activity hiking you’re going to be working up a sweat.

Merino wool has a natural ability to resist odors so your hiking buddy won’t mind being downwind from you a few hours later. 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

Synthetic fibers are great at wicking sweat from your skin and transporting it to the outer surface of the fabric but they don’t breathe as well as Merino wool.

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 exhibit superior durability. They’re not great in really cold or hot weather though. This is 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.​

Best Base Layer Reviews

We checked out a number of good base layers and narrowed the list down these great options:​

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

This great base layer from Minus33 provides excellent insulation and the 18.5 Micron wool is really soft and feels great against the skin.

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

If it gets colder than this 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 up perspiration and breathes extremely 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. 

It can be worn over another tight layer if you want to. As with most wool garments you’ll want to avoid wearing abrasive layers over this and wash it 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+ giving 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 claim that the special way that they treat their wool makes it resistant to shrinking and itch-free.

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

While their competitors may not agree, after wearing this top there’s no denying the excellent performance this base layer exhibits in each of these areas.

If you’re looking for a base layer that will perform throughout a range of temperatures then this is an exceptional choice.

This mid-weight shirt is really warm but the zipper and its great breathability ensure that it remains comfortable even during more intense activities.

The clever way they’ve used panels means they’ve avoided top shoulder seams and made 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
  • If you are allergic to wool, then avoid this one

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

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

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

This makes it an ideal choice for trail running or other high activity hiking. The superior temperature regulation is achieved by their use of 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 accelerate 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 does. The synthetic component does a great job of reducing the weight but the wool does mean 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 doesn'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
  • If you are sensitive or allergic to wool, you might want to avoid this one

If you’re going to be alternating between working up a sweat and 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 fabric doesn’t breathe very well but it has excellent wicking performance and shifts that sweat really fast.

It dries quickly too. Being 100% Polyester makes it really lightweight but we were surprised at how warm it was.

We were grateful for the venting the chest zipper provided when the sun came out.

If your skin is too sensitive for wool then you’ll love this shirt. The micro-fleece feels really good 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 

  • Flatlock seams prevents 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

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

This garment traps air between the two layers of the fabric to provide excellent insulation.

The blend of Merino wool and synthetic fibers results in a soft, warm garment that has excellent anti-odor performance.

If you up your activity level it also does a fairly good job of getting rid of sweat and not overheating. 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.

Fabric: Inside 100% CPT Performance Polyester/ Outside 70% Polyester, 30% Merino Wool​

Weight: 12 oz / 340 g​

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

This very lightweight base layer is the ideal choice 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 up the sweat quickly and dries really fast too.

Some synthetic fabrics have a tendency to get smelly after only a few hours. This fabric 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!

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

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

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

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