How to Choose the Best Base Layer (Thermal Underwear)

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.

Recommended read: Top 12 Great Hiking Gadgets Every Hiker Needs This Year

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 cozy in cold climates. 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 Syncrino Base

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 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:

• Hardshell Layer – keeps the rain off your inner layers
• Insulation Layer – traps body heat in and keeps cold out
• Wind proof Layer – Blocks wind
• Base Layer – Wicks moisture away from the skin

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

Convection

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.

Radiation

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.

Conduction

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.

Evaporation

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, and 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.

Comfort

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 flatlock 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.

Fit / Mobility

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 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.

Durability

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.

Backpacking

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

Wool

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.

Synthetics

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

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.

Blends

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-made should be hang-dried. Wool is especially prone to shrink if it’s dried in a clothes dryer.

Recommended read: Conquer Any Mountain: 15 Best Women’s Mountaineering Boots 2024

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:

Hoods

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.

Zippers

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 a 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 layer 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 too. We’re normally pretty careful about putting suncream on our exposed skin but UV rays can also penetrate lighter fabrics.

Other Considerations

Price

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 has 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.

Recommended read: Top 12 Great Hiking Gadgets Every Hiker Needs This Year

Frequently Asked Questions FAQs

Conclusion

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 product. Whether they’re right for you depends on the conditions you expect and the activities that you’ll be engaging in. Make sure to bring a sleeping bag for winter camping. This will keep you warm and help prevent hypothermia too.

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.

Need More Advice on Gear and Footwear?

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Last update on 2025-03-21 / Affiliate links / Images from Amazon Product Advertising API