There’s no arguing with the effectiveness of thermal layers in cold weather. Fitting close to your skin, they trap body heat to keep you warm and wick moisture when you sweat.
However, there are many products out there, as well as a wide range of different materials used in their manufacturing.
To help you in your search we’ve reviewed the most effective thermal underwear on the market and assessed them against a range of important criteria including material, weight, and style.
This review and comparison can help you to find the best base layers for your next cold-weather adventure.
Best Overall: Icebreaker Everyday 175 Merino
Material: 100% Merino Wool
Weight: 175g (grams per meter²)
Pros: Versatile, comfortable, designed for mobility and comfort, 100% Merino provides exceptional warmth and breathability
Cons: More expensive than synthetic options
Icebreaker are well known for its high-quality garments made 100% from New Zealand merino wool. The Everyday Long Sleeve Crew certainly doesn’t disappoint.
This is a very versatile base layer, it is constructed for mobility, and has a soft and comfortable feel on the skin.
One of the great things about this wool is how odour-resistant it is, meaning it can be worn for multiple days in between washes.
This is perfect for longer trips and, for many, features like these make it worth paying the extra money for wool rather than synthetic fabrics.
At 175gsm, this is one of the lightest options on our list. You probably want something more heavy-duty for extreme cold, however, this will be a perfect first layer for skiing, hiking, hunting, fishing, and many other outdoor pursuits.
The Icebreaker Everyday 175 range also includes leggings (or long johns) so you can pair your base layers for ultimate warmth.
Also Great: Ottie Australian Made Thermals
Material: 95% Merino, 5% Elastane
Weight: 200g (grams per meter²)
Pros: Made in Australia, soft and comfortable, all the benefits of merino, great all-rounder
Cons: More expensive than synthetic, wide neck
Ottie’s high-quality thermals are made in Melbourne with Australian merino wool.
They’re made from a mid-weight 200gsm merino, which is odour resistant, moisture wicking, and temperature regulating. They also include 5% elastane to provide extra comfort and stretch.
The tops are available in both Men’s and Women’s fits and Ottie also makes unisex merino bottoms (with pockets!).
These thermals are perfect for hiking, skiing, or any other outdoor activity where you need a base layer to keep you comfortable and toasty.
When compared to the Icebreaker thermals, the Ottie top has a slightly wider neck, a longer fit, and is slimmer in the forearms.
We like the fact that these thermals are made here in Australia, as opposed to in China. Ottie is also accredited with Ethical Clothing Australia.
Best Value: Macpac Geothermal Base Layer
Material: 100% recycled polyester
Weight: 180g (grams per meter²)
Pros: Affordable, easy-care, brushed fleece backing for comfort
Cons: Synthetic materials don’t resist odours as well as merino wool, nor provide the same breathability
These Macpac base layers are perfect for keeping you warm on your next hike or ski trip.
Made from 100% recycled polyester fabric, they’re warm and lightweight, yet soft on your skin thanks to a brushed fleece backing.
They maximise warmth and moisture management so you can stay comfortable all day long, even during physical activity.
These base layers are fast drying, durable, and easy to care for (you don’t need to do a special wool wash like merino!).
Best Mid-Weight: Icebreaker 260 Tech Merino
Material: 100% Merino Wool
Weight: 260g (grams per meter²)
Pros: Very warm and comfortable, temperature regulating, seams designed to prevent friction
Cons: More expensive than synthetic options
For a warm, midweight option for winter layering, try the 260 Tech Long Sleeve Crewe by Icebreaker.
It’s made with 100% merino wool, one of the softest fibres in the world, to keep you warm and comfortable in chilly temperatures. The fabric also resists moisture and is highly durable so you can wear it over and over again with extended use.
Good for Low-Intensity Activities: The North Face Warm Poly Crew
Material: Recycled Polyester and Elastane
Weight: Not listed
Pros: Low cost, soft, traps heat well
Cons: Not very breathable, doesn’t offer highly technical features
The North Face Warm Poly Crew is a synthetic base layer that is well suited to hiking, skiing, or as an everyday layer for low-intensity activities. This thermal underwear is warm, soft and is one of the most affordable options on our list.
The downside to this base layer is that it is not as breathable as the other options listed. Although it doesn’t offer quite as much breathability, it does keep the heat in well. So, if you feel the cold easily and want long underwear that is going to hold onto heat and keep you warm, this is a good choice. It’s also great if you are going to be somewhere cold but you aren’t going to be doing too much activity.
All in all, this is a good layer if you don’t have requirements for a highly technical kit.
Best High Performance Option: Kathmandu KMDAscent Long Sleeve Top
Material: Polygiene (Polyester, Merino Wool and Polypropylene)
Weight: 215g (grams per meter²)
Pros: Polygiene provides odour control, very warm and comfortable, quick-drying, lightweight
Cons: More expensive than purely synthetic options
The Kathmandu KMDAscent combines merino, polypro, and polyester in the ultimate technical base layer.
The benefits of natural wool along with synthetic fibres mean you get warmth when you need it while moisture is wicked away from your body when you sweat. Polygiene offers superior odour control by preventing bacterial growth, meaning you can wear this thermal layer day after day.
Super-warm, comfortable and lightweight, the KMDAscent should be near the top of your list if you’re in the market for a high-performance base layer.
Best for Warmth in Winter: XTM Merino Crew Neck Thermal Base Layer
Material: Merino Wool
Weight: 230g (grams per meter²)
Pros: Very warm, thumb holes, comfortable
Cons: Wider neck may not suit some people, more expensive than synthetic options
A great option for those who really feel the cold, the XTM is a high quality, 230gsm, winter weight thermal from one of Australia’s favourite snow sport brands.
Made with 100% Australian Merino wool, this top will work well as a base layer or on its own, with the properties of the natural fibres regulating body temperature and providing breathability when you need it most.
We appreciate the addition of thumb holes that make it easy to pull your sleeves down and stop cold air from getting between your gloves and thermal underwear.
Great for Active Days: Icebreaker 260 Zone Midweight Thermal Base Layer
Material: Merino Wool and Lycra
Weight: 260g (grams per meter²)
Pros: Excellent mobility, suits a range of activities, midweight base layer
Cons: High price
The 260 Zone is another excellent mid-weight option from Icebreaker. This is a technical long-sleeve crew ideal for active days in cold weather such as climbing or skiing.
Unlike the Icebreaker Everyday, this midweight baselayer is a combination of merino wool and lycra to give a lightweight, stretchy and odour-resistant top.
The features of this thermal underwear include thermally-mapped fabric zones to trap and release heat as needed, eyelet merino mesh offering ventilation, flatlock seams for comfort, and gusseted panels for improved mobility.
Pair this with the Icebreaker 260 Zone leggings for a high-performance thermal underwear set that provides comfort and warmth when you need it.
Best for All-Day Comfort: Smartwool Merino 250 Base Layer Crew
Material: Merino Wool
Weight: 250g (grams per meter²)
Pros: Comfortable, durable, flatlock seam, machine washable
Cons: Expensive
This long-sleeved top from Smartwool is made with 100% merino wool that wicks away moisture and is odor resistant. You could wear this for cold weather activities and feel comfortable all day.
This top has a slim fit design but it doesn’t seem to be as tight as other options so it doesn’t hug your arms as tightly. It also has merino-covered shoulder panels and a flatlock seam to help minimise chafing.
This Smartwool Merino base layer is warm yet thin, weighing 250g it is a good winter weight option that isn’t too thick or heavy.
Quick Drying: Macpac Geothermal Base Layer
Material: Recycled Polyester
Weight: 200g (grams per meter²)
Pros: Affordable, recycled materials, durable, fast drying
Cons: Doesn’t resist odours as well as merino wool
This thermal top from Macpac is made from 100% recycled polyester so is a sustainable winter option. It has brushed backing which is soft and warm and is ideal for layering or wearing alone.
One of the key features of this layer is the fast drying recycled polyester fabric. As well as this, the top offers excellent moisture management. If you are going to be getting wet, this is a good synthetic option that will dry quickly.
If you are looking for synthetic thermal underwear, the MacPac Geothermal base layers are good for hiking and general outdoor use.
Affordable Merino and Polyester Blend: Mountain Warehouse Merino Thermal Top
Material: Merino Wool and Polyester
Weight: Not listed
Pros: Comfortable, warm, good price
Cons: Thin compared to some options
The final product on our list is this high-performance base layer from Mountain Warehouse. This thermal underwear combines the best of both worlds by using 80% merino wool and 20% polyester.
This blend makes the fabric breathable, moisture-wicking, and durable. The blend of natural and synthetic make this long sleeve similar to the Kathmandu KMDAscent option that combines polyester, wool, and polypropylene.
The antibacterial properties of the fabric not only mean this top will keep you warm but it will also stay fresh and free from odours for longer too.
This is a good option, especially if you want the quality and properties of a merino wool base layer without the high price tag.
Thermal Underwear Comparison Table
| Thermal Underwear | Our Verdict | Material | Weight | Thumb Loops? |
| Icebreaker Everyday 175 | Best Overall | Merino Wool | 175gsm | No |
| Ottie Thermals | Also Great | Merino, Elastane | 200gsm | No |
| Macpac Geothermal | Best Value | Recycled polyester | 180gsm | No |
| Icebreaker 260 Tech | Best Mid-Weight | Merino Wool | 260gsm | No |
| The North Face Warm Poly Crew | Good for Low Intensity Activities | Recycled polyester and elastane | Not listed | Yes |
| Kathmandu KMDAscent Long Sleeve | Best High Performance Option | Polygiene | 215gsm | Yes |
| XTM Merino Crew Neck | Best for Warmth in Winter | Merino Wool | 230gsm | Yes |
| Icebreaker 260 Zone Midweight | Great for Active Days | Merino Wool and lycra | 260gsm | Yes |
| Smartwool Merino 250 | Best for All-Day Comfort | Merino Wool | 250gsm | No |
| Macpac Geothermal | Quick Drying | Recycled polyester | 200gsm | No |
| Mountain Warehouse Merino Thermal | Excellent Merino and Polyester Blend | Merino wool and polyester | Not listed | No |
Thermal Underwear Buying Guide
The best thermal underwear is going to be used again and again, keeping you warm whatever the weather.
Taking your time to choose your thermal underwear carefully can mean you are not only protected from the cold but also comfortable and dry throughout your adventure.
This guide takes a closer look at the material, breathability, and fit of thermal underwear so you can find the most effective option for you.
What is the Base Layer?
Base layers are all about long underwear and thermal underwear. It’s the layer that is going to be worn next to your skin and it is going to keep you warm.
This layer is going to be your first defence against sweat so it needs to be moisture-wicking and breathable.
No matter what type of outdoor adventure you are going on, a good base layer is an essential piece of kit.
While in this article we are focussing on thermal tops, don’t overlook the importance of thermal bottoms (or long johns) in cold conditions.
A thermal underwear set (top and bottom) is important for keeping warm during winter activities.
Natural Vs Synthetic Material
The first choice you need to make with thermals is between synthetic and natural fibres.
Common synthetic materials are polyester and polypropylene, or “polypro”, while the most popular natural material is merino wool.
Synthetic
Generally, synthetic materials will be lighter, more durable, quicker to dry, and cheaper.
This material is good for its moisture-wicking properties and the ability to withstand many washes without losing its shape, comfort, or important properties.
You can generally find many synthetic options that are thinner and lighter than wool base layers which gives you more choice if this is an important factor for your trip.
Synthetic base layers are fantastic for high-intensity activities as they dry quickly. If you are going to be running or participating in similar activities then look at a synthetic thermal underwear set.
Wool
Merino will usually be warmer, more breathable, and feel nicer against the skin. A wool base layer is naturally heating and cooling and helps you self-regulate your temperature to keep you comfortable in all conditions.
One of the key properties of wool is how odour-resistant it is, its ability to wick away moisture means you can wear your thermal underwear more than once without an issue.
Wool is great for a range of activities and temperatures, this is why it is a popular choice for the majority of outdoor activities.
Some people worry that wool is itchy but the itchiness occurs because of the fibres and because merino wool has much finer fibres than traditional wool garments you shouldn’t experience any itchiness from wearing this material.
Rest assured, most people comment on how soft and comfortable merino is, which lines up with our own experience wearing merino wool products for many years.
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Warmth
How warm the thermals are will usually come in the form of the manufacturer’s rating e.g. heavyweight/winter weight, midweight or lightweight.
They will also usually include a reference to how heavy the fabric is, measured in grams per square metre e.g. a winter-weight merino thermal is likely to be between 200-300gsm.
Your personal preferences and requirements will determine which is best for you.
For skiing and snowboarding, you may be tempted to choose the heaviest option, but do keep in mind you will be physically active while on the slopes, so a mid-weight option may be suitable.
Keep in mind that warmth generally translates to more weight, so if you’re going to be carrying your base layer in your pack, it’s worth thinking about this trade-off.
Breathability
Breathability is a fundamental feature of your base layer for two main reasons.
Firstly, while thermals are designed to trap your body heat in colder temperatures, they also need to allow excess heat to escape during physical activity or in warmer conditions.
Breathability is also essential for wicking moisture away from the skin, in order to retain warmth even when you sweat.
Natural fibres like Merino tend to perform better in breathability than any synthetic alternatives.
Comfort
Thermal underwear is meant to fit snug, with enough stretch to allow a wide range of movements. In terms of skin feel, merino is likely to be the most comfortable, especially over longer periods, making it a popular choice for hikers.
Think about the activities you intend to use your thermals for and what features are most important to you.
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Fit
The fit of any thermal underwear base layers needs to be tight. If the clothing is loose then they are going to be less effective and you will struggle to stay warm unless you have other layers to make up for it.
Look for a compression fit or form fitting design as this will ensure your body is kept warm and the base layer is doing its job. The material should be flexible enough that you don’t feel restricted or uncomfortable in your movement.
Style
The most common styles of thermal underwear are high neck, round neck, and crew neck. These may look the same but high necks are, as the name suggests, higher, round necks are looser and crew necks are ideal for layering.
Most of the thermal underwear we have looked at have been round or crew neck styles as these are the most versatile and comfortable for hiking and similar activities.
It is easy to pair a crew neck with a scarf to help protect your neck from the cold or sun. This style works best as a base layer.
Care and Maintenance
Thermal underwear and base layers are working hard to stop you from becoming sweaty, cold, and uncomfortable. It’s important you clean them regularly to keep them in good condition and stop them from becoming smelly.
The best thermal underwear is an investment so it is important to look after it properly. Fortunately, most thermal underwear can be washed in a washing machine but always check the care label for instructions based on the fabric used.
Wool is generally thought to be a high maintenance material that needs to be hand washed but you can usually machine wash it on a low heat or a wool/ delicate cycle.
This guide can help you keep your merino winter clothing clean and ready for action. Different products have different care needs but most thermals do well with a light wash.
Extra Features
Some products come with features such as chin guards or hoods. We’ve outlined the more common features you may see on thermal underwear below:
Thumb loops
Some of the best thermal underwear and base layers have thumb holes so you can pull the sleeve up over part of your hands. You can pair this with some gloves and you won’t feel the wind in your sleeves.
In freezing temperatures, this small and seemingly insignificant feature can really increase your overall comfort.
One of the best things about thumb loops is you don’t have to use them all the time but they are always there when you are starting to feel cold.
Zips
Some thermal underwear includes a neck zip that can help you regulate your body temperature throughout the day.
You can keep it zipped all the way up for minimum chill and to keep you extra warm or you can unzip it to help with ventilation and cooling down after an intense uphill.
Pockets
The clothing may include zip pockets for easily storing items while on the go. Pockets are convenient, especially if you only plan on wearing your base layer when conditions warm up.
Longer Body
Some base layers are designed to be longer in the body which makes them easier to tuck in. This helps to keep you warm and prevent the cold winter air from sneaking in.
