The great summer weather is the perfect time for long cycling trips alone or with friends. Although for many people, the bike is simply a means of communication in the city, it is worth seeing more of its
Activity zone 4F
In this zone, your heart rate rises at the mere thought of the next workout. You'll find the best ways to exercise effectively, discover new disciplines, and choose an outfit that suits your needs. We'll also guide you through the clothing care process and advise what to buy for enthusiasts of various sports. Our blog is a guide for everyone who wants to feel motivation 4 times more!
Published: 1/12/2026💡 Tips|🏂 Winter sports
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Thermal and base layer underwear for winter and summer
Most athletes wear thermal underwear regardless of the weather, as it keeps you comfortable, which is a must during many activities. But how to choose the right style that matches the season and personal expectations? What to wear under thermal underwear? Do terms base layer and thermal really denote the same thing? Read on: here’s what you should know before choosing the perfect underwear for yourself.
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Base layer vs thermal underwear - are they the same or completely different?
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How to choose underwear for winter and summer - basic types of thermal underwear
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Thermal underwear vs base layer underwear pieces - activity type:
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Base layer pieces vs thermal underwear. Which one provides superior moisture wicking?
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How to take care of thermal underwear and base layer pieces?
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Thermal underwear and base layer pieces - sports taken the pro way
Table of content
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💧 Thermal underwear is the first layer made of synthetic fabrics that wicks sweat and moisture away from the skin, keeping you dry and reducing the risk of hypothermia (cotton is not a good choice for activities).
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Thermal ≠ thermal: thermal underwear primarily warms and insulates, but does not wick away moisture like thermal underwear, so it works better for lower intensity activities and in cooler weather.
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🌤Choose according to the season and intensity of activity: light thermoactive for summer and intense workouts, medium year-round as a base layer, and thick/fleece for winter or as an extra layer in low temperatures.
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🚵 How to choose for your activity: for running/fitness, choose thermoactive (ventilation, quick drying), for winter sports, thermoactive models made of merino or synthetics will work well, and for walking/sightseeing in winter, thermal underwear is often enough for thermal comfort.
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🪥 Care matters: wash your underwear after each use and avoid fabric softeners (they can weaken its properties) – this helps to preserve its functions for longer.
A brief summary
What is thermal underwear?
Thermal underwear is a type of clothing manufactured from synthetic knitted fabrics that efficiently wick sweat and moisture away from human skin, leaving it dry even during intense exercise. Moisture causes the body to lose heat about 20 times faster than in dry conditions. Thanks to the specialised construction of the fibres and material, thermal underwear prevents this process. It wicks sweat away to the outer layer, reducing heat loss by the skin.
Cotton, for example, which is not recommended for any physical activity, absorbs as much as 14 times more moisture than synthetics and has very low moisture wicking parameters. Because of this, you may experience unpleasant sensation of cold and dampness during intense workouts, thus putting yourself at risk of hypothermia. This is why a sweat-wicking T-shirt should not be made of cotton.
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It has elaborate external structure.
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It absorbs moisture more slowly than it is wicked away.
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It is flexible and fits the surface of the skin snugly.
Thermal underwear effectively wicks sweat away because:
Base layer vs thermal underwear - are they the same or completely different?
Base layer and thermal clothing are completely different terms. Many people use them interchangeably, thinking about the latter type - that’s why you may hear different opinions about them. However, these two terms refer to a completely different type of clothing and technologies used.
Base layer underwear pieces protect the body from hypothermia, but unlike thermoactive pieces, it does not wick the moisture outside. Base layer underwear is therefore not recommended for summer days - it’s all due to high temperatures that usually require sweat be wicked away from the skin.
It is perfect for those who do not participate in very intensive activity that increases perspiration, while their primary concern is good thermal insulation. That’s the reason why mountaineers, skiers, and those trekking in low temperatures enjoy wearing it. It is also recommended by snowboarders, ice skaters and those enjoying the winter outdoors. This type of underwear ensures thermal comfort. Paired with other clothing it forms a set that not provides comprehensive protection against cold, moisture or strong gusts of wind.
What are the functions of base layer pieces?
Base layer underwear pieces have many functions. This is one of the reasons why it is so popular among active people. Here are the most important ones:
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Allowing breathability and keeping the skin dry - as we wrote earlier, this type of clothing wicks moisture away faster than it would absorb it. Sweat-absorbing T-shirt and similar pieces are perfect for anyone involved in intense sports such as running, skiing, cycling. It elevates the comfort without exposing the body to cold.
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Providing excellent thermal insulation - the outer layer of underwear regulates body temperature. How? Firstly, it prevents cool air from permeating inside, and secondly, it traps the heat generated by the body. To sum up, professional thermal underwear provides excellent thermal insulation, even at low temperatures.
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Guaranteeing unrestricted movement - thermal underwear is manufactured using special methods that make it adjust to the body and provide unrestricted movement. Moreover, most pieces are seamless, protecting the skin against abrasions and eliminating the unpleasant sensation of pressure.
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Reduces odours - some thermal underwear pieces are made with the use of silver threads or polypropylene fabrics. Their bactericidal properties reduce sweat odour.
How to choose underwear for winter and summer - basic types of thermal underwear
Thermal underwear can be conventionally divided into 3 types - lightweight, medium and thick. This is your starting point; you should select underwear that matches particular physical activity and conditions that you plan on using it in.
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Lightweight underwear - best as a first layer and as a summer training outfit. Perfect for running, trekking and gym training. Thermal underwear for summer effectively wicks moisture away without warming up the body. This type of apparel includes thermal T-shirts and short or 3/4 leggings.
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Medium underwear - clothing that can be worn all year round. It will also work as base layer. It is perfect for all physical activities, but sometimes requires putting on extra layers of clothing. Browse this category to find thermal long sleeves and leggings. What to wear over thermal underwear? Put on a warm fleece, softshell or functional jacket.
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Thick underwear - unmatched as the second layer of clothing. Most often it is made of soft microfleece, which retains body heat in a superior way. This is practical sports thermal underwear for winter. There’s more: it can be worn as thermal underwear for physical work. Over it, you should put a warm jacket, which will take even better care of your thermal comfort. This type of underwear will work well during various winter activities. Thick thermal socks, warm leggings or long sleeves are some pieces of this type.
€25.90
The lowest price in the last 30 days before the reduction
€34.99
€30.90
The lowest price in the last 30 days before the reduction
€39.99
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€25.90
The lowest price in the last 30 days before the reduction
€29.99
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€30.90
The lowest price in the last 30 days before the reduction
€34.99
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How to buy base layer pieces and thermal underwear?
When choosing the right underwear, pay attention to its cut and quality. Want to use it as the first layer? Remember that it should fit your body perfectly without restricting movement. That's why choosing a size in line with the size chart matters so much. High-quality products are designed using modern technologies - look out for them while shopping. This will serve as a tip regarding the conditions that these pieces can be worn in .
Next step? Check underwear fabric - at 4F we differentiate between thermal underwear, seamless underwear, quick-drying and fleece underwear.
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Quick-drying underwear - as we’ve mentioned before, this makes for perfect summer apparel. It’s made of polyester with elastane admixture and has no warming properties. It owes its perfect sweat-wicking properties to extra technology used.
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Thermal underwear - also called seamless underwear in our store - includes pieces with an extensive range of uses and that work as base layer. These are made of polyamide , polyester and elastane. The first two materials provide superior insulation and sweat-wicking properties while the last fabric improves underwear flexibility.
Remember: don't be guided by appearances and clothing composition, as it takes on different features through the use of various technologies. For seamless underwear available in the 4F store, we use the 4F Skin system. In addition, selected pieces, such as thermoactive shirts, have the 4F Dry technology, which quickly wicks moisture away to keep you feeling dry.
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Base layer underwear - which sometimes serves as the second layer of clothing. Made of polyester with elastane admixture to increase its flexibility, or polyester alone. However, this type of material is not used to wick moisture away, so it is referred to as thermal clothing. Search for Microtherm or Microtherm Extreme fleece underwear to find it in our store . Fleece underwear is made of soft microfleece, so it works even in particularly harsh weather conditions.
When shopping, make sure the underwear you choose has the following:
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Reflective elements - priceless when running after dark.
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Fixed or adjustable stand-up collar - providing extra protection against the wind.
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Superior ventilation zones based on body map - such underwear has superior sweat-wicking properties.
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Bacteriostatic system - this type of underwear neutralises sweat bacteria that causes unpleasant odour.
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Special seams - thanks to advanced technology, they cannot be felt and do not cause chafing or damage to the skin during intense activities.
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Zipper - for pieces that serve as the second layer, it makes it easier to put it on. Moreover, it provides better ventilation.
Thermal underwear vs base layer underwear pieces: which one to choose depending on physical activity type?
When choosing functional underwear, consider your sports activity of choice. Keep in mind that it’s worth choosing different underwear depending on various workout intensity.
Thermal underwear vs base layer underwear pieces - activity type:
1. Intense workouts - running, team sports, crossfit
Opt for thermal underwear. It will keep you comfortable even during an intense workout. Such pieces efficiently wick sweat away and provide decent breathability. Long story short, they do not cause overheating. During intense exercise, you will appreciate their low weight and flexibility.
2. Outdoor activities of moderate intensity - Nordic walking, trekking, recreational cycling
Thin thermal underwear will wick sweat away and adjust to your body during moderate-intensity workout sessions. If you're after quick-drying properties, purchase a version made of synthetic materials.
3. Winter sports - skiing, tobogganing, snowboarding, outdoor walking
During winter activities, thermal underwear made of merino wool or synthetic materials will be your best bet. Such clothing will wick moisture away and prevent heat loss.
4. Light workouts on cold days: walking, sightseeing
In this case, wear base layer pieces underneath your clothes. Why? When you engage in light activity, the body sweats moderately. The underwear is to primarily ensure your thermal comfort and unrestricted movement.
5. Working out in variable conditions - cross-country running, hiking in the mountains
When venturing into the mountains, prepare a set of medium-thick thermal underwear. Its flexibility and breathability will keep you comfortable, and you'll appreciate the extra thermal layer when you stop.
6. Summer workouts - fitness in the outdoors, bootcamp, kayaking
On warm days, go for thermal underwear. Its sweat-wicking properties will keep your skin dry.
Which pieces work well for the youngest ones?
Wondering what underwear will be a good choice for your child? Below you will find tips to make your choice easier:
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Fit - both base layer pieces and thermal underwear are supposed to fit snugly without being overly tight.
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Material - kids’ sets are often made of polyester, polyamide or have elastane admixture. These fabrics ensure quick drying, breathability and flexibility. If your child has irritation-prone skin, look for versions with a blend of natural fibres, such as merino wool.
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Purpose - for a busy child who plays outside a lot, thermal underwear will be the right choice. And thermal underwear will work well for less intense activities, such as a winter walk.
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Comfort - make sure kids' underwear is comfortable to wear. Flat-felled seams, elasticated cuffs or cut ensuring freedom of movement are all details worth noting.
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€12.90
The lowest price in the last 30 days before the reduction
€14.90
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€14.90
The lowest price in the last 30 days before the reduction
€17.90
€14.90
The lowest price in the last 30 days before the reduction
€17.90
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€12.90
The lowest price in the last 30 days before the reduction
€14.90
€12.90
The lowest price in the last 30 days before the reduction
€14.90
)
)
)
€14.90
The lowest price in the last 30 days before the reduction
€17.90
€12.90
The lowest price in the last 30 days before the reduction
€14.90
€14.90
The lowest price in the last 30 days before the reduction
€17.90
€14.90
The lowest price in the last 30 days before the reduction
€17.90
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€39.90
The lowest price in the last 30 days before the reduction
€49.90
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€34.90
The lowest price in the last 30 days before the reduction
€42.90
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€30.90
The lowest price in the last 30 days before the reduction
€34.90
Base layer pieces vs thermal underwear. Which one provides superior moisture wicking?
If you are looking for underwear that quickly wicks moisture away, make sure your wardrobe includes thermal pieces. They are designed to move sweat away from the body, towards the outer layer of the material. As a result, the skin remains dry. Thermal underwear does not offer this good moisture-wicking properties; its most important task is to keep you warm.
How to take care of thermal underwear and base layer pieces?
Wash your base layer pieces and thermal underwear after each workout to ensure these pieces retain their original properties for a long time. Skipping this step can decrease its effectiveness by up to three times!
Wash both base layer pieces and thermal underwear using ordinary washing powder. Importantly, its formula should not include lanolin or other softeners . And this is because they could damage the fibres, affecting adversely the properties of functional textiles.
Read more about how to wash thermal and thermoactive underwear!
Thermal underwear and base layer pieces - sports taken the pro way
Base layer pieces and thermal underwear are two types of clothing designed for anyone who appreciates comfort regardless of the weather outside. If you don't want to sacrifice comfort during a workout, including an intense one, stock up on such products. When choosing functional underwear, follow your preferences, check the technologies used in a particular piece and consider the conditions you plan to use it in. After all, each sport requires slightly different preparation.
Remember that underwear is important, and in harsh weather conditions - it's plain crucial.
€25.90
The lowest price in the last 30 days before the reduction
€34.99
€30.90
The lowest price in the last 30 days before the reduction
€39.99
€25.90
The lowest price in the last 30 days before the reduction
€29.99
€30.90
The lowest price in the last 30 days before the reduction
€34.99
FAQ – thermal underwear vs. thermoactive underwear
1. How does thermoactive underwear differ from thermal underwear?
2. What underwear should I choose for winter and what for summer?
3. Does thermal underwear really prevent hypothermia?
4. How to choose underwear for the type of activity?
5. How to care for thermoactive and thermal underwear so that it retains its properties?
4F Editorial Team
The 4F editorial team is a group of experts who live and breathe sports and value an active lifestyle. Drawing on the experience of the 4F brand, we show that exercise is a source of energy, balance, and well-being. We provide reliable training advice, follow trends in sports fashion, and support our readers in discovering their motivation to take action.
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