Do Thermals Actually Keep You Warm?

What are Thermals?

Thermals are form-fitting layers of clothing designed to help retain body heat and provide insulation against cold environments. The term “thermals” comes from the concept of thermal insulation—using materials with low thermal conductivity to reduce heat transfer between the body and the external environment.

The history of thermals dates back centuries, with natural materials like wool providing warmth for people living in cold climates. Modern thermal underwear emerged in the late 19th century with the development of moisture-wicking synthetic fibers like polyester. Today, thermals come in two main styles:

  • Long johns – One-piece, skin-tight, full-body undergarments.
  • Underwear – Tops and bottoms made from thermal fabrics.

Both styles trap body heat through insulation and moisture management. They are made from wool, cotton, silk, or synthetic blends. The snug fit increases insulation by reducing air circulation next to the skin.

How Thermals Work

Thermals are designed to trap body heat and retain it close to the skin to maintain warmth. The material and construction of thermals allows them to act as insulators for the body.

Thermals are made from materials that have low thermal conductivity, meaning they do not allow heat to pass through them easily. Popular materials for thermals include wool, silk, and various synthetic fabrics like polyester, nylon, and spandex. The materials are woven or knitted in a way that creates small air pockets, since air is a poor conductor of heat. The trapped air pockets act as insulation to retain body warmth.

Some key factors that influence the insulating properties of thermal materials are:

  • Fiber density – Higher density fabrics have more tightly packed fibers that retain more warmth.
  • Thickness – Thicker materials provide more insulation.
  • Moisture wicking – Materials that wick moisture away help keep skin dry for better warmth.
  • Tight fit – Thermals are designed to fit closely to the body to maintain a warm layer of air close to the skin.

The combination of insulating thermal materials and the snug fit creates an effective base layering system to retain body heat. As the body warms the air inside the thermals, the heat is trapped by the fabric rather than being released into the colder external environment.

When to Wear Thermals

person wearing thermals while skiing

Thermals are designed to provide an extra layer of insulation and warmth, making them ideal for wearing in cold weather or environments. Some of the most common situations when thermals can be useful include:

Cold Weather – Thermals make excellent base layers when it’s cold outside. They help retain body heat that would otherwise be lost through your outer layers. Thermals are commonly worn during winter months or in cold climates to provide extra warmth.

Outdoor Activities – Active outdoor sports like skiing, snowboarding, hiking, hunting, and camping can benefit from wearing thermals underneath your outer gear. The extra insulation helps regulate body temperature and prevents chilling.

Under Regular Clothes – For everyday cold weather wear, thermals can be worn underneath regular pants, tops, dresses, and suits to provide an extra layer of warmth without being visible. This allows you to wear your normal wardrobe while staying insulated.

Inactive/Stationary Situations – Thermals are useful when you will be relatively inactive or stationary in the cold. For example, wearing thermals under a uniform or work clothes if working outside in the cold. Or wearing thermals while sitting at a sports event or outdoor concert.

Pros of Wearing Thermals

One of the biggest pros of wearing thermals is that they provide warmth without bulk. Thermals are made from lightweight, breathable fabrics that trap heat close to the body. This allows you to stay warm without having to wear multiple bulky layers. The thin profile of thermals makes them easy to layer under regular clothes.

Another advantage of thermals is their moisture wicking properties. Thermals pull sweat away from the skin to keep you dry. This is important for maintaining body temperature. If sweat gets trapped against the skin, it can cause chills. The moisture wicking fabrics in thermals prevent this.

Thermals are also extremely affordable compared to other insulators like down jackets. You can buy high quality thermals for a fraction of the cost of a down jacket. This makes thermals accessible for people on any budget.

Lastly, thermals are versatile. You can wear them under your regular clothes for everyday use. Or layer them under snow pants and jackets for winter sports. Their thin profile makes them easy to use in a variety of situations.

Cons of Wearing Thermals

While thermals do provide warmth, they also come with some drawbacks compared to regular clothing. The main complaints about thermals are that they can feel tight and constricting, especially for active wear. The tight fit that helps contain body heat also restricts movement and flexibility. For some people, thermals can feel too clingy and uncomfortable.

Thermals are also less breathable than cotton or athletic fabrics. The materials used to make thermals, like polyester and spandex, don’t allow airflow and ventilation as well as looser, natural fabrics. This can lead to a hot, stuffy feeling during activities, as sweat and heat gets trapped close to the body. The lack of breathability is a particular issue during high exertion activities like sports and exercise.

In very cold weather, thermals alone may not provide enough insulation. Layering is required on top of thermals to block wind, snow, and freezing temperatures. The need for layering reduces the convenience factor of thermals. Having to wear thermals plus multiple outer layers makes getting dressed more complex.

How Effective Are Thermals?

Thermals have been around for decades, providing warmth and comfort to outdoor adventurers, athletes, and anyone who needs an extra layer of insulation. But just how well do they work? Let’s take a look at some research on the effectiveness of thermals.

Over the years, there have been several studies analyzing the heat retention capabilities of various fabrics and weaves used in thermals. These studies generally look at metrics like clo values (a measure of insulation) and compare the effects of different materials and thicknesses.

In one study published in Textile Research Journal, researchers tested moisture-wicking thermals made from polyester versus cotton. They found the polyester thermals provided significantly more insulation and warmth while active. The polyester fabrics retained warm air better than the cotton.

Other studies have looked specifically at wool versus synthetic thermals. Wool is known for its natural insulating properties. In a study published in the Scandinavian Journal of Medicine & Science in Sports, participants wore merino wool, polyester, and cotton thermals while exposed to cold temperatures. The wool provided the most insulation, followed by polyester and cotton.

When it comes to thickness, thermal experts recommend opting for a midweight style in most circumstances. Heavyweight thermals can restrict motion while lightweight may not provide enough insulation for colder activities. A 200-300 weight merino or synthetic thermal offers a good balance of warmth and breathability.

In real-world tests, outdoor gear testers and winter athletes generally find today’s polyester and wool thermals effective at retaining body heat. They emphasize choosing the right weight and fit for your intended use. With a snug but comfortable fit, quality thermals can make a significant difference in keeping core body temperatures up, compared to regular clothing.

Thermals vs. Other Insulators

Thermals provide insulation through their tight-fitting, breathable construction that traps body heat. Compared to other common insulators, thermals have advantages and disadvantages:

Down: Down insulation, often from goose or duck feathers, is extremely warm and lightweight. However, down loses insulation value when wet. Thermals retain warmth even when damp from sweat.

Wool: Wool is a natural fiber that provides excellent insulation, even when wet. But wool can be itchy for some people. Thermals are usually made from smooth, synthetic fibers.

Fleece: Fleece and thermal underwear are both synthetic insulators. Fleece is extremely soft and comfortable but less fitted than thermals. Thermals’ snug fit increases warmth by reducing air pockets.

Cotton: Cotton is affordable but loses almost all insulating properties when wet. Thermals maintain insulation despite moisture from sweat.

Overall, thermals strike a balance between insulation, moisture wicking, breathability, comfort and affordability. For cold and active pursuits, they are often a better choice than down, wool, fleece or cotton alone. But layering thermals with fleece or wool outer layers can provide additional warmth as needed.

Best Materials for Thermals

When it comes to the best materials for thermals, there are a few top options to consider:

Merino Wool

Merino wool is one of the most popular materials for thermals. It’s a natural fiber that provides excellent insulation and helps regulate body temperature. Merino is also moisture wicking, breathable, and odor resistant. It’s a great choice for active thermals.


Polyester is a synthetic material that is commonly used in thermals. It dries quickly, is durable, and provides insulation. Polyester thermals are typically more affordable than wool. However, polyester doesn’t breathe as well as natural fibers. It’s a good budget option for thermals.


Silk thermals provide ultra-soft insulation. Silk is one of the warmest natural fabric options, while still being breathable. It’s also moisture wicking. The downside is that silk thermals are quite expensive.


Cashmere is an extremely soft, fine wool that makes incredibly warm yet lightweight thermals. It insulates well without overheating. But cashmere comes at a luxury price point, making it an investment piece.

When selecting the best materials for your thermals, consider warmth, breathability, moisture wicking, odor control, and budget. Synthetics like polyester provide an affordable option, while premium natural fibers like merino wool, silk, and cashmere offer superior comfort and insulation.

Caring for Your Thermals

Taking proper care of your thermals will help extend their lifespan and performance. Here are some tips for washing, drying, storing, and mending your thermal underwear:


Check the care tag first to see the recommended washing method. In general, wash thermals in cold water on a gentle cycle. Avoid using fabric softener as it can affect the thermal properties. Turn them inside out before washing to protect the exterior fabric.


Lay flat to dry or tumble dry on low. Do not dry clean thermals as the chemicals used can degrade the fibers. Avoid hanging thermals as the weight can stretch them out.


Fold thermals neatly and store in a clean, dry place like a drawer or bin. Do not stuff them tightly in a hamper. Store away from direct sunlight which can fade colors. Consider storing with cedar blocks to help repel moths.


Inspect thermals regularly for holes, torn seams or loose threads. Hand stitch minor holes and tears to avoid runs. For larger holes, use an iron-on patch on the inside of the fabric. Replace thermals with excessive thinning and pilling that affects their insulating ability.

The Future of Thermal Technology

Thermal technology is constantly evolving as researchers and manufacturers seek to improve the performance and versatility of these garments. Here are some key areas of innovation to look out for in the future:

New Materials: Scientists are developing new synthetic fabrics and treatments that maximize insulation while remaining lightweight and breathable. Some future materials may include aerogels, microfibers, and phase change materials that adapt to ambient temperatures.

Improved Moisture Wicking and Breathability: Current research aims to improve moisture wicking performance to keep wearers drier. New fabrics will also have enhanced breathability to prevent overheating and allow vapor from sweat to evaporate quickly.

Companies are experimenting with additions like graphene and carbon nanotubes to make thermals more effective. Spacer fabrics with air pockets are another innovation to watch. The end goal is thermals that insulate without bulk and keep wearers comfortable in more diverse temperatures and activities.

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