If you are a seasoned trekker, you are probably aware of what exactly base layer thermals are and when and where to use them. However, if you are someone who hikes once in a blue moon (or probably someone who plans short camping trips in the mountains once a year), there is a good chance that you have spent sleepless nights in the tents/home-stays feeling unbearably cold(and uncomfortable/itchy) even after wearing multiple layers of jackets and sweaters. That’s probably because most of the body heat escapes from the layer closest to the skin and you are satisfied with a normal shirt, hoping that the heap of jackets and sweaters and pullovers would keep you comfortable enough in those freezing temperatures!

As a result, it might indeed be a good idea to invest in the layer closest to the skin when hiking out in subzero conditions, even if you do that only once or twice a year. Additionally, with the advanced fabric blends available nowadays, clothes for the winter season have become versatile and suitable for large temperature ranges, thus ensuring that your investment in buying a base layer is not only for an annual/biannual trip. We’ll also see later how extreme cold weather thermals for both men and women should ideally be versatile to combat significant temperature changes/moisture absorbed and various other challenges of winter inner-wear in tough conditions.

What exactly is thermal wear?

Thermal inner wear (or long johns as they are also called) are a combination of thin long pants and thin tops with long sleeves that are designed to be worn directly against your skin(almost like a second skin) with other layers of winter clothing to keep you warm in extremely cold climates. Unlike standard inner-wear, thermals must be absorbent on the inside to wick away moisture from the body quickly. They are also designed to be fast drying to prevent the chill factor due to body sweat. Hence good quality thermals are required in extreme cold weather conditions, especially if you are planning to be active(walking, running, jogging) so that you don’t feel cold when moisture lingers against your skin.

Decoding the magic of merino wool: fiber by fiber

The best thermals for subzero temperatures are made of merino wool. Merino wool comes from merino sheep who live in the Southern Alps of New Zealand and survive extreme temperatures of 35°C in the Summer and -20°C in the Winter. As these sheep have to survive in such extreme conditions, their body coat is temperature regulating meaning the sheep are kept cool in the Summer and warm in the Winter. Thus merino is an active fiber that takes in factors such as the wearer’s body temperature and external conditions, making them the best fit for making thermals. The few properties of merino wool which make it the best choice for thermal wear are listed below:


Unlike regular wool, merino wool doesn’t cause skin irritation. These fibers are so fine that when in contact with the skin they bend. They are also hypoallergenic in nature.

Moisture absorbing properties

Merino wicks moisture away from the skin into the atmosphere, keeping you dry for a longer period of time. It can also absorb about 35% of its own weight before feeling wet.

Lasts long

Merino wool clothing lasts much longer than ordinary wool, primarily because merino wool fibers can bend over 20,000 times without breaking so won’t lose shape or get holes. This is also one of the reasons merino wool fabrics are so expensive.

The secret blend for extreme conditions

Although 100% merino wool base layer thermals are highly effective in subzero conditions, they can still be itchy at times because of the nature of the wool and have almost no aesthetic features. Additionally, if you are running or hiking from the bottom of the mountain(with a temperature above 10°C) to the summit(with temperatures below -5°C), you would naturally sweat a lot and pure merino wool may not always be the best fabric for absorbing the moisture, hence making you feel colder at the top of the mountains. It’s also worthwhile to note that thermals are not easy to remove or change as they are the innermost layer of winter clothing. In such cases, it makes sense to blend merino wool with a fabric that not only is soft and lightweight but also has enhanced moisture-wicking properties.

With the advance of fibre technologies, Kosha, a premium winter wear brand based in Mumbai has now made it possible to blend merino wool fibres with bamboo fabric, made from the pulp of bamboo plants, which are actually the fastest growing grass species in the world.This has been made possible through extensive Research and Development on the subject, and incorporating both the properties of merino wool and bamboo fibre in the blend.

Bamboo fiber is soft and comfortable against the skin, as it is naturally smooth and round even without any chemical treatment. It has no sharp spurs to scratch against the skin, is a highly absorbent material,has quick drying properties and people who are allergic to other natural fibers like wool or hemp will usually experience no allergic reactions from wearing clothing made of bamboo fibers, making it the ideal choice to blend with merino wool.

The best part about this blend is that it doubles up as “winter t shirts “, apart from being soft,durable with enhanced moisture wicking and antibacterial properties making it the ideal choice for seasoned hikers or adventure seekers. These are made up of 47.5% merino wool, 47.5% bamboo and 5% elastane making it suitable for a temperature range of 15°C to -20°C.

Thus it’s safe to say, after studying both the qualities of merino wool and bamboo, that you should definitely invest in thermals not only because you need them when you are encounter extremely cold climates, but also because they are suitable for wide temperature ranges and have enhanced moisture-absorbing properties so would come handy in a wide range of situations!