1. Expert reviews

Top 4 Best Thermal underwear of 2021

Anna Lavfors

Updated March 6, 2021

We tested thermal underwear and name the Reima Lani as best in test. Reima's sporty underwear is warm and, thanks to flat seams, very comfortable.

Our best budget choice is the Kuling Outdoors, which is made from merino wool. This good value underwear is really soft against the skin and also keeps the heat in really well.

Top 4 Best Thermal underwear of 2021

How we did the test

We carry out all our tests ourselves and all products are used as they are intended to be used in reality. In our test of thermal underwear, we focused on the following things:

  • Fit:

How well do the garments fit on the body, are they comfortable to move around and play in, did the child like wearing them?

  • Function:

Does the underwear keep the wearer warm? Does the child get sweaty in the underwear? Do they feel comfortable or do they itch?

  • Durability:

How well did the underwear resist wear and tear? Did it develop bobbles or holes? How well did the underwear stand up to washing?

In addition to these properties, we also considered the price of the underwear in choosing the winner of the test.

1. Reima Lani – BEST CHOICE THERMAL UNDERWEAR 2021

Comfortable underwear with flat seams, keeps the child warm and dry

Design: Sold in sets of top and trousers Material: 43% Polyester THERMOLITE, 53% polyester, 4% Sorona Washing: 40°C, wash inside out, do not tumble dry

Reima Lani Thermal Set - Blue (536183-6680)

The Reima Lani is a slightly sportier set of underwear with marked seams, good elastic at the waist and a top that’s extra long at the back. The design of the underwear is great for play and movement, and the materials succeed well in keeping the child warm without them becoming sweaty. If you look closely inside the garments, you will also see a structure similar to that found in more functional garments, designed to dissipate sweat and moisture. Our testers found the underwear to be really comfortable. The seams are flat and never chafe on the skin. Instead of a label at the neck, the top even has its washing instructions printed directly on the fabric. The same applies to the trousers, even though both garments have tags on the side for information about the material.

The fit is good and holds up well even after washing. Size 130 is perfect for a child who otherwise wears size 122/128, with a bit of room for growth. However, the wide elastic at the waist can feel a bit hard and makes the trousers feel tight across the stomach for some children. There were some signs of bobbling in the trousers, after one winter using the underwear, but not very much. The inside was still as smooth and comfortable as when the Reima Lani underwear was new and unused. The underwear can be machine washed but not tumble dried, but it dries easily overnight if hung in a warm room. The Reima Lani is a set of underwear that works for most situations and is made to be played in.

Keeps the child warm and dryflat seams make it comfortable
Elastic at the waist can feel a bit stiff and tight

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2. Kuling Wool Underwear – BEST BUDGET CHOICE

Affordable, soft and comfortable thermal underwear with good heat-retaining ability

Design: Trousers and top sold separately Material: 100% merino wool Washing instructions: 30°C, do not tumble dry

Kuling Wool Tee - Grey Melange

The Kuling Wool is a surprisingly affordable set of thermal underwear, particularly if you care more about function than appearance. After an entire winter season using the Kuling Wool thermal underwear, our testers said that it was really soft and comfortable underwear which, despite its very close fit, never feels itchy or uncomfortable for the child. The seams are also soft. The only thing that chafes is the washing instruction label in the side of the top. In that respect we would have preferred Kuling to stick to the previous year's idea of having patches sewn on the outside of the garments. Not as nice to look at, but much more comfortable for the child.

The underwear is relatively thin but still feels warm and works well as layer 1 – in other words the layer the child should have closest to their body. Often it was enough to wear just the underwear indoors, and outside with just a snowsuit over the top. The garments are especially suitable for warm children, who can easily get sweaty from more compact underwear. The garments can be machine washed at 40°C but must drip dry. They remained clean and fresh throughout the winter, despite rather hard use in the preschool environment. What is noticeable is that the outside has become quite bobbly. But if you can ignore a certain amount of aesthetic wear, the Kuling Wool Underwear is as warm and comfortable after a year of use as it was when the garments were brand new. So you really get a lot of underwear for your money.

Very soft and comfortable to wearkeeps the heat ingood value
Gets bobbly from washing and uselabel itches

3. Reima Taival

Really warm underwear with flat seams, comfortable to wear directly against the skin

Design: Sold in sets of top and trousers Material: 80% wool, 20% polyamide Washing instructions: 30°C, wash inside out, do not tumble dry

Reima Taival Thermal Set - Navy (536181-6983)

The Reima Taival is an aesthetically pleasing set of underwear in jacquard knitted wool. Both trousers and top are fully patterned in nice colours and shapes. This underwear is perfect for children who feel the cold easily, or for those who are going to be outdoors often and for a long time. Because wool acts as a temperature regulator, it should withstand all seasons. In practice, however, the children found it to be a bit warmer than the thinner underwear in our tests. But when the winter was at its darkest and coldest, the Reima Taival under a snowsuit was the perfect combination of clothes for playing in the snow.

The fit is good on both trousers and top. The garments sit close to the body without feeling too tight, and the flat seams can’t really be felt against the skin. Instead of size and washing labels at the neck and waistband, Reima have printed the information directly onto the fabric. This is a clever move, as such labels can often itch. The elastic at the waist keeps the trousers firmly in place, but never feels rigid or uncomfortable. Overall, our tester children were happy to wear and use the Reima Taival. The only thing we did notice is that the seat of the trousers became a bobbly quite quickly. But the material didn’t get thin and otherwise there were no apparent effects from washing or use. The Reima Taival should definitely keep children warm over several winter seasons.

Really warmvery comfortable to have directly against the skinflat seams
Gets bobbly around the seat quickly

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4. Bergans Fjellrapp

Soft, thin underwear that breathes well and fits well

Design: Trousers and top sold separately Material: 100% merino wool Washing instructions: Wool program 30°C, do not tumble dry, should be stretched while wet

Bergans Fjellrapp Youth Shirt - Primula Purple

The Bergans Fjellrapp is a thin, airy set of underwear that’s intended to cool the wearer when the body gets warm, and warm them when the body gets cold. The material should also repel both dirt and odours. Whether this is true in practice is difficult to tell, because when children wear clothes while playing in a group or eating, stains occur regardless. But it’s very clear that the underwear works well and keeps them warm, as on many winter days the Fjellrapp alone works well as the only garments the child needs under their snowsuit. The design is both sporty and functional.

The trousers and top are sold separately but are available in matching colours and patterns. The total purchase price is relatively high. The top has many visible seams, and even if they’re flat and the wearer can’t feel them very much, there do appear to be more seams than necessary. The merino wool is very comfortable against the skin and the fit is good. The garments follow the shape of the body well without being tight. The waist of the trousers consists of a fairly broad elastic. This means the trousers won’t slip down, but it can also mean they feel a little tight around the stomach for some children. As the merino wool is airy, Fjellrapp underwear dries quickly after washing, but it’s always a bit of a chore to have to stretch the garments while they’re wet. The Bergans Fjellrapp is both durable and bobble-free, which proves that this is underwear of the highest quality.

Very thinsoft materialgood fitboth warm and breathable
Expensivesomewhat complicated to washtoo many decorative seams

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