Best thermal underwear: Base layers to keep you cosy all winter
When the temperature drops, the best thermal underwear is sure to keep the whole family snug and toasty
As the days get shorter, darker and colder, we all inevitably start to rummage around in our wardrobes looking for extra layers to keep us warm. And while an extra jumper is the most obvious go-to once the cold hits, knitwear can be quite bulky and uncomfortable even when going about our day-to-day errands. However, there is a better option: thermal underwear.
Usually consisting of a long-sleeved t-shirt and leggings-style bottoms, a thermal underwear set is made from a soft material that’s able to retain heat. A good set of thermal underwear is comfortable, lightweight, and will keep you warm all day – even if you’re out in the blistering cold.
For those who believe that any old T-shirt or leggings under your clothes will suffice, think again. Thermal underwear is designed to hold on to heat, keeping a layer of warm air against your skin. These light and comfortable separates are often worn by professional athletes such as skiers, snowboarders, hikers and mountain climbers.
There are two main considerations when shopping for thermal underwear. It’s important that this innermost layer fits like a second skin and that it’s made from materials that won’t irritate. This may sound daunting, but it’s easy to find thermal underwear that meets both your needs and your budget. Below we explore the key considerations when purchasing good-quality thermal underwear, while also listing the best thermal underwear available to buy now.
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Best thermal underwear: At a glance
- Best thermal underwear overall: Smartwool Classic Thermal Merino Bottom | £100
- Best budget thermal underwear: Heatwave Men’s Thermal Underwear Set | £16
- Best silk thermal underwear: John Lewis Thermal Silk Leggings | £45
- Best thermal underwear for kids: John Lewis Kids’ Thermal Long Sleeve Top | From £10
How to choose the best thermal underwear
What types of material is thermal underwear made from?
There are a few materials you’ll see when shopping for thermal underwear:
Merino wool – This is one of the most popular materials for any type of warm and comfortable clothing, including socks and thermal underwear. If you’re having flashbacks of old, itchy wool jumpers, don’t worry – merino wool is ultra-fine and soft to the touch. As a natural fibre it wicks moisture well, retains heat, cools when needed, and is odour-free. Merino wool items tend to come in at a higher price point, and require proper care if they’re to last some time.
Silk – Like merino wool, silk is a natural fibre that retains heat, holding warmth against the skin. It’s moisture-wicking, too, although less so than merino wool, and is priced higher than items of other materials. Note that silk also holds odours, so will require more frequent laundering.
Cotton blend – Shopping around for thermal underwear, you’ll frequently come across items made from a combination of cotton and synthetic materials. For those with a more modest budget, this is the best option. Since cotton is a natural fibre, it’s good for heat retention, although it doesn’t have the wicking abilities of some of the other materials on the list. And it’s in this regard that synthetic material such as polyester, nylon or viscose is useful, since it wicks moisture well. However, they’ll need frequent washing.
Synthetic – Fully synthetic thermal underwear options come with many benefits. They tend to be the most affordable and are usually quite durable, too. They’re excellent at moisture-wicking and can be comfortable, particularly if they include spandex.
As well as material type, it can also be helpful to consider the weight; thicker fabrics might sit awkwardly under outfits and tend to be less breathable than lighter fabrics. However, for a long trek in the cold, base layers made from a thicker fabric will certainly keep you warmer.
What fit should I look for in good thermal underwear?
Thermal underwear should fit as snugly as possible, while still remaining comfortable. It needs to sit close enough to your skin to act as an insulating layer, trapping the heat between the material and your body. Note, too, that moisture-wicking fabric has to be touching skin in order to work properly. So, while your thermal underwear shouldn’t feel constrictive, it should be form fitting and rest close to the skin.
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The best thermal underwear to buy in 2022
1. Smartwool Classic Thermal Merino Base Layer Bottom: Best thermal underwear overall
Price: £100 | Buy now from Smartwool (women’s) | Buy now from Smartwool (men’s)
Merino wool is one of the best fabrics for keeping you warm in colder weather. The slim fit allows it to be functional, no matter your body shape. This style also uses a flatlock seam to prevent chafing.
As a breathable fabric, merino wool holds the warmth against your skin. At the same time, it pulls moisture from your skin, wicking away sweat to keep you dry. The interlocking knit of the wool allows the skin to breathe comfortably while also delivering thermoregulation.
There’s no doubt that these bottoms are one of the pricier options on this list, but in our opinion it’s well worth the extra cost. It works as intended, and with proper care will last an incredibly long time. The men’s bottoms here include a merino-covered elastic waistband and a standard fly design. Overall, Smartwool has some of the most comfortable and warm thermal underwear available.
Key details – Material: 100% merino wool; Sizes: XS–XL (women), S–XXL (men); Colours: Black, Black Cherry Heather, Blueberry Hill Heather, Twilight Blue Heather (women); Black (men); Fit: Slim
2. Heatwave Men’s Thermal Underwear Set: Best budget thermal underwear
Price: £16 | Buy now from Amazon
For a more affordable way to keep warm in colder temperatures, look to Heatwave. For under £20, you can get two full sets of men’s thermal underwear, with each set comprising a thermal T-shirt and full-length pants. Arriving with a 0.45 TOG rating, it keeps more warm air against your skin.
The cotton and polyester mix delivers a comfortable fit, the ribbing helping the items shape to your body. A brushed interior feels super-soft against the skin, and the tops have seam-free sides to prevent any irritation.
Since this set is designed for men, the long johns have an open fly design. These thermals are machine-washable and can be tumble-dried, too, for super-easy maintenance. If your budget is tight, this double thermal underwear set is a perfect choice.
Key details – Material: 50% cotton, 50% polyester; Sizes: S–XXL; Colours: Black, Blue, White, Charcoal; Fit: Comfort
Heatwave® Pack of 2 Men's Thermal Underwear Set, T-Shirt & Pants Baselayer Set Thermals, Small Black
3. Heatgen Plus Thermal Long Sleeve Top: Best thermal underwear top for women
Price: £16 | Buy now from Marks & Spencer
This thermal top is a lightweight layer to keep you warm and cosy on particularly chilly nights. This long-sleeved layer has a slim fit, but stretch means it’s never uncomfortable. Made from a combination of acrylic, viscose and elastane, it’s brushed on the inside to give an ultra-soft feel against the skin.
Using Heatgen Plus technology, this thermal top retains the heat that’s generated by your body. Its snug fit also means bulkiness won’t be an issue once you’ve added a second layer over it.
Available in six colours, you’ll easily find one to match just about any outfit. It also comes in a fantastic range of sizes, making this thermal suitable for just about anyone who needs an extra layer of warmth this winter.
Key details – Material: 64% acrylic, 28% viscose, 8% elastane; Sizes: 6–28; Colours: Rose Quartz, Black, Light Cream, Dark Raspberry, Navy, Dusty Green; Fit: Added stretch
4. Heat Active Medium Warmth Thermal Top: Best thermal underwear top for men
Price: £15 | Buy now from Argos
This thermal option from Argos is heat-activated, meaning that the long-sleeved top itself generates heat from your own body’s warmth, keeping you warm and comfortable in the cold. This particular thermal comes with a “medium warmth” rating, so you’re unlikely to overheat with this on.
The acrylic blend material is brushed, giving the top both a soft feel and adding to the warmth it delivers. Its high, round neckline ensures you’re insulated all the way up to your neck, and the top is machine-washable, thus easy to care for.
It’s available in a number of different sizes, but colour options next to other items in this list are limited. That said, a well-fitting, neutral-coloured thermal layer is the perfect piece upon which to build almost any outfit.
Key details – Material: 56% acrylic, 37% viscose, 7% elastane; Sizes: XS–XXXL; Colours: Black, Grey; Fit: Slim
5. John Lewis Thermal Silk Leggings: Best silk thermal underwear
Price: £45 | Buy now from John Lewis
Perfect for year-round wear, silk leggings are an investment worth making. In the winter months, these pure silk leggings will keep the cold at bay, holding a layer of warm air against your skin, while naturally wicking away any moisture. Their tight weave means they do an excellent job of delivering thermal protection.
Silk is a natural fibre that’s naturally breathable, too, which means you can also wear these leggings in the summer months to keep you cool. Thermals made of silk are basically the only thermal option that can be worn year round.
Sporting a glossy finish, these leggings add a little touch of glamour, even though they’re unlikely to be seen. Conveniently, they’re machine-washable, although we advise reading up on how to care for silk properly in order to keep these leggings in their best condition. Overall, they’re a great option.
Key details – Material: 100% silk; Sizes: 8–18; Colours: Black; Fit: Slim
6. John Lewis Kids’ Thermal Long Sleeve Top, Pack of 2: Best thermal underwear for kids
Price: From £13| Buy now from John Lewis
Suitable for both boys and girls, this two-pack of long-sleeve thermal tops will keep any child toasty through their winter play. The knitted fabric is a soft, cotton blend with subtle ribbing, giving them a cosy feel. Like silk, cotton is a natural fibre and so holds a layer of warmth against the skin while remaining breathable.
The tops are heat-generating, yet lightweight. This means that children will be able to keep hold of their body heat without feeling smothered. And as far as care is concerned, it couldn’t be simpler: throw them into the washing machine inside out and then straight into the tumble dryer.
A slim fit means they’ll sit as comfortably under a school uniform as they will under a snowsuit for outdoor play. There are options to fit almost any age, from toddler to teenager. There are no colour options to choose from, but this isn’t a biggie when they do their job so perfectly.
Key details – Material: 50% cotton, 50% viscose; Sizes: 2–16 years; Colours: White; Fit: Slim
