As a keen snowboarder and cyclist, I’ve always valued a good thermal layer, whether it’s a tech-packed long-sleeved t-shirt or a thin hooded top that can be worn under jackets before doubling as a lightweight layer for fireside apres-ski drinks.
But first things first. What exactly is a thermal garment? It’s basically a hi-tech base layer – one that is often designed to be worn against the skin, and one that’s engineered to trap heat and wick away sweat without making you feel like you’ve been locked in a sauna. The most common materials are polyester and merino wool which, despite wool’s natural origins, does all of the above incredibly well. The downside? Merino wool is more expensive, and it’s less durable than polyester. But its moisture-wicking properties are hard to beat – a decent merino wool garment can absorb 30 per cent of its weight in moisture and still feel dry to the touch.
Former Olympic athlete Sally Gunnell advises prioritising practicality, along with a few added extras. “Thermals are all about comfort and practicality,” says Sally. “Choose ones that breathe, give you good coverage and feel great under your layers. For tops, I love a design with wrist-length sleeves and thumbholes to keep everything in place when you’re on the go.”
The best thermals for 2025 are:
- Best overall – Berghaus women's MTN guide long sleeve half zip: £85, Berghaus.com
- Best for flattering fit – Finisterre women's Ria seamless baselayer: £75, Finisterre.com
- Best for softness – Ronhill women's core thermal 1/2 zip: £55, Ronhill.com
- Best for extra-cold climes – Rab women's conduit hoody: £75, Rab.equipment
- Best for comfort – Keela Outdoors women’s merino zip neck top: £64.95, Keelaoutdoors.com
How I tested
Lots of early morning bike rides, walks and runs were involved when it came to testing the best thermals – although I should point out that, after years of living and working in colder climes, I’ve become a bit of an expert on the features worth shelling out for and the ones that aren’t. Factors I took into consideration include price, comfort, fit and material, and while add-ons such as thumbholes can be a godsend, I made a point of exploring their design – a thumbhole becomes redundant if it’s too small or the material near the wrist is paper-thin. I wore the garments alone and beneath jumpers and jackets to see if there were snag points or other layering-related issues, while also checking the garment’s opacity – if you’re going to be wearing your base layer at the bar, thin material which leaves you unexpectedly exposed might be a winter warmer, but it’s never going to be a winter winner.
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Berghaus women's MTN guide long sleeve half zip
Best: Thermal overall
Main material: Polartec Power Wool - merino wool and synthetic fibres
Why we love it
- Flatting fit
- Top quality material
- Fab design
Low necks and miniscule zips are two of my bug bears, but Berghaus’s fantastic tops had neither. The incredible softness of the material means the high collar didn’t chafe or rub, and the zip, which was approximately half the length of the top, was smooth as silk and easy to grasp. The generous length of the sleeves banished any chance of cold wrists (another pet hate) on a frosty morning run, and the precision-engineered material (thinner in the places where I wanted a little more breathability, thicker in the areas most prone to chills) was very much appreciated.
The material, by the way, is Berghaus’s polartec power wool, which is a sweat-wicking, quick-drying, anti-microbial fabric that kept us dry, comfortable and smelling fabulously fresh all day long. A final point to flag – it’s not just quick drying when being worn, either – after a wash (it can be cleaned at 40C but not tumble dried), it air-dried surprisingly fast.
Buy now £85, Berghaus.com
Finisterre women's Ria seamless baselayer
Best: Thermal for a flattering fit
Main material: Merino wool and polyester
Why we love it
- Slimming areas of darker materials on the side
- Top quality material which still feels breathable
Take note
- Treat with care – it’s 40 per cent wool
Everything about this garment screams quality. Yes, you’ll need to treat it with a little extra TLC, but it’s hardly surprising given the quality of this top and the luxurious thickness of the merino wool. I was smitten with its tailored fit and stylish design (the different sections of material didn’t just add breathability where it was needed most, but were also incredibly flattering), and it was also one of the smartest technical tops I’ve come across – I’d be just as happy wearing this at my favourite apres-ski bar as I would be wearing it under a ski jacket.
Merino wool is the main material, so it’s hardly surprising that it’s insanely comfortable (a long, sweaty bike ride was my testing ground), although its seamless design ramped up the comfort even more. The warning that colour transfer could occur when wet was a bit ominous, although I didn’t experience this after a gentle hand wash (the instructions say it can be washed on a wool cycle, but I played it safe).
Buy now £75, Finisterre.com
Ronhill women's core thermal 1/2 zip
Best: Soft thermal
Main material: Polyester
Why we love it
- Ridiculously soft
- Spot-on fit
Take note
- Material had a strong chemical whiff, but it dissipated quickly!
This is the kind of top I’ll slip on under my jumpers when I’m not sure exactly which way the weather’s going to go, barring the fact that I’m not going to be skiing through a blizzard. In other words, don’t rely on this during a sub-zero snow storm, although I’ll point out few garments are designed to cope with such a weather event anyway.
It’s one of the softest thermal garments I’ve come across, and the lightweight polyester did a great job of sweat-wicking, while also being fabulously breathable. Added extras such as a zip cover, thumb holes and a reflective logo at the rear (although I wish the latter was slightly warmer) were very much appreciated. I should flag that I was surprised to find, despite the fact its mainly polyester, a long list of don’ts when it came to care. According to the guidance, it can’t be tumbled dried (standard) or used with biological detergents and fabric softener. Full disclaimer – I used it with both of the latter and it emerged just fine.
Buy now £55, Amazon.co.uk
Rab women's conduit hoody
Best: Thermal for extra-cold climes
Main material: Recycled Thermic G fleece
Why we love it
- Feels luxuriously thick without impeding freedom of movement
I’ll just say it – wearing this top feels akin to wearing a cloud, albeit a warm one. I’m wary of thermals with hoods, simply because they can often add too much bulk when worn under jackets. Luckily, this top uses Rab’s 92 per cent recycled thermic G fleece – a hi-tech material that traps warmth without adding unnecessary weight or compromising on comfort – and the hood could even be worn under my snowboard helmet.
It’s truly precision-engineered – I loved the dropped hem at the back to provide extra warmth, as well as the fleece’s gridded design, which enhances its sweat-wicking capabilities. There’s plenty for those prioritising sustainability, too – not just the use of PFAS-free fabric and the use of recycled materials, but the fact that Rab relies heavily on renewable energy when it comes to production. It’s also pretty low maintenance when it comes to care – it needs to be washed at 30 degrees, but it’s fine to tumble dry.
Buy now £75, Rab.equipment
Keela Outdoors women’s merino zip neck top
Best: Thermal for comfort
Main material: Merino wool
Why we love it
- Ergonomic fit
- Wind-blocking high neck
- Great value
Take note
- Would have loved to see more colours – not just shades of grey
This ultra-comfortable thermal top is a winter essential – and one that will definitely go the distance, too. Features such as the zip cover enhance its durability, while the length of the zip was spot-on – long enough for a spot of bespoke temperature control on days when the weather was unpredictable, but with a solid construction that ensured no breezes worked their way in.
I loved the generous length of the sleeves too – cold wrists simply weren’t a possibility with this top, and its overall length prevented accidental builders’ bum-related incidents. Care-wise, it can be washed on a delicate cycle at no more than 30 degrees. I’m also pleased to report it was another top that dried incredibly quickly.
Buy now £64.95, Keelaoutdoors.com
Peak Performance spirit crew AOP synthetic baselayer women
Best: Thermal for style
Main material: Polyester
Why we love it
- Retina-burningly bright design
Take note
- Be prepared to find yourself lusting after the leggings, too
Don’t get me wrong – I’m all for a bit of colour blocking, but who doesn’t like to mix it up with a bit of multi-colour fun? That’s not the only reason to grab Peak Performance’s colourful thermal top though. It has just the right amount of stretch to maximise comfort while keeping out the cold, along with various features I wish I saw more of, including thumbholes on the sleeves (almost always found on ski jackets, but less so on thermals).
It’s also proof that, while I love a bit of merino, synthetic materials, when used the right way, can be hard to beat. Another easy-care garment, it can washed on a delicate cycle and tumble dried – but again, opt for the delicate cycle.
Buy now £70, Peakperformance.com
Finisterre women's Ria seamless leggings
Best: Thermal leggings
Main material: Merino wool
Why we love it
- Quite possibly the cosiest leggings in the world
Take note
- High price tag (although worth every penny, we should point out…)
I love these leggings’ circular seamless construction, resulting in a chafe-free fit which did a great job of keeping us toasty on an expectedly cold day. A top tip – you’ll want to check the weather before donning these, because the thicker material means they’re not the type of garment you’d slip on in a “just in case” scenario – do so and you could well end up overheating. But on bone-chilling, wind-blasting days? These gorgeously soft leggings will weatherproof your day, and the thicker, ribbed construction in certain areas (including, we’ll just say it – the bum) ensures great coverage. When they need a clean, a delicate cycle is recommended (there’s also that familiar warning about possible colour transfer when wet, but we escaped this predicant) and it needs to be air-dried.
Buy now £75, Finisterre.com
Q36.5 base layer 4 plus long sleeve
Best: Thermal for a splash of colour
Main material: Synthetic fabric
Why we love it
- Great colour
- Tailored fit
- Top quality material
Take note
- Consider sizing up
Is there something similar to a shoehorn but for clothes? Because if so, it might well come in handy for wearers of this top. Don’t get me wrong – I absolutely love it, but it’s also on the tight side, and my first impressions were that it was actually some kind of Spanx-type garment for my upper half. In reality, I think the sizing is just a little off, and the top-quality material has much more stretch than I first assumed (and yes, I could still breathe).
Okay, I tried it on a frosty day rather than a sub-zero day in the French Alps, but with its high-performance blend of polyester and PA EMANA (a smart-yarn with bioactive properties) this is one seriously tough top, which won’t let you down on the coldest of cold weather days. And – hoorah!You can machine wash (albeit at a maximum temperature of 30) and tumble dry it, too.
Buy now £114, Q36-5.com
Rab women's syncrino base LS tee
Best: Thermal for sweat-wicking and odour-busting
Why we love it
- Great for working up a sweat without worrying about the associated smells
Take note
- Expensive
I love the killer combo of materials used for this top – ultra-soft merino wool in the areas where comfort matters most, paired with fast-trying synthetic fibres which block out the wind and cold without compromising on comfort. While it’s not entirely seamless, low-profile seams ramped up the comfort, and the garment was also incredibly soft. In summary? It’s a great all-rounder that can be worn alone on warmer days or slipped on beneath a jacket or jumper for those occasions when the weather takes a turn for the worse. As an added bonus, its lightweight, packable design means it’s a breeze to cram in a backpack pocket. And, to end on a high note, it can’t just be machine washed and tumble dried but ironed too (a rarity in the world of thermals).
Buy now £65, Trekitt.co.uk
Bamboo Clothing thermal sculpting jacket
Best: Multi-purpose thermal garment
Main material: Bamboo
Why we love it
- Really, really warm
Take note
- Sorry to sound repetitive, but more colours please!
The fact that my minor complaint is that there are just two colourways (more specifically, two very similar shades of blue) for this top shows how much I love it. It’s brilliant value, incredibly comfortable and has my favourite added extra – thumbholes. I was especially impressed by the ultra-flattering fit, achieved not through the use of elasticated panels, which make us feel as though we’re being squeezed by some kind of torture device, but by contouring lines on the front and back. I loved the slimline pockets, too – proof that thermal garments can indeed have room in which we can stash our stuff. Just make sure you don’t wash it at temperatures higher than 30, and although care instructions were minimal, I’d advise air-drying it.
Buy now £75, Bambooclothing.co.uk
What are the best thermals?
Berghaus’s women's MTN guide long sleeve half zip was stylish, comfortable and durable, and looked fantastic, too. It’s worth pointing out that plenty of brands brag about patented materials, but Berghaus’s polartec power wool really does cut the mustard, thanks largely to extensive testing in the great outdoors. The design of Finisterre’s women's Ria seamless baselayer ensured all-day comfort, whatever the weather, and Ronhill’s women's core thermal 1/2 zip was also a joy to wear, and the perfectly-placed thumbholes did exactly what they were meant to do – keep our wrists warm while preventing our jacket sleeves rolling up.
How I selected the best thermals
When I testing each item, I used the following criteria to come to a verdict:
Why you can trust IndyBest reviews
Tamara Hinson has covered every category of adventure travel gear for IndyBest including camping pillows, base layers and suitcases. Tamara Hinson is a keen skier and snowboarder (she used to be a snowboard instructor and was also a qualified ski and snowboard technician) and spends a large amount of the winter months in the French Alps, so she knows good kit from bad. As with all of our IndyBest reviews, each piece of thermal clothing has been trialled in a range of different conditions, and she’s only included items that past the rigorous testing process.
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