Layering is the key to staying warm in the outdoors come winter, so it makes sense to start with a great base layer worn next to the skin. 

Thermal tops and bottoms are designed to be worn beneath ski jackets and salopettes on the slopes, under waterproof hiking layers on the trail or for sleeping in on cold weather camping trips, and will also double up as standalone pieces for outdoor pursuits once spring comes.

We recommend trying base layers on in person. Pick a long-sleeved top and full-length trousers – both should hug your body and offer good stretch, with no restriction anywhere, but still feel snug enough around the waistband, neck and sleeves to trap warmth in. 

Look for some stretch to the material, and a wide, comfortable waistband on bottoms. 

Thermal tops usually have a crew neck or a high, zipped neck – the latter can double up as a handy neck warmer.  

You may see base layers listed with a number, such as 140 or 220 – this refers to their weight in grams, and the higher the number, the warmer, but heavier, the base layer. 

Base layers are made either with synthetic materials such as polyester or natural materials such as merino wool. 

Natural materials feel best against the skin, and we really rate merino wool, which is warm, soft and naturally moisture-wicking – merino base layers are pricier but worth investing in if you’re going to use them regularly. 

Look for a blend of merino or 100 per cent merino. A good new option on the market are bamboo base layers, which are antibacterial and soft to wear.

As well as simple designs in plain colours, base layers increasingly come in bright colours, stripes and patterns – these work well as standalone tops for casual wear or for a spot of apres on the slopes as well as for the more serious business of keeping you warm all winter long.

We tested out the latest base layers on multi-day cold weather hikes and compared them for warmth, breathability and comfort.

You can trust our independent reviews. We may earn commission from some of the retailers, but we never allow this to influence selections, which are formed from real-world testing and expert advice. This revenue helps to fund journalism across The Independent.

Icebreaker merino Oasis 200 top and bottoms: £75 and £70, Icebreaker

It’s definitely worth investing in merino base layers – this wonder wool regulates your body temperature, wicks away moisture, breathes when you sweat and resists odours, making it ideal for a week-long ski holiday or for regular outdoor use. Icebreaker offers a great range of merino base layers in different weights (the higher the number, the warmer the base layer) but our pick are the excellent 200 Oasis leggings top and leggings. Versatile, beautifully cut, comfortable to wear all day – there’s nothing not to like here. A high neck version of the top is also available.

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KJUS trace half zip thermal top: £95, KJUS

This smart microfleece top is wonderfully soft to wear and to touch. Pull it on and you’ll immediately feel it warming you up and trapping your body heat. It’s not too warm, though – we found it breathable enough to deal with fast-paced skiing and hiking without overheating, and the zipped neck comes in handy for regulating your temperature. It’s nicely cut to move as you do and looks flattering, too. We reckon this is a top worth splashing out on.

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BAM bamboo snuggly thermal bottoms: £42, BAM

Snuggly by name and nature, these soft-as-anything leggings were the cosiest bottoms we tested. You’ll never want to take them off – and they double up as pyjama bottoms on really cold nights as well as working for skiing, winter hiking – even yoga. The wide, supportive waistband stays put whatever shapes you throw. BAM’S soft athleisure pieces are all made from bamboo, which is a bit of a wonder material – antibacterial, breathable and an ecologically sustainable resource. A versatile pick, especially if you value great comfort.

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Mountain Warehouse talus top and bottoms: Both from £9.99, Mountain Warehouse

Can you own a great set of base layers for under £30? It seems so with this simple but effective set from Mountain Warehouse. Both feel soft to touch and warm to wear, but aren’t breathable enough to deal with hard-charging skiing or warmer spring days – save this set for colder weather, as its Isotherm technology traps in heat well. A great choice for a first ski holiday, as a spare set of base layers or just if you’re on a budget. You may need to go for a size down for a slim, heat-retaining fit.

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Patagonia capilene midweight crew top: £60, Patagonia

This top comes in a variety of colours and patterns, and we reckon it’s smart enough to wear with jeans as a normal top when you aren’t out in the mountains. It still packs a technical punch, though – the honeycomb-like material traps in warmth but is still breathable when you’re working up a sweat. The top is treated for odour control, which does prove effective on test – this is a good choice if you’re going ski-touring away from showers for a few days. The cut is boxy, so go a size down if you prefer a slim fit.

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Howies Deryn merino thermal top: £55, Howies

Sailor top or technical base layer? We’re big fans of the lovely looks of British brand Howie’s stripy Deryn top, which comes in four colour versions. This may look like your favourite Breton top, but it’s also a high-performing base layer, and is made with 100 per cent merino wool to keep sweat, odour and bacteria at bay. The longer-length cut is also great for keeping your torso warm. A brilliant choice for regular use both on the mountains and off.

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The North Face summit L1 top: £100, The North Face

This gorgeous grey number was one of the best-looking base layers we tested, and looks just as smart worn with jeans as under your ski jacket. The North Face has constructed this with a very slim fit, and we found we needed to go a size up for it to fit properly on the shoulders. But once you’ve got the right size, that body-hugging cut does make it easy to layer up comfortably, and the articulation makes it move effortlessly with you, even on fast-paced mountain adventures. The fabric feels cool and moisture-wicking, but wasn’t the softest on test. At a hefty £100, this is one to grab in the sale.

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Berghaus thermal tech top: £45, Berghaus

We love the reassuringly thick fabric of Berghaus’ thermal tech top, which has a silky, comfortable feel against the skin. It has a boxier fit than other slim-cut base layers we tested, but still does a good job of retaining heat. We found this top to be one of the more breathable tops on test, and it wicks away sweat and odours well. It’s also quick-drying – a good choice if you tend to overheat when wearing base layers.

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OOSC Every day is a Saturday base layer set: £80, OOSC

In a sea of plain, sensible base layers, new ski brand OOSC’s weird and wonderful prints really stand out. Its thermals come in a variety of bright and cheerful patterns, but we love Every Day is a Saturday’s eye-catching geometric pink and blue design. These base layers have a flattering, slim fit, a silky outer material and a delightfully cosy fleecy inside, and the trousers incorporate a comfortable waistband. OOSC’s offerings weren’t the warmest base layers on test – wear these for spring skiing and then show them off at apes ski. After all, they look so nice it’s a shame to cover them up with outerwear.

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Dare 2b confirm thermal top: £34.95, Dare 2b

You’ll probably either love or hate the eye-wateringly busy geometrical print here. If you’re in the former camp, then you’ll get a comfortable and warm base layer for well under £50. This was one of the warmest, but least breathable, base layers on test – good for really chilly mornings. We really rated the neck, which is cosy without feeling restrictive, with a handy half zip for ventilation. The top also has stretchy side panels that make it flattering and form-fitting.

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Smartwool merino 250 zip top: £91.95, Alpine trek

There’s lots of lovely stuff packed into this top, and for a top-end price you get a high-performing 100 per cent merino top, with a zippable, comfortable neck, a flattering, heat-holding slim fit and reliable warmth and breathability. We like the nicely-cut shoulder panels, which don’t rub if you’re wearing a backpack. This is our top pick if you’re shopping for 100 per cent merino base layers, and it comes in a range of handsome muted shades that work well for daily use.

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30Seven ski mesh base layer heated top: £146 (heating element sold separately £42), 30Seven

And now for something completely different. 30Seven’s clever, breathable mesh fabric is heated with rechargeable lithium-ion batteries and can keep you toasty warm for up to seven hours. This gilet-style top can be worn with or without the battery, but although it’s called a base layer, we would wear a thin long-sleeved top underneath for comfort. That heating tech makes the 30Seven pricy, but if you feel the cold or have to work outdoors for extended periods of time, adding this to your winter layering system could really help to keep the chill at bay.

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The verdict: Base layers

You can’t go wrong with Icebreaker’s excellent merino base layers in various weights. For comfort, we love BAM’s snuggly leggings and for something a little different, go for OOSC’s eye-catching prints.

IndyBest product reviews are unbiased, independent advice you can trust. On some occasions, we earn revenue if you click the links and buy the products, but we never allow this to bias our coverage. The reviews are compiled through a mix of expert opinion and real-world testing.