Best hiking base layers for 2025: Our experts pick their top long-sleeved layers for cold weather

Our favourite material for base layers is merino because it's temperature regulating, naturally anti-odour, and soft. Where 100% merino base layers sometimes fall short is with durability, and because they're generally more expensive than synthetic options, the value isn't the best.

What many brands have done is employ the use of a merino-synthetic blend to address the issue of durability, and Scottish-based Loomi Activewear's take on this is the best we've come across so far.

We've used this base layer for running, hiking, and everyday use, and it's superb. We wore it for a week and it still didn't smell; on runs and hikes it was very breathable and effectively regulated body temperature; and it's durable.

We were curious how Loomi managed this and its founder told us about the surprisingly complex way the merino, nylon, and elastane fabrics are knitted together in a technique called BetaSpun.

The BetaSpun fabric feels thicker than a standard lightweight merino base layer, but is still stretchy and soft, and holds its shape very well. We liked skin-tight but unrestrictive fit, crew neck (we find zips can be annoying), and sleeves that aren't too long.

Loomi is so determined to make a long-lasting, high-performance base layer that addresses sustainability issues that plague the clothing industry. Loomi, you can give yourself a pat on the back, this is a brilliant base layer.

This base layer is ideal for those with sensitive skin, particularly if you don’t like the clingy, static feel of synthetic fabrics or if even the finest merino wool feels a bit itchy. The fabric is a super soft and comfortable blend of bamboo and organic cotton, with some elastane for added stretch and improved shape retention.

Bamboo is naturally anti-bacterial and hypoallergenic. At 210gsm, this is heavier than most base layers, but provided us with decent warmth. Initially, the use of cotton in the blend was a slight concern to us. That’s because cotton holds moisture, easily becoming waterlogged and chilling the skin – hence the old Mountain Rescue adage ‘cotton kills.’

But actually, this base layer performed well. It felt a little damp under the arms at times but generally wicked efficiently and never got saturated, even during strenuous climbs.

The fit is excellent. It’s nice and long in the body and arms and has other practical features like thumb loops, a raised collar and a zip neck. It is also a very eco-friendly choice, which is a big plus. Though it probably wouldn’t be our first pick for the most demanding mountain days, it’s a very comfortable option for general hillwalking.

Check out the women's version here

This hybrid base layer employs two different zoned fabrics, combining the benefits of natural and synthetic fibres. The main body is polyester, which has been treated with silver ions to neutralise body odour. Underarm panels are made from a blend of natural merino wool and hard-wearing nylon. It stayed noticeably fresh, even after several wears. As such, it’d be a great choice for multi-day backpacking trips. It also offers great wicking performance, plus decent warmth and durability despite its light weight.

It feels well made, which is evident in the details like low-profile merrow stitching, which resists snags better than conventional flatlock seams. There’s also a stand-up collar and a practical neck zip, though it’s a shame the cuffs lack thumb loops. The Rho has a little stretch but retains its shape well. Admittedly, it only provides moderate warmth, so works best in cool rather than very cold conditions.

But this base layer really excels in day-to-day wearability. It just looks and feels great, so became our go-to pick of all the options here. Having said all that, it’s still overpriced. And there’s no women’s version yet.

Artilect is a relatively new US outdoor brand from Colorado. It makes a complete range of outerwear based on its A/SYS layering system, the foundation of which are its innovative Nuyarn base layers. This is a method of spinning that ‘drafts’ merino wool alongside a core filament fibre – here, nylon.

The results when compared to normal merino are impressive. The brand claims a 5x faster drying time, 35% more stretch, 35% more loft, 50% more durability and 20% less raw materials. Though it’s hard to assess all those, this layer does seem to punch above its weight in terms of warmth, while drying noticeably quicker than other merino rivals. The jury’s out on the durability claim – a lighter 125gsm Artilect base layer we previously tested didn’t last very long, though this thicker top does seem to be going the distance so far.

A zoned construction enhances breathability, with extensive perforated panels in areas like the back and underarms. Construction is first class, with flatlocked seams, thumb loops and a raised collar with a very deep neck zip. The latter is great for dumping heat fast. Fit-wise, the Darkhorse is snug and sculpted, maximising next-to-skin performance and thermal efficiency. It’s undoubtedly an impressive bit of kit, but it is expensive.

Check out the women's version here

Like the Artilect Darkhorse 185 Zoned ¼ Zip base layer, this one from Black Diamond uses Nuyarn. However, the merino used here is even finer – so the Black Diamond Long Sleeve Rhythm T-Shirt weighs just 121g.

For a garment so lightweight, we were very impressed with its performance. It’s excellent at regulating temperature and is superbly comfortable thanks to the soft fabric and high level of breathability and fast wicking. You could use this for trail running as well, if you wanted to.

The fit is slim, rather than skin tight like many base layers. So, it is quite versatile as a result, but not ideal for really cold conditions.

Since it’s so lightweight, it doesn’t feel particularly rugged. Nuyarn is supposed to be more durable than core spun merino, and indeed we’ve found it to be, but in such as lightweight garment that doesn’t make it bombproof.

Check out the women's version here

This short sleeve is part of a wider Adidas Terrex merino base layer range of varying weights and including leggings.

On test, this provided the temperature regulating and anti-odour properties we love merino for. Being a mid-weight fabric, it was decently warm too, and excels in quite cold (and wet) conditions.

A lot of short sleeve base layers go too far with the short sleeves. We liked the Xperior fit because the sleeves covered most of the upper arm. We liked the fit, which is slim, but not as tight as some.

Being 100% merino, the price reflects that, but it's very well made, you do get a lot of performance in return.

Check out the women's version here

Norwegian brand Helly Hansen has been making base layers since the early 1970s. Today, its Lifa layers featuring those distinctive arm stripes are still worn by hikers, skiers, mountaineers and sailors alike. The extensive range includes synthetic, merino wool and hybrid options – including this practical midweight merino option. It’s more than just merino alone though, being lined with fast-wicking polypropylene fibres for superior moisture management.

It proved a great layer for active use in cold weather. We wore just this under a shell on sub-zero winter mountain days and stayed toasty. The two-layer construction seems to outperform most merino fabric blends, as well as the zoned construction of other hybrid layers. It’s comfortable too, not at all itchy with a nice fit. It isn’t baggy but doesn’t feel like it’s been sprayed on either. We’d like a little more length in the arms and body to keep wrists and lower back fully covered during dynamic activity, but this won’t be an issue unless you’re especially long limbed.

There aren’t any thumb loops either, and we’d like a deeper neck zip for ventilation. For durability this layer held up well though, still looking and feeling great after several wears and washes.

Check out the women's version here

This cosy synthetic base layer is made from grid stretch fleece that offers great next-to-skin warmth and softness but also wicks extremely well to keep you dry and comfortable. The polyester fabric has a little elastane in the mix to aid freedom of movement. It’s a shame there’s no recycled content though. However, Sprayway is a Fair Wear Foundation member.

In terms of fit, we found it’s reasonably trim, with plenty of length in the arms and handy thumb loops to prevent sleeves riding up. Stretch-bound cuffs hug the wrists, and a raised collar guards against chills. The half zip at the neck isn’t the deepest but did offer us a little ventilation if we started to overheat.

The smooth face of the fabric also slides easily under other layers. Seams are flatlocked, but perhaps a little bulkier than rivals, and don’t feel truly invisible like some. We were conscious of the two seams across the shoulders when wearing a pack, for example. Otherwise, it performed well on test in terms of warmth and moisture management, especially on colder days.

The only real drawback we found is one common to many cheaper synthetic layers – it starts to smell a bit after heavy use, and lacks any in-built odour treatment to counteract this. Also, though it’s a minor niggle, the logo started to peel off the chest after a few washes.

Check out the women's version here

Rab’s Syncrino is a 125gsm merino blend base layer tee, made from recycled polyester and merino wool. It offers more warmth than a similar weight synthetic top but has less insulating ability than any of the other options here – you might want a cosier and chunkier layer if you tend to feel the cold.

The fit is fairly relaxed, offering good freedom of movement but perhaps at the slight cost of thermal efficiency. On the other hand, that means it’s a bit more wearable and versatile if you just wear it as a long-sleeved T-shirt in milder spring or autumn weather, rather than as a technical layer. It is soft and comfortable, with minimal itch factor. Seams are low profile, sensibly placed and well finished. The body and sleeves are both nice and long, ensuring full coverage. There are no thumb loops and just a simple crew neck, which does leave the nape a little exposed.

But on test, this layer performed well in the mountains. It’s stretchy enough to permit dynamic movement. It wicks fairly well too, though we were conscious of some dampness under the arms and across our back when wearing a pack. Still, it dried off reasonably quickly. It didn’t pick up unpleasant odours either, even after a week’s continual wear.

Check out the women's version here

Jöttnar is a British brand with a Scandi name, founded by two former Royal Marines Commandos. It has built a rep for uncompromising technical mountain gear, and this base layer is no exception.

The Erling has a slim, sculpted fit to maximise next-to-skin performance, with plenty of length in the arms and body ensuring full coverage even during dynamic activity. All the features you’d expect from a technical layer are present and correct: you get thumb loops, a stand-up collar, a very deep neck zip for venting, and unusually, even a zipped chest pocket. Flatlocked seams are well placed to avoid irritation. The fabric is a midweight 205gsm polyester that offers plenty of stretch. Unlike cheaper synthetics, it has a Polygiene anti-microbial treatment to ward off odour too.

On test, there was lots to like. It wicks fast, dries quickly and offers enough warmth for cool to cold conditions. All in all, if you’re looking for a trim-fitting technical layer and don’t mind paying a bit more for quality, this ticks all the boxes.