We’ve spent the past few months putting Isobaa’s heavyweight Borg fleece through its paces — from Cornish cliff paths to chilly nights in the Beacons — and it might just be our new favourite midlayer.

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Isobaa Borg fleece

Ideal for: Hiking, wildcamping, backpacking

Not suitable for: Fastpacking, trail running

The Isobaa Borg fleece is a 420gsm merino-blend midlayer that combines the natural warmth and breathability of wool with the durability of recycled polyamide. At 530g, it’s not ultralight — but it’s impressively warm, well-built, and so stylish enough you can literally wear it anywhere. With an RRP of £180, it rivals the Patagonia Better Sweater on both price and performance, while offering the natural benefits of merino. Designed for British conditions, it’s warm, breathable, and endlessly versatile.

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The Good

Outstanding warmth

Breathable for weight

Beautiful natural fabric

Superb everyday styling

Durable elbow patches

Comfortable raglan cut

Sustainable materials

The Bad

Slightly coarse out of the box (though it quickly softens)

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Isobaa Borg fleece midlayer review

Founded in 2016, Isobaa has quietly become one of the UK’s best homegrown outdoor brands. Based in London and inspired by the rugged landscapes of Britain, they specialise in merino wool clothing that’s as sustainable as it is functional — bridging the gap between city living and mountain life. Everything they make feels considered: simple, natural, and built to last. We’ve tested plenty quite a bit of their kit this year, from merino base layers to socks, and the quality has consistently impressed us.

Their Borg fleece sits at the top end of the range — a heavyweight midlayer designed for year-round adventures in cool to cold weather. Made from a 420gsm blend of 64% merino wool and 36% recycled polyamide, it’s naturally breathable yet superbly warm. At 530g in a men’s medium, it’s not light, but it’s robust, cosy, and feels premium the moment you pull it on. Retailing for £180, it’s very much a performance-meets-lifestyle piece: equally at home on a windswept summit as it is in a pub beer garden.

In terms of market positioning, the Borg fleece sits alongside heavy hitters like the Icebreaker Descender Realfleece, North Face Futurefleece, and Patagonia Better Sweater. Compared to those, the Isobaa feels more natural, warmer, and less plasticky — with genuine eco-credentials thanks to its non-mulesed merino and recycled components. It also has the aesthetic edge: a classic, almost retro look that never looks out of place whether — you’re in a Scottish both or Shoreditch craft brewery. Think of it as a blend of the Patagonia Retro-X and the Icebreaker Descender — but arguably better suited to British life.

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How we tested the Isobaa Borg

We tested the Isobaa Borg fleece across a handful of classic British adventures. First came a multi-day hike along the south coast — a mix of steep cliff paths, salty air, and that persistent Cornish drizzle that seeps into your soul. The fleece was warm and breathable, even when layered under a waterproof shell. And when we dropped into town for a well-earned pint, it didn’t feel remotely out of place — a rare trick for a technical midlayer.

Later, we wore it on a chilly two-night wild camping trip in the Brecon Beacons, tackling the Horseshoe loop over Pen y Fan. With temperatures dropping into single digits after sunset, it became the first thing we reached for once the boots were off. Around camp, it hit that sweet spot: warm enough to cook and chat under the stars, yet breathable enough that we never overheated. It’s a hefty bit of kit compared to a lightweight fleece, but it never felt cumbersome. For this kind of trip — long days, cool evenings, and comfort at camp — it was near-perfect.

Features and design

The Isobaa Borg fleece feels different from the moment you pull it on. Made from a 420gsm blend of 64% merino wool and 36% recycled polyamide, it’s unmistakably substantial — more jacket than midlayer, and a far cry from the wafer-thin fleeces that so often underwhelm. The deep pile gives it that plush, natural look you only get from merino, but it’s backed by enough structure to feel reassuringly tough. Straight out of the box, there’s a touch of coarseness to the texture — not scratchy, just natural — and after a few wears it softens up beautifully, settling into that supple, broken-in comfort you only get with natural fibres.

The warmth is another area where the Borg really shines, which hits you almost immediately. This is not a layer for half-hearted spring days — it’s the fleece you’ll be grateful for when the wind turns and you’re still a mile from camp. It holds its own against the likes of the Patagonia Better Sweater, and even nudges close to the burlier Retro-X, but with far better breathability and none of that synthetic cling. You can feel the merino doing its job — holding warmth when the temperature drops, but never suffocating you when you start to move. On climbs or quick dashes across a windswept ridge, it wicks moisture fast enough that you don’t overheat, and yet when the wind bites, it holds its warmth with impressive consistency.

Then there’s the fit and finish — a reminder that someone at Isobaa has really thought about how this will be worn. The raglan sleeves make it a dream under a rucksack, with no seams rubbing on your shoulders no matter how heavy the load. The stand collar rises neatly to seal out drafts without digging into your chin, and the stretch binding at the cuffs and hem hugs just tightly enough to keep the warmth in while still letting you pull it on and off with ease. The zipped hand-warmer pockets are exactly where you want them: high enough not to clash with a hip belt, deep enough for cold hands or a phone, and lined with a soft material interior that’s dangerously easy to linger in on cold mornings.

And those subtle elbow patches — a small detail, but one that really completes the design. They protect the merino from wear when you’re kneeling by a stove or leaning on a damp rock, and they lend the whole piece a faintly retro air — like a nod to classic outdoor gear, reimagined for the present day.

All of it feels deliberate. The recycled polyamide adds durability where merino alone might wear thin, while the non-mulesed wool and recycled packaging reinforce Isobaa’s quiet, conscientious approach. Together, these details make the Borg fleece feel less like a fashion item and more like a long-term companion — something built for years of hill walks, pub stops, and cool evenings by the fire.

Performance in the British outdoors

After weeks of wear across wildly different conditions, we came away genuinely impressed. For general UK use — be that hiking, wild camping, or just living in something warm and reliable through autumn and winter — the Borg fleece hits an almost perfect balance between comfort and performance.

It’s exceptionally warm, easily coping with single-digit temperatures, but also remarkably breathable. Even during steep climbs, it never got uncomfortably hot. When paired with a light windbreaker or waterproof, it becomes a formidable cold-weather combo — perfect for Britain’s unpredictable climate.

It’s not, of course, an ultralight technical layer. If you’re heading into the Alps or doing long multi-day treks where every gram counts, something like the Futurefleece or a thinner Polartec layer will make more sense. But for 99% of real-world UK adventures — whether you’re out in the Lakes, camping in Eryri, or just walking the dog in December — this is the kind of fleece you reach for instinctively.

And then there’s the style. Most technical fleeces look, well, technical. The Borg feels different — more like something you actually want to wear day-to-day. Even after several muddy hikes and a week of travel, it still looked good enough to wear to the pub. That’s not something we can say for most gear we test.

Value for money

At £180, the Borg fleece isn’t cheap. But considering its build quality, natural materials, and versatility, it feels fairly priced. The North Face Futurefleece costs roughly the same, uses synthetic fabrics, and isn’t half as warm. The Patagonia Better Sweater is cheaper, but nowhere near as breathable. Meanwhile, the Icebreaker Descender feels lighter but less substantial — more of a base-mid hybrid. The Borg bridges all those gaps beautifully.

We’ll admit, we initially thought £180 was a lot for a fleece. But after living in it for weeks — both on adventures and around town — we’ve changed our tune. It’s one of the few layers that genuinely feels like an investment.

Isobaa Borg Fleece FAQs

• How warm is the Isobaa Borg fleece?

  Extremely warm. This 420gsm merino blend traps heat brilliantly while still breathing naturally. It’s easily one of the warmest midlayers we’ve tested — ideal for autumn, winter, and chilly spring days. Even on windy ridgelines, it provides plenty of insulation without feeling stuffy.

• How breathable is the Isobaa Borg fleece?

  Surprisingly breathable for such a thick fleece. The merino content regulates temperature exceptionally well, wicking away moisture and preventing that clammy feel typical of synthetic fleeces. It’s perfect for stop-start activities like hiking or camping, where you need warmth without overheating.

• Is the Isobaa Borg fleece worth the money?

  Yes. At £180 it’s an investment, but a fair one. You’re getting a sustainable, durable, and beautifully made piece that can replace several other fleeces in your wardrobe. It’s warmer than the Patagonia Better Sweater, more natural than the Futurefleece, and far more versatile than most midlayers we’ve tested.

Isobaa Borg Fleece jacket: our verdict

After months of use across the British hills and coasts, we’ve come to trust the Isobaa Borg fleece completely. It’s the layer we instinctively reach for when the weather turns, when the wind picks up over the ridgeline, or when the day ends beside a camp stove. It delivers that rare blend of performance and comfort that makes it just as relevant for long days outdoors as it is for evenings back in civilisation.

It’s not pretending to be ultralight or technical in the way something like The North Face Futurefleece is — this is a different kind of midlayer altogether. The Borg is about real warmth, natural materials, and effortless wearability. It performs when you need it to, looks good when you don’t, and stands up to months of use without showing signs of fatigue. Even the details — those smooth zips, the raglan sleeves, the elbow patches — feel like they’ve been designed by people who spend their weekends out there too.

For us, it sits right at the sweet spot between the technical reliability of the Icebreaker Descender Realfleece and the everyday comfort of the Patagonia Better Sweater. But it also brings something neither of those quite manage — a certain understated charm, a sense that it belongs as much in the British countryside as it does in the British wardrobe.

So, if you’re after the best wool fleece for life in and out of the hills — one that’s naturally warm, impressively breathable, and made with genuine care — this might just be the one.

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