Sea to Summit Ascent -1 sleeping bag review: brilliantly built for British bivis
One of the best all-round down bags we’ve tested — warm, packable, smartly designed and a dream for spring-to-autumn fastpacking.
Sea to Summit Ascent -1
Ideal for: Three-season wildcamping, fastpacking, bikepacking
Not suitable for: Deep winter wildcamping (though there’s the -9 version for that!)
The Sea to Summit Ascent -1 is a high-performance, lightweight down sleeping bag designed for camping across the UK’s cooler seasons. With a comfort limit of -1°C, a total weight under 800g, and premium features like a free-flow zip system, hydrophobic down and clever baffling, it’s a serious tool for wild camping, fastpacking or multi-day treks. It’s not the lightest on the market — and the price tag sits firmly in the ‘premium’ bracket — but as one of the best three-season sleeping bags for UK conditions, the versatility, comfort and performance are spot on. We’ve tested it across valley floors, exposed ridgelines and frosty bivis — and we’d happily take it everywhere again.
The Good
Excellent warmth-to-weight
Packs into a 30L running vest
Clever free-flow zip system
Hydrophobic, responsibly sourced down
No cold spots thanks to baffling
Comfortable relaxed mummy fit
Internal pocket for small items
Strong sustainability credentials
The Bad
Premium price tag
Not ultralight category
Compression sack not waterproof
Sea to Summit Ascent -1 sleeping bag review
Sea to Summit is an Australian brand with a global reputation for engineering high-quality, lightweight outdoor gear. From inflatable mats and packable cookware to sleeping bags and liners, their kit is built to work hard in real conditions — and last for years. We’ve previously tested their Etherlight XT sleeping pad, Ether Light XT Extreme, Aeros Ultralight Pillow, and Ether Light XR sleeping mat — all have been fantastic. With that track record, expectations were high going into our review of the Sea to Summit Ascent -1.
The Ascent -1 is a versatile three-season down sleeping bag built for lightweight adventures — whether you’re wild camping in the Lakes, fastpacking in the Brecon Beacons or bivvying on the coast. With a weight of just 782g (regular) and a pack size that cinches down to the size of a football, it’s designed for those who want warmth without weight. At £375 RRP, it’s positioned at the premium end of the three-season sleeping bag market — but the spec justifies the spend.
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When compared with other down sleeping bags we’ve tested, the Sea to Summit Ascent -1 stands out as a well-balanced all-rounder. It’s lighter and more packable than the heavy-duty OEX Leviathan EV 900, which offers similar warmth but at almost double the weight and without the Ascent’s clever baffling, hydrophobic down, or versatile zip system. It’s more adaptable than the winter-focused Therm-a-Rest Parsec 0, which excels in deep winter but feels like overkill for summer and shoulder-season camping. And it’s far simpler — and packs much smaller — than the modular Zenbivy Light Bed, which wins on comfort but comes with a bigger price tag, a bulkier packed size, and more setup faff.
How we tested the Ascent -1
We put the Ascent -1 through the kind of adventures it was built for — back-to-back weekends of wild camping across the UK, from the Yorkshire Dales to the clifftops of South Wales. In April and May alone, we spent at least one night out each weekend, using it in roomy tents, exposed bivis and even one night just metres from the sea, zipped in tight with the wind howling. We paired it with everything from the Etherlight XT to closed-cell foam mats, and it never faltered. The real proving ground came on a two-day fastpack in the Lake District with a 30L running vest — the Ascent -1 packed down, stayed warm in 1°C, and didn’t compromise comfort.
Why it works: Features that actually matter in the wild
This bag is more than the sum of its specs — though they’re worth noting: 750+ fill power RDS-certified down, 300g of fill, and a non-PFC hydrophobic treatment that genuinely works in the damp, unpredictable climate we call home. On paper, it sounds impressive; in practice, it’s the sort of bag you forget about once you’re in it — because it just does the job without fuss.
We took it everywhere from the clifftops of South Wales to high fells in the Lake District, and the warmth rating felt bang on. Rated to -1°C, it shrugged off frosty April mornings even when we were camped high without a tent, wrapped only in a bivi. All it took was a merino base layer and a beanie to stay comfortable. That hydrophobic treatment isn’t a gimmick — you can feel the difference when you brush off early-morning dew or deal with condensation inside a bivi. The loft stayed high, the down stayed fluffy, and within seconds of shaking it out after compression, it looked like it had never been packed away.
Packability is another strength. The Ascent -1 slides into its 8L compression sack and cinches down to the size of a cantaloupe, which made it a dream to carry on our two-day fastpacking trip across the Lakes. We managed to fit the bag, a mat, stove, and bivi setup into a 30L running vest without any compromise in comfort or warmth. For multi-day treks where every litre of pack space matters, this kind of compressibility is gold.
The zip system is where Sea to Summit’s design chops really show. You get three separate zips — a full-length one on the right, a half-length on the left, and another at the footbox. This gave us every option in the field: quilt mode on warmer nights, vented feet during muggy evenings, and a fully sealed cocoon when the wind was howling on a ridge. It’s intuitive to use, too — no wrestling with snag-prone sliders or awkward zip placements.
The fit is spot on for UK conditions. Sea to Summit call it a relaxed mummy shape, and that’s exactly what it feels like — close enough to trap heat efficiently, but with the extra space you need to change clothes or sleep comfortably on your side. Those extra centimetres in the knees and shoulders made all the difference during a night in the hills when we had to wriggle into dry base layers without leaving the bag.
Details are thoughtful and functional. The internal pocket is perfectly placed for keeping a phone, GPS, or headtorch warm and accessible. The baffling mixes vertical sections across the torso — which stops the down migrating and keeps warmth where you need it most — with horizontal ones in the legs for even coverage. The build feels robust despite the light weight, and while the compression sack offers some splash resistance, we’d still recommend pairing it with a drybag for prolonged exposure in wet wild camping conditions.
And it’s not just about performance — the sustainability credentials are rock solid. The down is certified to the Responsible Down Standard, the fabrics are recycled, and the whole thing meets the Recycled Claim Standard. This isn’t greenwash; it’s a well-made piece of kit designed to last years, not seasons. That’s something we can get behind.
How the Sea to Summit Ascent -1 performed in the UK
Over the course of testing, the Sea to Summit Ascent -1 proved itself across the wide range of conditions you’re likely to encounter between March and late October in Britain. It kept us warm during still, frosty bivis on the high fells of the Lake District, coped with the damp salt air of a clifftop camp in South Wales, and handled cool, breezy nights in the Yorkshire Dales without us ever wishing for a heavier bag.
Moisture management was a real standout. Whether it was dew soaking into the bivi fabric or condensation pooling near the footbox, the hydrophobic down stayed lofty and warm. That’s not something we can say for every down bag we’ve used in the UK — where even the driest forecasts often come with a side order of dampness.
Pack size and weight meant it was never a burden. For long train journeys to trailheads, bikepacking weekends, and fastpacking runs, it disappeared into our pack and left room for the rest of the kit. And because it’s warm enough for sub-zero shoulder season use yet ventable enough for mild summer nights, it’s one of those rare bags that feels like the right choice nine times out of ten in Britain.
Sea to Summit Ascent -1 sleeping bag FAQs
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The Sea to Summit Ascent -1 is rated to the EN 13537 standard, with a Comfort rating of 4°C, a Lower Limit of -1°C, and an Extreme rating of -17°C. That places it squarely in the three-season range — ideal for UK conditions from early spring through to late autumn.
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In testing, we slept comfortably through frosty spring mornings in 1°C temperatures, both in tents and bivis. The down lofts quickly and holds its warmth, and the fit traps heat well. A base layer and beanie were all we needed even in exposed, tentless setups.
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Yes — at 782g and packed into an 8L stuff sack, the Ascent -1 is light and compact enough for serious fastpackers. We fitted it, along with a mat, stove and tarp, into a 30L running vest on a two-day trip without issue.
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If you’re serious about multi-day treks, fastpacking or wild camping in shoulder seasons, absolutely. The price reflects the quality, performance and design features, and it’s a bag built to last for years of real-world adventure.
The Sea to Summit Ascent -1: A true all-rounder for UK adventures
The Sea to Summit Ascent -1 isn’t just a spec sheet dressed up in a fancy stuff sack — it’s a bag that’s been thought through from every angle. The relaxed mummy cut hits the sweet spot between heat-trapping efficiency and the freedom to move, and when you open those full-length and half zips together, the whole thing unfolds like a clamshell. That means you can sprawl out under the stars on warm summer nights, batten down the hatches for a frosty ridge bivi, or dial in the airflow anywhere in between. Combined with its ability to shrug off damp, pack down to the size of a melon, and deliver serious warmth without excess weight, it’s one of the most versatile sleeping bags we’ve ever tested. Yes, it’s a premium buy, but if you want one down bag to cover 90% of the UK camping season — and do it with ease — this might just be it.
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