Sea to Summit Ascent ACI Down Sleeping Bag review: Versatile, packable all-rounder

Stay cosy across in the Sea to Summit Ascent I, which zips open into a quilt or can be cinched up to offer the warmth of down

Sea to Summit Ascent ACI Down Sleeping bag review
(Image credit: Sian Lewis)
T3 Verdict

Sleep how you like in Sea to Summit’s adaptable Ascent, which works just as well as a traditional sleeping bag as a roomier quilt and is stuffed with a pleasingly soft and comforting down filling—a lovely three-season all-rounder.

Reasons to buy
  • +

    Versatile zip design

  • +

    Warm down filling

  • +

    Female-specific shape available

Reasons to avoid
  • -

    Not ultra light

  • -

    Fans of narrow mummy bags may find the Ascent too roomy

  • -

    Down bags require careful storage and must be kept dry

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In need of a new sleeping bag? Sea to Summit’s unisex Ascent and female-specific Altitude down-filled sleeping bags are designed to keep you cosy on camps from spring through to autumn and sport a clever triple zip system to help you regulate heat during the night or even convert the bags into a quilt. Are these among the best sleeping bags out there for three seasons of adventure? We tested the Ascent out in chilly conditions to see how it performed.

Sea to Summit Ascent ACI Down Sleeping bag review: Specifications

  • RRP: from £320
  • Gender: unisex (Ascent) and female (Altitude)
  • Size options: regular, long, women’s
  • Length: 183cm (regular)
  • Weight: 848g (regular)
  • Fill: Utra-Dry premium duck down
  • Comfort level: 2°C
  • Comfort limit level: -4°C
  • Zip: left

Sea to Summit Ascent ACI Down

(Image credit: Sian Lewis)

Sea to Summit Ascent ACI Down Sleeping bag review: Design and features

The Sea to Summit Ascent sleeping bag range offers campers real comfort in the wild. This soft down-stuffed bag stands out for its triple zip system, which allows you to unzip the bag around the torso and free your arms to cool down (or sit and make a cup of tea in the door of your tent on chilly mornings), fold the top of the bag down like a blanket or open up the bag fully into a quilt (ideal if you’re not sure if quilt-only camping is for you yet). There’s a variety of shapes and sizes to choose from, too: as well as the female version of the Ascent, the Altitude, there’s a longer version, which at 198cm will fit taller campers who are up to 2m/6’6" in height.

The generous filling of the Ascent is ULTRA-DRY Down 750+ Loft, RDS 90/10 Premium Duck Down, and the shape sits in between a narrow mummy and a rectangular sleeping bag, allowing a bit more wiggle room and for the bag to function as a quilt, and making this a good choice for side sleepers who need that bit more room.  

At 880g this isn’t an ultralight bag, and although it packs down easily into its compression sack and is small enough to fit in a roomy backpack. All in all, Sea to Summit are setting the Ascent up as an adaptable all-rounder bag for backpackers.

Sea to Summit Ascent ACI Down Sleeping bag

(Image credit: Sian Lewis)

Sea to Summit Ascent ACI Down Sleeping bag review: Warmth and performance

I tested the Ascent out on a chilly few nights in the Lake District and Dartmoor, with temperatures dropping to 2°C, the bag’s comfort limit. While I didn’t have much need for the cooling options the bag’s zips provide, I did find that, when zipped up snugly and cinched in, the bag’s 750+ loft down filling immediately traps in body heat and feels pleasingly squishy and comfortable to wrap up in. 

Vertical baffles over the chest section ensure the down insulation cannot migrate or shift to the outside of the bag during sleep, thus avoiding cold spots. The wide hood is great too, and can be cinched in snugly around the face, where it stays snug even if you toss and turn. 

I liked the extra design features the Ascent boasts, such as a roomy phone pocket, an easy-to-use compression bag and oversized tubes around the hood designed to trap in warmth. 

You can also zip this bag to other compatible Sea to Summit bags, including the Ascent. This is definitely a good versatile choice to see you through to warmer weather - come summer the ability to switch to a quilt will definitely come in handy, and while it’s hard to test durability in the short term, the outer material of the Altitude feels tough and rip-proof to touch, and the zips are decent quality.

Sea to Summit Ascent ACI Down Sleeping bag

(Image credit: Sian Lewis)

Sea to Summit Ascent ACI Down Sleeping bag review: The female-specific Altitude

The Ascent ACI also has a women's version, the Altitude AtI -4°C (RRP: £360), designed specifically with the female form in mind - it features body-mapped down-filled chambers to maximize warmth, more room through the hip and knee, and a shorter length and narrower shoulder to reduce dead space, increasing thermal efficiency. I’m 170cm/ 5’7”, and I tried out the unisex Ascent and found it fitted brilliantly, but shorter women and female campers who find unisex sleeping bags too roomy or draughty may find this smaller design fits them perfectly.  

Sea to Summit Ascent ACI Down Sleeping bag

(Image credit: Sian Lewis)

Sea to Summit Ascent ACI Down Sleeping bag review: Verdict

A wear-it-your-way zip system, the ability to fold out fully into a quilt and warm comfort from plentiful down makes the Sea to Summit Ascent I a great all-rounder of a bag for three season camping (and you can get away with using this bag on mild winter nights, too). Unlike most three season bags, you won’t overheat in the summer as it’s easy to let air circulate, and there’s also a female-specific fit as well as a long version of the bag available.

Sea to Summit Ascent ACI Down Sleeping bag review: Also consider

Sea to Summit also offer the Ascent in two warmer versions, the Ascent ACII -10°C (RRP: £370) and the Ascent ACIII -18°C (RRP: £420). In terms of other brands, the Rab Solar 2 Ultra is an excellent three-season mummy bag, while the Kelty Cosmic Ultra is a reasonably priced alternative if you’re on a budget, and the Sierra Designs Night Cap 20 offers a similar duvet-style design.

The Therm-a-Rest Polar Ranger is the ultimate cold-weather sleeping bag. It's warm, extremely comfortable, sustainably made, and just a fantastic piece of outdoor equipment overall. The 800-fill Nikwax Hydrophobic Down filling is durable and made with Responsible Down Standard Certified down; it'll keep you toasty even in the coldest temperatures. Read Matt's full Therm-a-Rest Polar Ranger Sleeping Bag review.

Sian Lewis

Sian Lewis is an award-winning travel and outdoors writer, author and influencer. She's the author of popular blog and book The Girl Outdoors, and when

she isn't writing or travelling she spends most of her time hiking, cycling and wild swimming across Britain, testing out the latest adventure gear and clothing as she goes.