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christos nicolaou

Hunter Bierce
Outdoor geek
Hunter Bierce is a PSIA Ski Instructor and multidisciplinary outdoor professional.


Sea To Summit Ascent AcI Sleeping Bag

A dependable sleeping bag has a special place in the heart of anyone who spends more than a handful of nights outside each year. Sea to Summit’s Ascent series sleeping bags are versatile and feature-rich 3-4 season options for those of us keeping it below treeline in the winter.

The series has 3 bags, each rated for different temperatures: Ac1 (25℉), Ac2 (15℉) and Ac3 (0℉).

I had the chance to try out their 25ºF Ac1 with the Thermolite Retractor liner and feel the system is an excellent all-condition option that I’d be happy to call home on most any adventure.

Our Overall Review

We have thoroughly tested - and read reviews from other experts and users. In summary, this is what we think:


Reasons to buy

  • An objectively "well rounded" sleeping bag
  • 2.5 season option that can be stretched into 4 with use of a liner
  • Packed full of unique features and an adaptable zip system
  • Draft protection liners along the zippers and collar
  • Zip Coupling for combining bags into a double
  • Chest baffles to prevent down spreading
  • Tripple-zipper Free-Flow Zip System (1 full-length right side, 1 foot right side, 1 half left side)

Reasons NOT to buy

  • The relaxed mummy fit isn't as warm as a tighter bag
  • More zippers mean more opportunities for issues with your zippers

Weight and Bulk

We can’t start talking about sleeping bags without mentioning weight. Out of all of the Ascent series’s practical assets and exciting features, its low in-class weight might be the most significant. With the long version at just over 2 lbs, the Ascent Ac1 is far from the lightest bag on the market. But in comparison to similar relaxed fit bags in related temperature ranges, it’s near the top of the pack.

Even compared to the lightest bags on the market, the additional weight doesn’t add much bulk. Extra ounces are easily justified by packing the bag full of features. The Ascent Series isn’t for people seeking the lightest possible option (I’d point you in the direction of a down quilt), but it’s a far cry from your grandpa’s bulky synthetic bag. If forced to place them within the existing sleeping bag taxonomy, I’d call them “luxury lightweight.”

Now that we’ve talked about weight, it’s worth taking a look at how it all comes together to fit in your backpack. The Ascent Ac1 is a little wider and a little looser than your typical mummy bag, but this doesn’t mean it’s any bulkier on your back. With the compression sack, the sleeping bag can reduce in bulk to about the size of a small melon (see picture). For all of the features that go into the bag, it’s small and light enough to fit in a large-ish day pack for emergency use as well.

See the complete list of the best Sleeping Bags here!

Fill, Warmth, and Weather Resistance

The Ascent Ac1 is filled with 750+, sustainably sourced, hydrophobic grey duck down. This is a mouthful, so let’s break it down a little bit. According to the IDFL (International Down and Feather Testing Laboratory) report included with the bag, 90% of the fill’s makeup consists of down clusters. The rest is made from various fibers and feathers also sourced from waterfowl.

While the 750+ fill power of the Ac1 is just at the bottom edge of premium options, this doesn’t mean it’s any less insulative. I felt like the bag on its own provided warmth true to the 25º EN rating. If you plan on doing any Winter camping or being at altitude during the shoulder seasons, take a look into the AC2 and AC3 rated to 15º and 0º F respectively. They have all of the features and most of the packability of this lighter version of the bag, with a lot more down packed into the baffles.

It’s reassuring to have the results to verify the fill’s quality, but I’m also excited by the Ultra-Dry treatment that Sea-to-Summit uses to provide some weather resistance. I didn’t dunk the bag into a creek to give it a full test, but I did manage to spill nearly an entire water bottle near the foot box while ducking into my tent. The shell held on to some water, but the down felt dry and lofted after a few minutes in the wind, and more importantly, my feet were warm through the night.

The Ascent Ac1 and Thermolite Reactor Liner working to keep me warm.

Sea To Summit Ascent AcI Sleeping Bag Keeping Me Warm

Sea To Summit Ascent AcI Sleeping Bag: Key Specifications

  • Season Rating: 2.5 Seasons
  • Comfort Temperature: 25℉ (35℉ for women)
  • Bag weight: 1.89lb (Ac1 Reg), 2lbs (Ac1 Long)
  • Compressed Vol.: 4.9L (Ac1 Reg), 6.4L (Ac1 Long)
  • Compression Sack: S - 10L to 3.3L
  • Max Height fit: 6ft (Reg), 6ft6in (Long)
  • Internal Pocket: 1
  • 750+ Fill Ultra-Dry Down
  • 20 Denier Nylon
  • Relaxed mummy fit
  • Anatomically shaped hood with pull cord
  • Internal device pocket
  • Included storage cell and compression bag
  • Also available in 15ºF (Ac1I) and 0ºF (Ac1II) versions

Where to Buy:

The “Relaxed Mummy” Cut and Design Features

Despite the wider fit, I didn’t find myself wishing for any more restrictiveness. The Ascent series retains a tapered anatomical shape but is less aggressive than a full mummy. I appreciated the freedom of motion the bag allowed, as I tend to thrash around a little bit in the night.

Looser bags usually take a hit on warmth, but because of the draft-proof bevels around the hood and the zippers, I felt the normal heat loss was much-mitigated.

Strategic insulation around key areas goes a long way to making the Ascent series more competitive. The lofted foot box and shoulders were a standout. Usually, it takes nearly an hour for my feet to feel as warm as the rest of my body, and I felt a significant difference from the extra time that Sea to Summit spent looking at the area.

Ascent sleeping bags also have down baffling to keep the insulation in place. These vertical sections hold the down around your torso in a consistent spread and spread. The benefits of this technology will really show after extended use. The protection is twofold: baffling prevents your sleeping bag’s fill from getting jumbled to either end vertically, and from bunching after compression (i.e., a night of sleep).


3-Zipper Free-Flow Zip Design

One of my favorite things about the Ac1 is the zippers. Though this might sound weird at first, bear with me for a moment. The Ac1 has three separate zippers- one full right-side zip that extends the entirety of the bag down to the feet, a half zip on the left side, and an additional zipper that also extends the length of the foot area.

The full zip is a pretty standard affair, running the whole length of the bag’s right side. But instead of stopping a foot or so before the toe of the bag, it extends across the bottom. This lets you zip your bag flat open and effectively convert it into a quilt at your preference. I was able to keep myself and a couple of friends warm during a windy winter day at the beach.

If you combine the full zip with the half zip, you can alternatively fold down the bag’s torso section while keeping your feet covered. I sleep warm, so in the spring and summer, it’s a great way to dump a lot of heat while staying covered. This feature made it much easier to fiddle around inside the tent before I went to sleep.

It was convenient to grab my water bottle, turn on my headlamp, and do other miscellaneous tasks without continually adjusting my sleeping bag. It lets you sit around casually at camp without having to worm your way around.

I often find myself kicking and thrashing to free myself from the hot, claustrophobic feeling of a sleeping bag in the throes of a summer evening. The foot zipper has solved that problem. I no longer need to wrestle my sleeping bag in a desperate struggle to cool down. It’s as simple as leaving the bag’s bottom section unzipped and wiggling my toes out when the need strikes.

The ability to fold down the torso of the sleeping bag makes moving around camp much easier.

Sea To Summit Ascent AcI Sleeping Bag Zip Design

Hydrophobic Down Treatment

Not all down bags are created equal. For an all condition bag like those included in the Ascent series, it’s crucial to have additional water resistance for unpredictable weather. Sea to Summit’s in-house Ultra Dry can keep you dependably warm through the night should you encounter some sudden moisture. The treatment comes at the additional cost of a little weight, but it’s well worth it to keep you safe and comfortable in any conditions.

Internal Pocket

It’s not revolutionary to have a pocket in your sleeping bag. But when coupled with the Ascent series’s zip system, I thought it had potentially more utility than other sleeping bag pockets I’ve used in the past. I’m a pretty dynamic sleeper and tend to thrash around a bit throughout the night. I never found myself getting “lost” in the Ascent like I do in other bags and could find my pocket with minimal effort.

My one criticism of the pocket is that I’m not sure what to put in there. It’s not quite big enough to fit my phone with the case on, and I usually opt to stash my headlamp somewhere that I can blindly grab without the hassle of a zipper. I think if you had extra camera batteries or a ceramic water filter: freeze-sensitive things that you need to keep warm but don’t want to stash by your feet, it’s great.

Zip Coupling system

Double sleeping bags are a great way to stay a little warmer through the night with a partner, but they are also bulky and have limited applicability based on their size. Thankfully, you can combine Sea-to-Summit sleeping bags into a double provided you have compatible zippers.

Unisex bags such as the Ascent Ac1 couple with Sea to Summit’s right-hand women’s specific bags or any other with full-length zips on both sides. When paired, Zip Coupling bags retain all of the insulative benefits of a single Sea-to-Summit sleeping bag, meaning that while you can share warmth, you’re not beholden to your partner to keep you warm.

Unfortunately, I was provided neither an extra bag nor an extra body to give it the full test. But the prospect of not struggling to cobble together a “Frankenbag” or deal with the bulk of a double bag is appealing. If you’re looking into this feature to bunk up with your backpacking buddy, make sure that you pay attention to the zippers’ specific compatibility.

Compression Bag and Storage Cell

Compression bags are a real luxury while backpacking. There’s not much to write home about in terms of the one included with the Ascent, but it effectively turns your bulky bag into an impressively small package. The storage cell does the opposite. It’s a convenient place to keep your bag while not in use to prevent your down from getting clustered up.

Sea To Summit Ascent AcI Sleeping Bag Outside Setup

Thermolite Reactor Sleeping Bag Liner

Alongside the Ascent 25º, I also had the chance to try out Sea to Summit’s Extreme Thermolite Reactor Liner to add some warmth this exceptionally cold winter. In theory, this liner should add 25º of functional warmth, but I’m not convinced it performed anywhere near where an actual 0-degree bag would. I was warm enough to last the night with the addition of the liner but felt as though the combination of the two wasn’t as warm as proper 0-degree bags that I’ve tried in the past.

This criticism isn’t to discredit the liner but more to give a more realistic overview of the limitations of a 25º bag. I’ve been thrilled with the comfort and performance of the liner itself. Even though it undeniably adds some weight, nearly half as heavy as the bag itself, the benefits of having a liner far outweigh the costs.

On its own, the Thermolite Reactor makes a very capable standalone bag for warm weather. The liner also has the benefit of keeping your bag cleaner by creating an easy-wash barrier between your sleeping bag and your dirty feet.

The impressively compact Ascent 25º as it would be stored in your backpack.

Sea To Summit Ascent AcI Sleeping Bag Compression Bag

Buyer’s Guide

Buying a sleeping bag, like most pieces of outdoor gear, is a big commitment. There are many factors to consider, not limited to the kind of camping you do. Below we’ll outline just a few of the factors you should consider when deciding on your next sleeping bag. For a look at our top picks and a little more of a buyer’s guide, check out our best sleeping bag article.

Sleeping Bag Ratings Explained

Warmth ratings on sleeping bags are important. It’s one of those things that’s so important to get right that the industry has standardized the system by which they’re rated. EN (European Norm) and ISO (International Organization for Standardization) are the two existing rating systems. ISO is the more contemporary of the two systems, but the measurements are so similar they’re sometimes used interchangeably. There’s a lot that you can learn about your sleeping bag from the EN/ISO ratings, but the Comfort Rating and the Lower Limit are the most important.

Comfort Rating- The lowest temperature that the average woman (who generally sleep much colder than men) can sleep comfortably.
Lower Limit: The lowest temperature the average man can sleep a full eight hours without waking up.

These rankings might seem a little subjective or confusing because they’re based on averages. The comfort rating is the coldest temperature that you can feel toasty. The lower limit is the coldest temperature that you’d want to take the bag out in without some kind of additional insulation.

Not all sleeping bags have an EN/ISO rating, but this doesn’t necessarily mean they’re low quality. Brands that don’t opt for the standardized rating system usually have some proprietary equivalent. You should be able to discern the lower limit of these bags through some careful investigation, but take care that you don’t end up assuming something will keep you warm below freezing when in reality, it will just keep you alive.

Down Versus Synthetic Sleeping Bags

There’s no doubt that down is the way to go for your outdoor gear when it comes to weight and warmth. The only performance benefit that synthetic fill could possibly offer is more resilience to moisture, down being worse than useless when it’s wet. But, with the rise in popularity of hydrophobic down treatments, this is less relevant.

The one upside of a synthetic bag is the cost. You can get a dependable synthetic bag for a competitively low price. However, if you plan on doing any substantial amount of camping or backpacking, I’d recommend making the commitment and going for a water-treated down bag.

Considerations for Down Sleeping Bags

Fill Power

Fill Power is essentially a measure of the quality of your down. Also commonly referred to as the “loft” of the down, your sleeping bag’s fill power is just how fluffy your down clusters are. The higher the loft, the less down is needed to keep things warm, meaning a lighter, warmer sleeping bag with the same amount of fill. On a scale out of 1000, the Ascent scores a 750, putting it on the upper end of the spectrum just behind true top-of-the-line sleeping bags.

Fill Weight

Fill weight is just a measure of how much insulation is in the sleeping bag. Fill weight can be a good clue for determining the warmth and weight for similarly rated bags or bags without an EN/ISO rating. Narrower mummy bags or relaxed mummies like the Ascent will be warmer with a lower fill weight than higher-volume sleeping bags.

See the complete list of the best Sleeping Bags here!

Sea To Summit Ascent AcI Sleeping Bag Scenic View

What We Like

  • An objectively "well rounded" sleeping bag
  • 2.5 season option that can be stretched into 4 with use of a liner
  • Packed full of unique features and an adaptable zip system
  • Draft protection liners along the zippers and collar
  • Zip Coupling for combining bags into a double
  • Chest baffles to prevent down spreading
  • Tripple-zipper Free-Flow Zip System (1 full-length right side, 1 foot right side, 1 half left side)
  • Highly compressible and reasonable light for backpacking
  • Considerations for long term use despite its low weight
  • Certified Responsible Down

What We Don’t Like

  • The relaxed mummy fit isn't as warm as a tighter bag
  • More zippers mean more opportunities for issues with your zippers

Where to Buy:

Where to Buy:

FAQ – Frequently asked questions about Sea To Summit Ascent AcI Sleeping Bag

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    Is Sea to Summit a good brand?

    Sea to Summit is primarily known for lightweight, high-quality dry bags. But they’re also renowned for a wide range of outdoor products ranging from full sleep systems to camping kitchenware. You can count on Sea to Summit to keep you covered for all of your camping needs.

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    How to wash my sleeping bag?

    The best thing that you can do for your sleeping bag is to try to keep it clean. Proactive and preventative care is always better than trying to scrub out stains after the fact. Consider investing in a sleeping bag liner, and barring that, at the very least, try to sleep with a clean pair of socks on. When you inevitably need to wash your bag, make sure that you read the care instructions as they may differ from bag to bag. You can wash most sleeping bags by following a few general rules.

    How to machine wash your sleeping bag:

    • Use any front loading washer or a top loader without an agitator
    • Set your washing machine to the gentlest setting and cold water
    • Use minimal soap or detergent, preferably as gentle as you can find
    • Make sure your sleeping bag is rinsed thoroughly before drying
    • Tumble dry on low, add tennis balls when it is nearly dry to keep the down from climbing

    How to hand wash your sleeping bag:

    • Fill a bin or large tub with cold or lukewarm water
    • Work a small amount of gentle soap or detergent into the entirety of the bag
    • Fill your tub or bin with water again to rinse the bag
    • Squeeze as much moisture as you can from the bag, leave to dry in the sun, and separate the down
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    How to store my sleeping bag?

    Despite how much space you may save by keeping your sleeping bag in a stuff sack or a compression bag, it’s better for your sleeping bag if you leave it stored in a loose cell. Keeping your bag compressed will cause your down to bunch up and reduce overall insulative properties.

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    What is a sleeping bag liner?

    If your sleeping bag is getting on in age or isn’t warm enough, you should consider investing in a liner to add a little more life. Sleeping bag liners are separate fabric sleeves that act much in the same way as a top sheet for your mattress. Some people think it’s much more comfortable to sleep with a liner. Your sleeping bag liner is also a whole lot easier to wash than your bag is and can protect your expensive down bag from wear over time.

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    Are down or synthetic sleeping bags better?

    Down sleeping bags have the edge on synthetic fill bags in terms of both weight and warmth. Synthetic fill bags are significantly cheaper and tend to stay much warmer when they’re wet. If you invest in a down bag with a waterproof treatment, you have the best of both worlds.

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    How to fold a sleeping bag?

    Unlike sheets or linens, you should never fold your sleeping bag. Folding sleeping bags can cause the down to bunch up in large sections, which in turn will keep you much colder than if spread out consistently. To store your sleeping bag, start at one end and stuff it into its sack piece by piece. Try to alternate how you stuff your bag to keep the down spread out randomly.


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