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Field Tested: Sea to Summit Spark SpI Sleeping Bag

A few months ago I had the opportunity to get my hands on Sea to Summit’s new Spark SpI sleeping bag. As an unabashed gear geek with a genuine affinity for all things uber-light, I couldn’t wait to get it into the backcountry to see how it performed. Given the stellar reputation Sea to Summit has within the outdoor industry I suspected it would perform perfectly, and with several nights in the SpI behind me, I can say it is by all accounts and exceptional bag.

Pulling the SpI from its elaborate storage cube, it’s hard not to marvel at its lack of heft. I could unleash the usual superlatives like featherweight, gossamer, or bantam, and they would barely describe how light and compact this bag is. At only 348 grams (in regular length) it is ideal for bikepacking and lightweight motorcycle travel. The compressed size is roughly equal to that of a standard one-liter water bottle making it an easy fit in any pack or pannier. I recently did a quick overnighter on my motorcycle and the Spark SpI fit into my rather small tank bag along with a full length sleeping pad and bivy sack, leaving room for a water bottle, and even one of Sea to Summit’s new inflatable pillows. The era of “light and compact,” just got even more so.


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The packed size is not just small, it’s tiny. The above bundle isn’t even fully compressed.



Many bags achieve their low weight by cutting back on the overall size of the bag. The Spark is generously cut for an ultralight bag.


The key to the Spark SpI’s diminutive size is its 850+ Ultra-Dry goose down fill. If you’re not familiar with Ultra-Dry Down, it is the revolutionary new process that makes down fibers virtually waterproof. It also improves loft and general durability of the down fibers themselves. Wrapped around that down fill is a semi-translucent 10 denier shell with a 15 denier lining that weighs a little more than 160 grams by itself. To achieve that weight the #3 YKK zipper has been shortened to only reach one third of the way down the side of the bag. That seemed like it could be problematic, but so far has not presented a problem. It’s a reasonable compromise given the mission of the bag––reduced weight. The hood has a simple drawstring closure to pull it snug against your head, and with that, the list of features is complete. Simplicity is the best way to keep weight to a minimum.




The hood and foot-box are well shaped although the fill in the hood is so minimal, it is only effective when pulled quite tightly around my head. Sea to Summit rated the Spark SpI at a conservative 46ºF. My first two nights in the Spark were slightly below that temperature, probably hovering around 40ºF, and while I wouldn’t say I was warm, I was comfortable nonetheless. The key to maximizing the warmth of this or any bag is the use of a good base layer with a hat and I even sleep in light merino wool gloves. Adding to the comfort is a reasonably generous shape, although it is by no means wide. I’m a relatively thin guy, so my experience may not transfer to users of all sizes.

For those in need of the lightest in summer sleeping bags, the Spark SpI should be added to your short list. As a travel bag, it’s nearly perfect and can easily slip into even the most austere luggage setup. It’s amazing where recent outdoor technology is headed. I can’t imagine sleeping bags getting any lighter than this, but I probably said that same thing just a few years ago as well.


MSRP: $299  www.seatosummit.com



– Obviously, the insanely small size and weight

– Ultra-Dry Down is the most significant new technology to hit the outdoor market in at least 20 years

– Seat to Summit quality is hard to beat, or even match

– Given the materials involved as well as the fill certification process, it’s a good value



– Some sleepers may not fit the Spark SpI

– The 1/3rd length zipper can be an issue for some, especially if used in a snug bivy sack


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As an added touch, each bag is shipped with a letter of certification confirming it’s fill-power rating by an independent lab. Not just per model of bag, but for each individual bag. That’s an impressive example of their attention to quality.

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Christophe Noel is a journalist from Prescott, Arizona. Born into a family of backcountry enthusiasts, Christophe grew up backpacking the mountains and deserts of the American West. An avid cyclist and bikepacker, he also has a passion for motorcycles, travel, food and overlanding.