Ultralight Sleeping Bag
Sea to Summit Spark Pro -9C/15F Bag
$649, 1 lb. 11 oz./765.4g (regular)
Sizes: unisex regular and long ($689)
seatosummit.com
We woke up from our first night in Colorado’s San Juan Mountains in mid-September, at well over 10,000 feet beside a creek that sang soft lullabys to us all night, to find ice crystals in our water bottles and one full water bladder that had been left outside the tent partly frozen. But the overnight temperature dropping to below freezing had hardly registered with me as I slumbered soundly zipped up inside my Sea to Summit Spark Pro -9C/15F sleeping bag—one of the warmest for its weight, most packable, and well constructed ultralight sleeping bags you’ll find.
I slept in the Spark Pro -9C/15Fbag for three nights backpacking in the San Juans in mid-September, with the other two nights at over 11,000 feet along the Continental Divide Trail, where the low temperature remained only slightly above freezing (no ice in our water).
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Stuffed with one pound, 0.9 ounces/480 grams of water- repellent, 950-fill Ultra-Dry Down (in the 15-degree regular; the 30-degree regular has 10.9 ounces/310 grams of down and the long versions of both bags, of course, have a bit more), the Spark Pro ranks among an elite group of the few very best down sleeping bags for warmth per ounce of bag weight, packability, and in the down’s resistance to water: Compared to standard down bags, it keeps you warmer when invaded by any moisture, whether condensation inside a tent, rain penetrating a backpack and the bag’s stuff sack, or condensation inside the bag from moisture released by your body.
The Ultra-Dry Down’s non-PFAS treatment also enables those goose feathers to dry much more quickly than standard down if the bag gets wet. Vertical baffles in the chest, horizontal baffles in lower bag’s top side and throughout the bottom side, plus box-wall baffle construction distribute the down evenly, avoiding cold spots. And this bag provides a visual demonstration of its high-quality down every time you pull it out of its stuff sack, quickly lofting up to about four inches thick.
And at just one pound, 11 ounces/765.4 grams (for the regular, on my scale), the total weight of the Spark Pro -9C/15F compares with many good-quality, 30-degree mummy bags.
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Sea to Summit uses an international standard called ISO 23537 to test and assign two temperature ratings to its sleeping bags: a lower limit and a comfort rating. The -9° C/15° F rating represents this bag’s lower limit; its comfort limit is -3° C/27° F.
On the night the temperature dropped below freezing, with the bag zipped up completely, I slept comfortably for most of the night in just a light wool T-shirt, underwear, and socks, only pulling on a long-sleeve top and a wool hat in early morning (the coldest time of the night and the time when your body often feels coldest because it needs more food). To me, that demonstrates this bag is true to its comfort rating. In temperatures at its lower limit, I would personally expect to need a base layer or two, top and bottom, plus a wool hat to feel warm enough.
I’ve pushed plenty of bags to their limits without getting miserably cold, but I sleep warm; people who don’t would definitely not enjoy pushing this or any bag to its lower limit, but would probably find the Spark Pro -9C/15Fidealfor nights no colder than several degrees above freezing Fahrenheit/three to four degrees above freezing Celsius. (See my “Pro Tips for Buying Sleeping Bags.”)
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Sea to Summit’s Free-Flow three-zipper design expands the versatility of the Spark Pro bags. A quarter-length zipper on the upper right side of the bag enables you to fold down the top of the bag like a quilt on mild nights. A footbox zipper ventilates that end of the bag—a feature I virtually never use in bags because my feet are rarely too hot, but which makes more sense in combination with the ability to fold the top of the bag down like a blanket. The two-way, full-length main zipper (left side) slides remarkably smoothly—never snagging, which isn’t true of most “no-snag” zippers in bags I’ve used.
Some ultralight sleeping bags achieve their low weights by using less insulation (read: less warm) or by reducing fabric and insulation weight through tighter dimensions (read: feels claustrophobic to some people). But the Spark and Spark Pro series bags have comfortable dimensions of 61x53x40 inches/155x135x102 centimeters (at the shoulders, hips, and feet, size regular), which compares with some of the better ultralight bags I’ve reviewed.
I found it has adequate space while maintaining thermal efficiency and without making the bag overly bulky: I could fit a long-sleeve top and pants in the foot of the bag (to add a little insulation and keep those clothes warm for when I put them on in the morning) and easily lift both knees to my chest while lying on my back in the bag, as well as dress in the bag with it zipped up. The 72-inch/185-centimeter length is standard for regular bags.
While the Spark Pro is not available in women’s sizes, and the unisex sizing likely favors the builds of more men than women, some women will find the unisex bag works for them because body shapes and sizes vary greatly within genders as much as between genders, rendering the fit of sleeping bags less gender-dependent. Sea to Summit recommends women use the comfort temp rating as their guide.
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In the smart features department, waterproof-breathable fabric in the hood and footbox repels moisture from condensation on tent walls in the areas of the bag most likely to brush against walls. The hood and collar cinch closely around your head using separate drawcords on opposite sides, which I find simplifies a task that’s often done in the dark when half-asleep; but if you’re a side sleeper (like me), either of those cords might brush against your cheek (as with many bags).
Stuffed inside the durable, 40-denier compression sack that comes with the bag, the Spark Pro 15F/-9C regular packs down to seven liters in volume, according to Sea to Summit—although I think I compressed it significantly smaller than that, to about the size of a soccer ball. Either way, that’s a very compact package for a bag with this much warmth. Sea to Summit gives the compressed volume of the 30F/-1C as 5.3 liters; based on my experience with the 15F/-9C, I suspect it can go smaller than that.
That’s due, in part, to the ultralight, 10-denier nylon lining and shell with a non-PFAS DWR (durable, water-resistant treatment), which is lighter and more packable than shell fabrics used in heavier sleeping bags, but also slightly less durable—although most users are very unlikely to tear a sleeping bag.
The Sea to Summit Spark Pro -1C/30F ($549, 1 lb. 5.8 oz./619g, regular) is a good choice for someone who doesn’t get cold easily and typically sleeps outside on nights around or well above freezing.
Another option, Sea to Summit’s Spark series of bags rated 45, 30, 15, and 0 degrees ($349 to $649), differfrom the Spark Pro primarily in that they use 850+-fill Ultra-Dry Down, lack the footbox zipper and short right-side zipper, and come in women’s sizes. In other words, at $100 cheaper for the same temp rating than the Pro series, you get an ultralight bag that’s still quite warm for its weight and packable—and actually a few ounces lighter than its Pro counterpart, due to having fewer features.
The Verdict
Featuring virtually the highest quality down available, giving it superior packability and warmest for its weight, as well as comfortable dimensions, excellent construction, and versatility in a wide range of temperatures, the Sea to Summit Spark Pro -9C/15F sleeping bag ranks among the few very best ultralight sleeping bags out there today.
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See all reviews of sleeping bags, air mattresses, backpacking gear, and ultralight backpacking gear at The Big Outside, plus my “Pro Tips for Buying Sleeping Bags” and “10 Pro Tips: Staying Warm in a Sleeping Bag.”
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NOTE: I tested gear for Backpacker magazine for 20 years. At The Big Outside, I review only what I consider the best outdoor gear and apparel. See The Big Outside’s Gear Reviews page for categorized menus of all gear reviews and expert buying tips.
—Michael Lanza