The Sea to Summit Spark SpI 40 is the lightest and smallest-packing bag we've ever tested by far. It's so light and tiny when stuffed into the provided stuff sack that it's our Top Pick for Insane Packability. This bag is ideal for those who measure weight to the second decimal place, or for those who want an emergency bag to throw in the pack. We also liked it for warm desert backpacking and summer bikepacking, but it didn't do great in the shoulder season temperatures. It's an amazingly light and packable bag that works great, as long as you're realistic about the temperatures you're using it in.
Sea to Summit Spark SpI 40 Review
Compare prices at 2 resellers Pros: Truly ultralight, packs down well, good zipper
Cons: Not very warm, tight around shoulders
Manufacturer: Sea to Summit
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Our Analysis and Test Results
The Spark comes in a range of temperatures, from 40°F to 5°F; we tested and assessed the 40°F version for this review. The first thing we noticed was how light this thing is! Sea to Summit's website listed the bag at 12 ounces, and our own scale measured it at 12.4 oz. This thing is crazy light and supremely packable. We ended up throwing this in our pack any time there was even a remote chance of a bivy, as the Spark is about the size of a liter of water.
The low weight does come with its drawbacks. We could only really use this thing in the summer months, as the EN 13537 rating lists the comfort temperature at 48°F, and the lower limit at 40°F, and we found that to be pretty true. However, for ultralight backpacking in the warmer months, we loved it.
Because the Spark is such an ultralight bag, warmth is its weakest category. Sea to Summit lists the fill weight at only 6.3 ounces, far below any other bag we have tested. Despite the 850+ down fill, there are definitely cold spots, in part due to the sewn-through construction of the bag. Sewn-through bags eliminate the weight of the baffle fabric, but it means that there are parts of the bag where two thin sheets of nylon are the only thing between you and the outside air. If we didn't sleep with a tent or bivy sack, which we often do in the desert, we could feel the wind blowing through the fabric on windy nights.
That being said, down to it's stated "comfort" temperatures, it did pretty well. Despite the minimal down and construction, the hooded mummy cut did keep in a lot of warm air, or at least more than we expected to when we first held up this little bag. For how tiny it is, it was adequately warm. Anything below about the mid-forties (F), though, and we could feel the cold start creeping in. During our frozen water bottle test, it also melted out before other 40°F bags, although it was warmer than the lowest-performing quilt.
We can't get over how gosh darn light this thing is! We've eaten burritos that were bigger. Did we mention that you could probably use a Nalgene as a stuff sack for this bag? While these are slight exaggerations, the size and weight of this bag are astounding. The included stuff sack adds an ounce, but it's a compression sack, which helps even more with the packability. With how small it is, you could get away with doing overnight trips with a daypack, maybe even an ultrarunning vest, or a single bike pannier. The possibilities are endless!
Sea to Summit achieved this weight with a hooded mummy by eliminating a lot of "extra features." On this bag, there's no draft tube, no draft collar, no baffles, only a half-length zipper, and only two connection points for pad attachment. This bag is minimalist and designed for light and fast missions.
Despite how light and small the Spark is, it was actually pretty comfortable. Our head tester for this bag is a broad-chested 5'8" male, who found the bag to be a touch tight around the shoulders, but not too bad. Our feet had enough room, and the bag rolled with us as much as any mummy.
It wasn't our absolute favorite fabric, but it doesn't feel as plasticky as some of the other bags we've tested. There also wasn't a bunch of dangly things around our faces, like long zipper pulls or drawstrings, which was nice.
In some ways, because the Spark SpI 40 is so freakin' light, it's really versatile. You can stuff it in just about any pack and bring it along any adventure, as long as the temperature is right. For summer backpacking, desert trips, bikepacking, alpine emergency bivies, summer packrafting trips, this bag is an excellent option.
On the flip side, its lack of warmth limits the time of year and environments in which the SpI 40 is suitable. It's great for warm summer trips, but we shivered all night long when we took it backpacking in late October when the temperatures dipped into the thirties. Pushing the Spark into three season applications would require extra clothing and an aggressively insulated sleeping pad to achieve the necessary warmth. Adding a liner could also boost its warmth and versatility, too. That said, these will all add pack weight, which is anathema to this bag.
Although the Spark SpI has pretty minimal features, the few included work well. We were pleasantly surprised by the zipper not catching on anything, and we liked the hood more than we thought we would. The zipper is only half-length, but it would have to be pretty warm outside to want to vent this thing. There isn't a draft collar or draft tube by the zipper, but a bag this warm (or cool, rather) rarely does. It does have one set of loops to connect the bag to a sleeping pad, but no included cord, and if that's something you really want, a quilt is probably more what you're looking for anyway.
As far as the sleeping bags in this review, the price of the Spark SpI 40 is somewhat middle of the road. It's a fairly high-quality bag, and the benefits of a superlight sleeping system are pretty sweet, but it can only be used in certain circumstances. We think this bag would be worth it if you're creating a quiver of sleeping bags. Otherwise, for a three-season bag, we think there are better options out there. This is a niche product that serves some needs extremely well but is a far cry from an all-around champ.
The Sea to Summit Spark SpI 40 is one crazy light, wildly packable bag, so much so that we had to single it out as our Top Pick for Insane Packability. This thing can fit almost anywhere, and you'll never notice that you're carrying it. With a sleeping bag so small, you could do all sorts of ultralight missions, where moving fast and being light are paramount. However, be aware of the conditions, as the Spark is really only great at summer temperatures. That being said, how cool is it to go backpacking with a 22-liter daypack? With the Sea to Summit Spark, things like that are actually possible.
— Ethan Newman