Offering the best warmth-to-weight ratio of any bag in our review, the Spark III provides a focused design that minimizes weight without compromising warmth. It shaved grams with tight (thermally efficient) dimension, super light 10D fabric, and only a 1/4 length zipper. While the Spark III wasn't quite the lightest bag in our review - that title belongs to the Marmot Phase 30 - the Spark III is only four ounces heavier and offers an equal compressible packed volume and but was far warmer.
Despite being the straight-up lightest bag in our review, the Spark III was on the warmer side of sleeping bags we tested. In our real world tests, we thought it was a pretty true 25-degree bag.
The Spark III is rated as a 25-degree bag. Our testing team felt even despite its impressively low weight, it was a true 25-degree bag, and on the warmer side of bags with similar temperature ratings. It features 14 ounces of 850+ fill power down that helps it achieve its 25F rating. Check out the chart below to see where the Spark III stands in the Warmth category.
The Spark III features a well-designed hood that helps capture heat with 14 ounces of 850+ fill power down. This bag is rated to 25F.
The volume of 850 fill-power down stacks up comparably with other bags, such as the 30 degree Western Mountaineering Megalite, which features 13 ounces of similar 850+ fill, or the slightly warmer 20-degree Western Mountaineering Ultralite which includes 16 ounces of similar quality/loft down. The Spark III is perfect for cooler weather backpacking trips, where evenings might dip down close to or below freezing, or for folks who just plain sleep on the cooler side.
The Spark III tipped the scales at an impressive 1 lbs 6 oz, making it the lightest bag we tested. It did this by featuring the lightest shell fabric in our review (10), high-quality down, and a short 1/3 length zipper (seen all the way unzipped here) that was also the smallest zipper featured on a sleeping bag in our review.
At one pound six ounces, the Spark III is the second lightest bag in our review. Despite being rated to 25F, it remains on the warmer side of all bags we tested. In fact, there are not many bags of comparable warmth that are even close to the weight of the Spark III. It achieves its best in class weight in four ways. The first way the Spark III minimizes its weight is by using a 10D UL Nylon exterior fabric treated with DWR; this makes up the backbone of the bag. The only bag that weighed less was the Marmot Phase 30, which was around 4.5 ounces lighter, but not as warm (we found the Phase to be closer to a true 35-40F rated bag).
The fabric featured on the Spark III is the lightest weight nylon used on any model we tested. We found this material was exceptionally durable. It's worth noting that while sleeping bags do not take a ton of external wear and tear, you do need to be careful with sharp objects inside of the tent, or with pointy rocks if sleeping out under the stars. The Spark III uses some of the highest quality down, minimizing the amount of extra insulation needed to achieve the same warmth. The cut is one of the tightest in the review, shaving off the few additional ounces that a more spacious bag would contain. Finally, the Spark III features a 1/3 length #3 YKK zipper; both the shortest and smallest sized zipper that we tested.
The Spark III came with the best compression sack among any sleeping bag we tested.
This contender is the second most compact bag in our review and was extremely close in packed volume to the Marmot Phase 30, which is not as warm.
Despite packing in a fair amount of warmth, the Spark III achieves this via several of the same reasons we just mentioned. This bag is an incredibly low weight option; it's made up of a super light 10D shell fabric, has a 1/3 length smaller than average #3 YKK zipper, is made of high-quality down, and has a tighter cut than many of the other contenders.
The Marmot Phase and Western Mountaineering Megalite pack down nearly as small, but the Spark III packs down 15-20% smaller. When compared to average 20-30°F bags, the Spark is much smaller, packing down to half the size of the Nemo Salsa 30, or even a third the size compared to The North Face Cat's Meow and the Kelty Tuck 20.
The Spark III (second from right) was the most packable sleeping bag we tested. This was even more impressive considering it was warmer than all the other sleeping bags pictured in this photo. From left to right: the Nemo Salsa, the Sierra Designs Backcountry Bed, the Marmot Helium, the Western Mountaineering Megalite, Sea to Summit Spark III and the Mountain Hardwear Hyperlamina.
Comfort, Spaciousness, and Fit
The Spark III is not very spacious; in fact, it was the tightest bag we tested. For those used to classic cut mummy-style bags, the Spark III feels marginally smaller than most. However, for those that struggle with tighter-cut bags, they will find the Spark's 57"/50"/35" dimensions feel like they just got into the bed and the sheets are WAY too tight. If you want a light bag, but wish it had a little more space, the Marmot Phase 30 offers slightly more space, with three inches more circumference in the shoulders (overall dimensions 60"/58"/39"). While the Phase's dimensions do not appear like they would offer much of a difference, we quickly noticed them as soon as we crawled in.
The WM MegaLite, our Editors' Choice award winner, featured far more room of any contender, with 64"/56"/39" overall dimensions. As a bonus, it's only two ounces heavier. It's worth noting that the Spark III feels a little smaller in the shoulder area than most bags; however, its leg area is where most folks noticed its smaller diameter cut. The lightweight fabric feels above average against our skin, and the Spark III was among the best feeling bags in our review.
Comparing the dimensions of three of the lightest bags in our review, from left to right: Spark III, Marmot Helium, and the widest of the three, the Western Mountaineering MegaLite.
The Spark III is excellent for trips where weight and packed size are at a premium and is a fantastic option for folks looking for a lightweight bag. Overall, the Spark III isn't a super versatile option. Its one-third length zipper doesn't allow for a ton of ventilation; this means that using it on warm summer night won't prove to be as pleasant as other options. Despite an overall tighter fit, the shoulder girth is only one inch smaller in diameter than several other non-comfort oriented bags. When using the Spark III, our broad-shouldered testers found that they could at least wear a light, or medium weight puffy, allowing this bag to be used in cooler temps down to 15-20 F.
The Spark III is a poor option for car camping or shorter trips where you might just be hanging out in the same camp for multiple nights in a row, where weight and packed size are likely to be less of an issue. It's a rad bag for all weight weenies; it offers enough warmth to make it adequate for light and fast trips, long-range backpacking trips, mountaineering and alpine climbing, or even the occasional spring ski mountaineering trip.
The slightly overthought Spark III storage sack. It has two size/volume options for storing your bag. We'd recommend using the larger of the two, but for road tripping or a cramped apartment the smaller, zipped down option is a nice compromise of maintaining your bag's loft while not taking up your whole closet.
Features and Design
This contender doesn't offer a lot of extra features, with its most appealing aspects being its minimal weight and fantastic packed size (while still offering surprisingly solid warmth). While it features the thinnest shell fabric in our review (10D), it did not "leak" more down than other bags in our tests. In fact, even after a few weeks of use, it leaked less feathery insulation than other bags using thicker nylon shells.
We loved that it came with a proper compression sack that was easily the most helpful stuff sack included with any sleeping bag we reviewed. It also fits perfectly into the compression sack, allowing the user to maximize the compressibility of the Spark III without having to throw down another $30 on a lightweight aftermarket compression sack.
Showing the Spark III's 1/3 length zipper all the way open. This partial length zipper was sweet for reducing weight but made it more challenging to regulate temperature on warmer nights.
This bag is best for any trip where weight and packed size are the most important factors. During the testing phase of our sleeping bag review, the Spark became our go-to for summer alpine climbs. We even brought it with us on a mid-spring ski mountaineering traverse in Washington's North Cascades. The Spark III would be a killer option for any long-distance or thru-hiking adventure. It would also be a great bag for folks who are camping in a colder climate or for those that desire a sleeping bag that offers more warmth.
The Spark III is on the more expensive side, but if you want a 3-season bag on the warmer end of the spectrum and still want a lightweight, packable bag, this model is hard to beat.
At $419, the Sea to Summit Spark III is one of the most expensive sleeping bags in our review. Despite its steep price, it offers several advantages with its minimal packed volume and exceptionally low weight. In fact, few, if any, options on the market can match it for its weight.
If you're on the hunt for a bag that is lightweight and packs down to an impressive size, look no further. This high performer excels across the board in all metrics and is an excellent model to bring along when going fast and light. While the Spark III does sport one of the highest price tags in our review, it does offer exceptional performance, no matter the adventure.