Sea to Summit
Sea to Summit Spark 15
No citable owner-review average published for this one, so no stars.
Sea to Summit is one of the few brands that prints both ISO numbers next to each other, and the Spark 15 is the clearest illustration on this site of why that matters: the bag is sold as a 15, the lab limit is 16 °F, and the lab comfort figure is 28 °F. Twenty-eight is the number a cold sleeper should plan around, and it is thirteen degrees off the name. Everything else about the bag is excellent — 850-fill goose down, a 10-denier shell, a half-length zip to save weight, and 25.7 ounces total with nearly 17 ounces of that being down, which is one of the best fill-to-total ratios in the catalogue.
Label against lab
Sold as a 15 °F bag; its ISO 23537 comfort rating is 28 °F, which is 13 °F warmer than the name suggests. Plan around the comfort figure, not the label. Why the gap exists
At that temperature the pairing table wants at least an R 4 pad underneath it.
Size run
One product, 2 sizes. The headline numbers on this page describe the Regular.
| Size | Weight | Approx. price | Length | Shoulder girth |
|---|---|---|---|---|
| Regular | 1 lb 9.7 oz | ~$549 | — | — |
| Long | 1 lb 11.7 oz | ~$579 | — | — |
Specifications
Highlights
Features
- ISO or EN rated
- Hydrophobic down
- RDS down
- Draft collar
- Half-length zipper
What it does well
- Publishes the ISO comfort figure (28 °F) as well as the limit (16 °F), which most of this catalogue does not.
- 16.9 ounces of 850-fill down inside a 25.7-ounce bag — about two-thirds of the total weight is insulation.
- The half-length zip saves real weight over a full zip and still vents enough for a warm night.
- 6.8 litres compressed is small for a hooded 15-degree bag.
- Non-PFAS Ultra-Dry down treatment, RDS-certified.
What it costs you
- $549 is at the top of the mainstream price band, and the cottage makers do 20-degree quilts for less.
- The half zip cannot be opened into a blanket and makes getting in on a cold night more of a wriggle.
- 10-denier shell is delicate; this is not a bag to use as a groundsheet or a couch.
- No shoulder girth published, so the fit is hard to judge without trying it.
Those two lists are drawn from Sea to Summit’s published specifications and from reviews that measured them. They are not owner sentiment — anything owners said appears under its own heading below.
What owners say
No owner discussion has been collected for the Sea to Summit Spark 15 yet. The pros and cons above come from Sea to Summit’s published specifications and from reviews that measured them — nothing on this page claims owners said anything. How that works.
Sea to Summit Spark 15, head to head
Others in the same band
More from Sea to Summit →
Therm-a-Rest
Therm-a-Rest Parsec 20
The Parsec is the lightest hooded mummy Therm-a-Rest makes, and one of the more honestly named bags here — it is called a 20 and it tests to a 29 °F comfort figure and an 18 °F limit, a nine-degree gap that only the Marmot Trestles Elite Eco beats among the 20-degree bags in this catalogue. Seventeen ounces of 900-fill duck down inside a 20-denier recycled shell gets the whole bag to 29 ounces. The zoned baffles put more down on top and less underneath, on the correct theory that the down you lie on is doing nothing, and the shoulder girth is a generous 62 inches for a bag this light.
Best for: Three-season backpackers who want a hooded bag near quilt weight
Full spec sheet
Feathered Friends
Feathered Friends Swallow YF 20
Feathered Friends sew every bag in Seattle and publish girth at four points — shoulder, elbow, hip and footbox — which tells you more about whether a bag will fit than any temperature rating can. The Swallow is their three-season workhorse: 900-fill-power Muscovy down, 16.8 ounces of it, in a 59-inch-shoulder mummy that weighs 29.2 ounces. Like Western Mountaineering they publish no ISO figure, and like Western Mountaineering they have a forty-year record that people buy on instead.
Best for: Three-season backpacking where fit matters as much as warmth
Full spec sheet
REI Co-op
REI Co-op Magma 15
REI revised the Magma 15 for 2026, and the new bag is a different proposition from the one most reviews describe: 850-fill goose down, 15.9 ounces of it, in a Pertex 15-denier shell, for 28.2 ounces total — about six ounces lighter than the previous version and $10 cheaper at $389. The ISO figures moved with it: a 28 °F comfort rating and a 16 °F lower limit, where the old bag tested to 21 and 9, and REI now prints 16 °F as the temperature rating even though the name still says 15. Read it as a lighter, cooler bag than its reputation: the limit sits within a degree of the name, but the comfort figure a cold sleeper should plan around is 28.
Best for: Backpackers who want a lab-rated 850-fill bag under $400 from a shop that takes returns
Full spec sheet