Sea to Summit Spark Down Sleeping Bag

The Sea to Summit Spark Down Sleeping Bag is an ultralight mummy-style bag filled with RDS-certified 850+ fill power goose down, built for backpackers and weight-conscious campers who refuse to sacrifice a real night's sleep. A 10-denier nylon shell and liner keep the bag exceptionally light while a non-PFAS DWR finish and Ultra-Dry Down treatment protect the insulation from condensation and moisture. The Spark is available in four temperature ratings (45°F, 30°F, 15°F, and 0°F) and in Regular, Long, and Double sizes to match a range of sleepers and conditions.

850+ Fill Power Down in an Ultralight Shell

The Spark's RDS-certified 850+ fill power goose down delivers exceptional warmth relative to weight. The higher the fill power, the more loft each ounce of down produces, which means less total down is needed to reach a given temperature rating. That translates directly to a lighter bag and a smaller packed size without giving up thermal performance on cold nights. The down is treated with Sea to Summit's non-PFAS Ultra-Dry Down finish, which absorbs less moisture and dries faster than untreated down, and the treatment lasts through at least 20 washings.

Shell and Construction Built to Shed Moisture

Both the shell and liner are woven from 10-denier nylon: thin enough to shave grams and breathable enough to prevent the clammy buildup that disrupts sleep. The shell carries a non-PFAS DWR finish, and the hood and footbox use a highly water-repellent fabric to resist tent-wall condensation where it matters most. A zipper draft tube runs the length of the zipper to block cold air infiltration, and YKK #5 zippers with a zipper plow provide snag-free operation in the dark.

Baffle Construction Matched to Temperature Rating

Sea to Summit varies the internal baffle construction by temperature rating rather than using one design across the line. The 45°F version uses sewn-through baffles to minimize weight. The 30°F version uses hybrid box baffles in the torso, where warmth matters most, and sewn-through construction in the leg area. The 15°F and 0°F versions use full box baffles throughout for maximum insulation loft. Choosing the right temperature rating means the bag's construction is optimized for the conditions it's rated for, not just stuffed with extra down.

Features

• RDS-Certified 850+ Fill Power Down: goose down with non-PFAS Ultra-Dry Down treatment; lasts through 20+ washings
• 10D Nylon Shell and Liner: non-PFAS DWR on shell; highly water-repellent hood and footbox fabric
• Technical Hood and Anatomical Footbox: ergonomic mummy shape with room to move despite ultralight construction
• YKK #5 Zippers with Zipper Plow: snag-free operation; zipper draft tube retains warmth
• Temp-Specific Baffle Construction: sewn-through (45°F), hybrid box/sewn-through (30°F), full box baffles (15°F and 0°F)

Specifications

Shell: 100% nylon. Liner: 100% nylon. Down fill: 90/10 goose down. Stuff sack: 100% nylon (excluding trims). ISO temperature ratings follow the ISO 23537 protocol. ISO Lower Limit is the rating for men; ISO Comfort is the rating for women.

Who This Is For and Isn't

The Spark Down Sleeping Bag suits weight-conscious backpackers and fastpackers who want a genuine down bag in a small packed size. The four temperature ratings (45°F through 0°F) cover three-season and shoulder-season use through serious winter camping, and the Regular, Long, and Double size options accommodate a wide range of sleepers.

The Spark does not have a footbox zipper, so it cannot be opened flat for use as a quilt. The fit is cut snug for ultralight efficiency rather than for comfort-first sleeping; buyers who want more interior room should consider a roomier mummy design. The ISO Lower Limit ratings are tested for men; women should reference the ISO Comfort column when selecting a temperature rating.

Common Questions

Can the Spark be opened flat and used as a quilt?

The Spark Down Sleeping Bag does not have a footbox zipper, so it cannot be opened flat for use as a quilt. Buyers who want that convertibility should look at the Spark Pro or Ascent sleeping bags, which include a footbox zipper for that purpose.

How roomy is the Spark inside?

The Spark Down Sleeping Bag is cut for ultralight efficiency, so the interior fit is snugger than a comfort-oriented mummy bag. The shape still provides enough room to shift position during the night, but buyers who prioritize interior space over weight savings will find a better match in a roomier design.

Can two Spark bags be zipped together for couples?

Spark and Women's Spark models that have full-length zippers can be zipped together. Not all Spark variants use a full-length zipper (the 45°F Regular, for example, uses a half-length side zip), so confirm the zipper configuration of the specific variant before purchasing with this use in mind.

What do the ISO temperature ratings mean for choosing the right bag?

Sea to Summit rates the Spark Down Sleeping Bag using the ISO 23537 protocol, which produces two figures: the ISO Lower Limit (the lowest temperature at which a man should stay warm) and the ISO Comfort rating (the lowest temperature at which a woman should stay warm). Unisex bags like the Spark are listed by their Lower Limit. Both ratings appear in the specifications table above for every variant.