Sea to Summit Spark II Review

Overview

The Spark II, also sold as the Spark SP II, is Sea to Summit’s three-season minimalist: a 10D-shelled mummy with 280 g of 850+ fill-power goose down, a technical hood and a zipper that stops a third of the way down. It was built for riders, racers and weight-obsessed backpackers who want a bag that disappears into a pack. The trade-off is a snug cut and a thin margin on cold nights. It’s now a legacy product, replaced by the Spark Down Sleeping Bag (30°F).

Key Specs

Spec	Detail
Weight	464 g (Regular) to 510 g (Long)
Fill	850+ fill-power goose down, 90/10, Ultra-Dry Down treatment
Fill weight	280 g (Regular), 305 g (Long)
Temperature rating	35°F / 2°C lower limit; 44°F / 7°C comfort
Packed volume	2.9 L
Shell	10D nylon with DWR
Liner	15D nylon
Zipper	1/3-length YKK #3
Baffles	Sew-through transverse
Fit	Regular for users under 6’0”; Long for users under 6’7”
Girth (chest / hip / foot)	Regular 57 / 50 / 35 in; Long 60 / 52 / 37 in

Performance

Weight and packability. This is where the Spark II earns its keep. At 464 g in Regular, it compresses to 2.9 liters, roughly the size of a one-liter Nalgene by one reviewer’s comparison. Its successor, the Spark 30°F, weighs 493 g in Regular and compresses to 4.5 liters, so the older bag is about 29 g lighter and takes up a third less room. Few 35°F bags matched that when it was current. A field tester on a New Zealand backcountry fly-fishing trip called the pack size minute and the warmth surprising for the weight.

Warmth. The rating needs careful reading. The 35°F figure is a lower limit, the point at which a sleeper is expected to be cold but not in danger. The comfort rating is 44°F. Testers back that up. ADV Pulse found the bag comfortable to about 45°F and said that below that, a liner becomes necessary. A mountain biker who bought it at the 35°F number, camping at 9,000 to 12,000 feet with overnight lows routinely below freezing, said he underestimated the cold. He would take the roughly 6 oz penalty of the warmer SP III instead. The Spark II suits summer and shoulder-season nights above about 40°F, not freezing alpine camps.

Fit and comfort. The cut is narrow. Reviewers describe it as not very spacious, and ADV Pulse cited the tight fit alongside the price as the main reasons some buyers will pass. The 15D liner feels soft against the skin, though ADV Pulse found it thinner than the typical lightweight bag. The hood has an elastic cinch. One bikepacking tester often left it slightly open and wore a wool hat on cooler nights.

Construction and down management. The sew-through baffles save weight but let down migrate. ExpediTom reported clumping around the head area that left cold spots with no insulation. Owners on retail sites also mention occasional zipper trouble. The short zipper saves grams and limits ventilation, so the bag is less forgiving when a night turns warm. Owners have also had success restoring loft by tumbling the bag in a dryer and shaking the down back through the baffles.

Weather resistance. The 10D shell is DWR-treated and the down carries a hydrophobic treatment. One tester found the shell keeps water out if it’s shaken off quickly, but it isn’t waterproof. Think condensation and brief splashes, not rain.

Choosing a Version

The Spark II comes in two lengths. Regular fits users up to about 6’0” and weighs 464 g with 280 g of fill. Long fits users up to about 6’7” and weighs 510 g with 305 g of fill. Both compress to 2.9 liters, so the extra 46 g buys more length and slightly more room at the chest (60 inches against 57), hip (52 against 50) and foot (37 against 35). Anyone near the top of Regular’s height range should take the Long, because a bag that’s too short compresses the down at the foot and loses warmth where it matters. Everyone else should stay with Regular and keep the weight savings.

Pros & Cons

Pros

• 464 g (Regular) and 2.9 L is a very small package for a 850+ goose down bag
• Treated down and DWR shell handle condensation and light moisture
• Soft liner with a technical hood and anatomic footbox
• Lifetime guarantee on the brand’s current Spark line

Cons

• Comfort rating is 44°F, nine degrees above the headline 35°F
• Narrow cut that restricts side sleepers and larger frames
• Sew-through baffles allow down to shift, especially around the head
• Short zipper gives little ventilation on warm nights
• Premium price when new ($359 Regular, $379 Long), and it is discontinued

Who Should Buy This

Bikepackers, fastpackers and summer thru-hikers who sleep above 45°F and prioritize pack volume over room are the match. It also suits anyone who runs a down jacket and hat to extend the range. Cold sleepers, side sleepers, and anyone camping near or below freezing should look at the current Spark 30°F or a bag with box baffles and a roomier cut.

Verdict

The Spark II delivered a compact, light, soft-feeling bag for mild nights, and it does that job well. Its narrow cut, comfort rating of 44°F and migrating down keep it from being an all-rounder. Buy it secondhand or on clearance for warm-weather use, and buy the Spark 30°F if the trip might dip toward freezing.