Down sleeping bag: read the label before you trust the number
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Quick answer
A down sleeping bag's warmth comes from fill power (how lofty the down is) times fill weight (how much is inside), and its temperature name is often the ISO lower limit, not the comfort rating. Sea to Summit's Spark Pro 30°F uses 900+ fill power and 310 g of down; its Circuit 30°F uses 650+ fill and 411 g. Both list a 30°F lower limit, but comfort is 36 to 37°F. Kelty's Cosmic Down 20 uses 550 fill. Wash with down soap, never lift it soaking wet, and dry it completely.
Fill power and fill weight work together
Fill power describes how much the down lofts; higher numbers trap more warmth per gram. Fill weight is how many grams of down are inside. A bag with higher fill power needs less down for the same warmth, which makes it lighter and smaller packed.
Sea to Summit's two 30°F bags show the trade:
| 30°F Regular | Circuit | Spark Pro |
|---|---|---|
| Down | 650+ fill power duck, 80/20 | 900+ fill power goose, 90/10 |
| Fill weight | 411 g (14.5 oz) | 310 g (10.9 oz) |
| Total weight | 820 g (28.9 oz) | 619 g (21.8 oz) |
| ISO lower limit | 30°F | 30°F |
| ISO comfort | 36°F | 37°F |
| Shape | Semi-tapered rectangle, room to side sleep | Mummy with technical hood |
The Spark Pro reaches the same lower limit with about 25% less down (101 g less) and weighs about 7.1 oz less in total. The Circuit's wider cut and heavier fabrics make it the roomier camp and travel bag.
Comfort vs lower limit: the number that matters
Sea to Summit labels both its ISO temperatures. The lower limit is the colder figure in the name of the bag; the comfort rating is warmer. On the Circuit and Spark Pro, the 30°F bags have comfort ratings of 36 and 37°F, and the 15°F versions list 23 and 27°F comfort.
For planning, treat the comfort rating as your guide and expect to need extra layers or a liner as nights approach the lower limit. Kelty lists the Cosmic Down 20 at 20°F (-7°C) without saying on the product page which value that is, so compare it carefully against bags that list both numbers.
Plan an insulated sleeping pad as part of the system rather than relying on the bag alone.
Three down bags compared
| Spec | Kelty Cosmic Down 20 (Regular) | Sea to Summit Circuit 15°F (Regular) | Sea to Summit Spark Pro 15°F (Regular) |
|---|---|---|---|
| Fill | 550 fill down, 464 g | 650+ duck, 715 g | 900+ goose, 480 g |
| Rating | 20°F (type not stated) | 16°F lower / 23°F comfort | 16°F lower / 27°F comfort |
| Weight | 2 lb 7 oz | 1,160 g (40.9 oz) | 784 g (27.7 oz) |
| Fits up to | 6 ft 0 in | 6 ft 1 in | About 6 ft 1 in |
| Notable | Trapezoidal baffles, stash pocket, semi-rectangular | Opens flat as a quilt, storage cell included | Zips to a Women's Spark |
All three list recycled or PFAS-free fabric treatments, and Sea to Summit lists RDS-certified down on both of its bags. The Spark Pro 15°F is 376 g (13.3 oz) lighter than the Circuit 15°F, but the Circuit is warmer at its comfort rating (23 vs 27°F).
Wash it the right way
Sea to Summit says quality down can be washed without harm; problems come from common mistakes. Its guide:
- Use a down-specific wash, not laundry detergent, whose fillers and brighteners keep down from lofting.
- Skip top-loading machines, which can shift down between chambers. Use a front loader on a delicate or wool cycle no hotter than 40°C (100°F), with an extra rinse and a long spin, or hand wash.
- Zip all zippers before washing.
- Never pick up the bag while it is full of water; the weight can tear the internal baffles.
- Dry on low heat or a delicate setting, with dryer balls rather than tennis balls, and no dryer sheets. Keep going until no clumps remain; a damp bag can grow mold or mildew.
- Air it out before putting it away.
A liner cuts how often you need to wash the bag.
Down in an emergency kit
- Store it loose. Keep a down bag in its large storage sack (the Circuit includes a storage cell) and save the compression sack for trips.
- Keep it dry. Both Sea to Summit bags use a water-resistant Ultra-Dry down treatment and DWR fabrics, which resist moisture but do not make a bag waterproof; Sea to Summit warns a damp bag in storage can grow mold or mildew.
- Pick the rating for your coldest likely night without heat at home, and use the comfort rating as your guide.
- Plan the pad and layers as part of the system, not afterthoughts.
What to look for when you buy
Ultralight example: Sea to Summit Spark Pro
Best for: Backpacking and go-bags where weight and pack size matter most
Sea to Summit lists RDS 900+ fill goose down, 619 g for the 30°F Regular (30°F lower, 37°F comfort) and 784 g for the 15°F version.
Watch out: Close-fitting mummy cut with 10D fabrics; handle with care around sharp gear.
See options on Amazon (opens Amazon in a new tab)Roomy example: Sea to Summit Circuit
Best for: Side sleepers, car camping, travel and home emergency use
Sea to Summit lists a semi-tapered rectangular shape, 650+ fill RDS duck down, a zip that opens flat as a quilt and a storage cell; 15°F version has a 23°F comfort rating.
Watch out: Heavier: 1,160 g for the 15°F Regular.
See options on Amazon (opens Amazon in a new tab)Classic three-season example: Kelty Cosmic Down 20
Best for: A first down bag for spring-to-fall camping
Kelty lists 550 fill down (464 g in the Regular), trapezoidal baffles, an internal stash pocket and recycled fabrics with PFAS-free DWR, at 2 lb 7 oz.
Watch out: The 20°F figure is not labeled as comfort or lower limit on Kelty's page.
See options on Amazon (opens Amazon in a new tab)Questions people ask
- What fill power is good for a down sleeping bag?
- Higher fill power gives the same warmth with less down. Sea to Summit's 900+ Spark Pro uses 310 g of fill for a 30°F lower limit; its 650+ Circuit needs 411 g.
- Is a 30 degree sleeping bag good for 30 degrees?
- Often only at the edge. Sea to Summit's 30°F bags are rated at their ISO lower limit; their comfort ratings are 36 to 37°F.
- Can you wash a down sleeping bag?
- Yes. Sea to Summit says use a down wash in a front loader on a gentle cycle or by hand, never lift it soaking wet, and dry it on low until no clumps remain.
- How should I store a down sleeping bag?
- Only when fully dry, aired out and loose in its large storage sack; keep the compression sack for trips. Sea to Summit warns that storing a damp bag can cause mold or mildew.
Sources
- Circuit Down sleeping bag: specs by rating and size, Sea to Summit
- Spark Pro Down sleeping bag: specs by rating and size, Sea to Summit
- Cosmic Down 20 sleeping bag: features and specs, Kelty
- How to wash a down or synthetic sleeping bag, Sea to Summit