It's sibling, the Kelty Cosmic Down 20, wins the Best Buy award in our Backpacking Sleeping Bag Review. The Cosmic 20 will be the best choice for anyone that doesn't mind another pound for much more warmth and comfort.
This is not the warmest bag. But, as mentioned above and below, it can be warm enough depending on how many clothes you wear. Most high-end sleeping bags, once you give them a few minutes out of their stuff sacks, inflate like the Michelin Man. Not the Cosmic 40. As you can see in the photo below, it never really puffs up and always looks a little sad and deflated. But it works!
This bag was revised in 2016/2017 to go from 550 fill to 600 fill DriDown. It also gets a neck baffle (shown in red below). Overall, we were not thrilled with the changes. They made the bag almost four ounces heavier without necessarily adding any more warmth. Yes, the other shell might be more durable, and the zipper won't snag as much at the newly reinforced point, but we would have preferred they move in the other direction and make the bag even lighter. Oh well, it's still one of the lightest bags for the money.
At about two pounds, this bag is relatively lightweight for its price but not particularly noteworthy compared to the most expensive bags in the test, which averaged 1.6 lbs. The other comparable bag in our review was the Nemo Salsa 30. It was warmer, more comfortable and about the same weight, but cost $220. This highlights the value of the Cosmic 40: it's not the best bag, but getting a better bag at the same weight will cost you.
This bag packs down about as small as any in our tests. The photo below shows it in its provided stuff sack, but it will pack down much smaller in a compression bag.
Cosmic Down vs. $400 bags
We first used this bag when OutdoorGearLab co-founder Chris McNamara went on a five-day cross-country High Sierra backpacking trip. Chris had access to many of the $300-500 top scorers from the ultralight bag review, including the top-rated Katabatic Gear Palisade. But the shipping costs of the Palisade across the country from the category editor, combined with the hassle, were about equal to the value of just buying the Cosmic Down 40 (on sale for $80 at Amazon). Also, Chris was interested to see if such an inexpensive bag could compete against the super-expensive custom bags. He had also tried the hoodless quilt style and wasn't sold on giving up a zipper and hood.
On the trip, Chris's friends had the two highest-rated bags, the Palisade and the ZPacks Classic. So how did they all compare? Both the Palisade and ZPacks were much warmer and 10 ounces lighter. However, the Cosmic 40 when worn with all of Chris's clothing was warm enough down to 30 degrees. The verdict: Chris's friends, frequent High Sierra ultralight backpackers, were very happy with their bags. And Chris, a once-a-year-at-most ultralight backpacker, got through just fine and saved $300!