Why It's CoolWhen considering an envelope rated to 40 degrees or higher, make sure it's light. The Butterfly weighs in at a pound and change. It shaves ounces by using a quarter-length zip, 700-fill down, and moisture-repelling Pertex Quantum fabric for the shell and liner. » An interior storage pocket doubles as the stuffsack. Although I couldn't figure out what I'd want to stash there (contacts?), it did guarantee that I'd never misplace the stuffsack.Hmmm . . .The bag purports to be "extreme," rated to 28 degrees, but anything past 40 degrees stretches its range.