If pack space and weight are your primary concerns, this is your bag. The 850-fill down lofts quickly and kept our testers warm right down to the 5°F rating when they wore a puffy jacket (a common mountaineering technique that allows you to carry a lighter bag). The Sphere has a trim, efficient cut in the legs and feet with a slightly wider torso area to accommodate those extra layers.
Mammut’s 20-denier prolightTX shell is extremely breathable, which, along with venting the 3/4-length zipper, allowed testers to stay comfortable up into the mid-40s. “The shell requires standard ultralight love and care, but it effectively repelled light drizzle and snow on Mt. Hood when we slept sans rainfly and woke to unexpected precipitation,” says one tester. However, it did wet out when pressed against a damp tent wall all night. It squishes down to the size of a small watermelon with the included compression stuffsack. Extra credit for the cozy microfleece face guard and the 14-by-14-inch mesh pocket inside the torso area, which is perfect for storing gloves, liners, or damp clothes that need drying. $550; 2 lbs. 8 oz.; reg. and long; mammut.ch