The lightest, most breathable option in Arc'Teryx's Insulation Collection, the Proton SL Hoody takes the guesswork out of warming up on cold climbs and other high-output activities in cool conditions. The knit insulation holds its loft and shape—no matter how often you stuff it into the bottom of your pack. More importantly, it allows your body to breathe, so you’re not left a sweaty mess after you have to step on the gas to get through the crux.