The L. L. Bean PrimaLoft Packaway Vest gets its warmth-to-weight ratio from PrimaLoft Gold insulation built with Cross Core technology, which uses the same Aerogel developed for NASA to trap heat without adding bulk. A Pertex Quantum shell blocks wind and resists weather over a fully recycled polyester build, so the vest holds up to real cold-weather use despite weighing next to nothing. When the weather warms up, the whole vest stuffs into its own built-in pocket for the walk home or the bottom of a pack. What Our Gearhead(R) Experts Are Saying:"I stuffed this in a jacket pocket at the trailhead not expecting much from something this thin, and it kept me warmer standing around at the summit than a vest twice its thickness has any right to."