Designed for use at the edge of human tolerance (-40c), this 850 fill goose down thermal expedition bag is the warmest available from The North Face®. We've constructed this bomber bag with the highest level of available protection - 20 denier Pertex® Endurance™ waterproof, durable shell -because being wet in extreme conditions can be life-threatening. As a bonus, the compression stuffsack doubles as a lightweight out-and-back summit pack when it's time go.