The Western Mountaineering Hotsac Vapor Barrier Liner is your most important companion for those technical winter trips. The liner works by stopping heat loss from evaporative cooling, and it also prevents your body vapor from degrading your insulation. This is most important during extended stays inside your bag. This Hotsac VBL goes one step further by stopping radiant heat loss with its reflective metal coating, while an elastic drawcord at the top of the Hotsac cinches around your neck to prevent convective heat loss (you're welcome for the thermodynamics lesson). Plus, the new Hotsac weighs only 4. 5 ounces and is light enough to carry on day trips as an emergency bivy.