The HotSac™ vapor barrier liner (VBL) is your most important companion for technical winter trips. A VBL works by stopping heat loss from evaporative cooling.  Vapor barrier liners also prevent your body vapor from degrading your insulation. This is most important during extended stays inside your bag, particularly if it has a laminated outer shell fabric. Our HotSac™ VBL goes one step further by stopping radiant heat loss with its reflective gold backing. An elastic draw cord at the top of the HotSac™ cinches about your neck to prevent convective heat loss. This proprietary fabric was specifically developed as a two layer lightweight ripstop with a reflective ultra-thin film designed to block moisture vapor transfer. The HotSac™ weighs only 3.5 ounces (101 grams), so light and compact it can be carried in a day pack for use as an emergency bivouac sack.