This fire engine red vapor barrier liner is your most important companion for those technical winter trips. A VBL works by stopping heat loss from evaporative cooling. A VBL will also prevent your body vapor from degrading your insulation. This is most important during extended stays inside your bag. Our Hot Sac™ VBL goes one step further by stopping radiant heat loss with its reflective metal coating. An elastic draw cord at the top of the Hot Sac™ cinches about your neck to prevent convective heat loss. This proprietary fabric is a new lightweight ripstop. The new Hot Sac™ weighs only 4.5 ounces and is light enough to carry on day trips as an emergency bivy.