The Hot Sac VBL 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 rip stop The new Hot Sac weighs only 4.5 ounces and is light enough to carry on day trips as an emergency bivy. Fits Up to 6'6".