This fire engine red vapour 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 vapour from degrading your insulation. This is most important during extended stays inside your bag. Western Mountaineering's 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.