This black and gold 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. Western Mountaineering's Hot Sac VBL goes one step further by stopping radiant heat loss with its reflective 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 under 4 ounces and packs down to fit into a tiny stuff sack, making it easy to pack on expeditions or bring along on day trips to use in an emergency for signaling, warmth retention or protection from the elements in bad weather.