This 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 vapour 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 6.4 ounces and is light enough to carry on day trips as an emergency bivy.
Western Mountaineering's VBL sleeping bag liner is a definite niche product, Vapour Barrier Liners are by definition non-breathable - that is their function afterall, use one in normal conditions and your sweat will condense on the inside of the liner and you will be soaked by the morning.
Its designed purely for very cold extended expeditions where evaporative sweat would condense into clothing layers making them wet with no prospect of getting them dry. The VBL prevents this but there is still a need to minimise sweating by adjusting insulating layers so that there is not a massive build up of moisture inside the VBL. This can only really be achieved in cold conditions.