On technical winter trips, every bit of warmth matters, which is why the Western Mountaineering Hot Sac Vapor Barrier Liner is a must-have. Whether we're hunkered down for the night or facing unexpected conditions, this liner works overtime to stop heat loss from evaporative cooling, keeping us warmer longer. The gold reflective lining goes even further by reflecting our body heat back to us, ensuring we're not wasting energy as temperatures drop.