Big Agnes Cinnabar -20°

The Big Agnes Cinnabar -20° is an expedition mummy bag for genuinely extreme cold: rated to -29°C and classed as 5-season/expedition gear, it belongs on high-altitude and polar trips rather than ordinary winter backpacking. Insulation is 850-fill-power down with a hydrophobic treatment that slows the loss of loft when moisture gets in — a critical property at these temperatures, since wet down is a genuine safety concern, not just a comfort issue. The 1148 g of down fill inside a 1816 g total weight shows how steeply bag weight climbs as the temperature rating drops. Vertical baffles (Insotect Flow) keep the down from migrating, maintaining even insulation along the bag's length and cutting down on cold spots cheaper constructions are prone to. The shell is water-repellent ripstop nylon paired with a polyester taffeta liner, backed by a draft collar and draft tube. At 51×28 cm packed, it's relatively compact for an expedition-class bag, though it still claims a significant share of pack volume. Materials are bluesign-certified and PFC-free, sized for users up to 182 cm.