The Marmot Col -20° is an expedition mummy bag rated to -20°F (-29°C), built around 1063 g of 800 fill-power hydrophobic goose down in a box-baffle construction that resists cold spots at high loft. RDS-certified, with down processed in the USA, it targets extreme winter and expedition use where a wide safety margin matters more than shaving grams.
At 2020 g with a fill efficiency of about 2.73 g of down per gram of warmth, the bag is reasonably efficient for its class, though still firmly expedition-weight. The shell is a two-layer 30-denier Pertex Shield membrane (2.3 oz/yd²) over a 30D Pertex Microlight liner (1.3 oz/yd²), giving real protection against condensation and blowing snow. A 3D anatomical hood, draft collar and draft tube along a center zipper cut heat loss at the seams, and an internal length of 211 cm leaves more room than Regular's usual fit.
Packed volume runs to 15 liters (41 x 19 cm), worth factoring into pack sizing, and it ships with a stuff sack and storage sack - gear built for serious cold.