Sea to Summit Spark III

The Sea to Summit Spark III sits a step warmer than the Spark II, with a -2 °C comfort rating, -8 °C lower limit and -26 °C extreme rating under EN 13537 testing — a genuine three-season bag that stretches into early winter on comfort alone. It weighs 665 g with 430 g of 850+ fill power, 90/10 hydrophobic goose down, and a PFAS-free Ultra-Dry treatment that helps the fill resist moisture without the cost of older DWR chemistries. Its warmth-to-weight ratio of roughly 12, close to double the Spark II's, and a weight-per-degree figure of about 83 g reflect a bag optimized hard for grams relative to warmth delivered. That comes from a box-baffle build with a 10D nylon shell and liner, while the full-length side zip with two sliders keeps ventilation flexible. At 665 g and packing to 35 × 15 cm, it suits summer alpine trips and autumn hikes to 3,500 m, but the trade-offs that keep it light — ultrathin 10D fabric, no liner reserve — make it a poor fit for winter bivouacs or cold sleepers.