Big Agnes Pluton UL 40°

The Pluton UL 40° is a warm-weather mummy bag built around a small fill charge — 227 g of 850-fill-power down — to hit a 454 g total weight. Big Agnes rates comfort at 4°C with a manufacturer-tested limit around -2°C, which places it firmly as a summer and high-summer alpine bag rather than a three-season all-rounder. The down carries a hydrophobic DownTek treatment, and both shell and liner use 100% recycled Pertex Quantum ripstop, a combination aimed at keeping loft intact if the bag gets damp overnight. Big Agnes skips the draft collar and draft tube found on colder bags to save weight, relying instead on a simple drawcord hood and a full-length two-way zipper that can vent from either end. The whole thing is bluesign-certified and PFC-free. For thru-hikers chasing a minimal base weight on summer sections, or anyone building an ultralight kit for warm nights, the trade-off is straightforward: excellent warmth-to-weight for the conditions it's rated for, and very little margin if temperatures drop below the stated range.