Therm-a-Rest Hyperion™ 20F/-6C Sleeping Bag

The Hyperion 20F/-6C packs 900-fill hydrophobic goose down (330 g of it) into a total bag weight of 567 g, tested under EN/ISO 23537 to a comfort rating of 0 °C and a limit of -7 °C, with an extreme rating down to -24 °C. That fill power is high enough that the down does most of the insulating work with little mass, which is why a bag rated well below freezing still comes in under 600 g. Insulation is zoned rather than uniform: roughly 70% sits on top of the sleeper, 30% underneath, since a sleeping pad already handles most of the ground-side insulation. A half-length left-side zipper follows the same logic, cutting weight versus a full-length version while still allowing venting. The 10D nylon ripstop shell and liner keep fabric weight minimal, backed by a DWR finish, draft collar, and draft tube against heat loss. At a 57 cm shoulder width it's a standard mummy cut rather than a roomy one — a reasonable trade for thru-hikers prioritizing low pack weight over sleeping-position freedom.