What Our Gearhead(R) Experts Are Saying: "The Spark 0F blows me away with how warm it is for the weight. The box-baffle construction and 850+ down loft give it legit winter performance without the bulk. The full-length zipper makes it far more livable than most ultralight winter bags, and the hood and footbox shed condensation incredibly well." We rely on the Sea to Summit Spark 0F when the trail leads into true cold and every ounce of warmth matters. Packed with lofty 850+ Ultra-Dry Down and wrapped in a 10D shell treated to repel condensation, this bag's construction is tuned for high-efficiency warmth in freezing climates. Full box baffles prevent cold spots, while a technical hood and draft tube seal in heat to create a cocoon of comfort through long alpine nights. Despite its winter rating, the Spark maintains the ultralight pedigree the series is known for. The breathable 10D liner feels soft against the skin, and the full-length zipper allows for easier temperature modulation when conditions shift. Highly compressible and surprisingly packable for a 0degF bag, it makes cold-weather missions more streamlined-and far more comfortable. For winter backpackers aiming to move fast without sacrificing warmth, the Spark 0F is a powerful ally.