Insulation is undoubtedly one of the biggest drivers in decisions regarding winter sleeping bags. It’s what the bag is made of, after all! And ultimately, you’ve got two options: down or synthetic.

Oex Leviathan Ev Hydrodown 900 | Sleeping Bag | George Fisher Oex Leviathan Ev Hydrodown 900 | Sleeping Bag | George Fisher

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Made from extremely light and compressible Responsible Down Standard-certified insulation that’s been treated to repel water, this bag has several distinguishing features. Snugpak’s polyester Softie insulation has been a mainstay in the British brand’s products for over thirty years. It utilizes superfine yarns bound together with special resins to mimic the properties of natural insulation such as down. The result is a really plush feeling bag that is easily warm enough for winter camping trips.

OEX Leviathan EV 900 Down Sleeping Bag | Millets

The Kelty Cosmic Down 20’s is unusual in that it uses trapezoid-shaped baffles to provide insulation. The dog approves (Image credit: Kelty) Insulation OEX list this bag as having a comfort temperature of -9 degrees Celsius, a lower limit of -16 and an extreme rating of -39. They tested the bag according to EN13537 (which is an industry standard means of testing how warm sleeping bags are), and having used the bag for over three years, we definitely have some thoughts on these numbers (which we cover below).

One clear difference when it comes to 4-season bags is that they will almost always come with hoods and baffles. Rectangular bags that don’t cover your head might be fine during the warmer parts of the year, but in the winter you really need a bag in which you can properly batten down the hatches. That’s why winter bags almost always come with hoods that can be cinched down tightly around the head. Small: 10.6in x 6.7in / 27cm x 17cm; Medium: 10.6in x 6.7in / 27cm x 17cm; Large: 10.6in x 6.7in / 27cm x 17cm