Best Sleeping Bag For Extreme Weather: REI’s -20 degree Expedition bag is a true four-season bed. A double zipper system allows you stay comfortable in any temperature; unzip the inner zipper to reveal a little extra space and dump heat, or keep both zippers closed when you’re high above the treeline. Water-repellent down and face fabric, as well as an independent hood and collar closure, firmly plant the Expedition as a powerhouse for heat retention. It might be overkill this summer, but if you’re heading anywhere high altitude, you’ll thank us while your partners shiver all night.
The lightest ultralight sleeping bag we tested in this review is the Feathered Friends Vireo at 13.9 ounces. This "half bag" eliminates all heavy features, and is designed to be paired with a warm down parka for light and fast alpine trips. The Hammock Gear Burrow 40 and Mountain Hardwear MTN Speed are the next two lightest models we tested. Although we have concerns about its durability, the 16-ounce Mountain Speed is the lightest hooded mummy bag we evaluated. The two bags that earned the highest overall scores in the review, the Katabatic Gear Palisade and Zpack's 20 Degree bag, deliver the most warmth relative to weight. At 19 and 17 ounces respectively, they are impressive ultralight bags.
So on one hand, this makes it sound as if a 0 degree bag would be perfect for me. On the other hand, I've frequently heard (including from the guy at REI today) that you should always buy a bag that's rated for 10-15 degrees colder than the temps you're planning on using it for, AND I'm a cold sleeper who doesn't sleep well on his back and would absolutely not mind paying another $100-$150 to have the ability to sleep in a partially-zipped bag in his thermals only in 10-15 degree weather. So now I'm thinking that maybe getting a -20 bag isn't as ridiculous as it seemed to me at first. On the other hand, I don't want the bag to be so hot that sweat into it when it's 20 degrees out and then am soaked for the rest of the night.
1) Will a 0 degree bag be only a little warmer than a 15 degree bag, that it's not worth paying another $300 just to basically tread water in terms of my ability to be comfortable during rare high-altitude/high-wind bivies?