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If I use two identical 20-degree-rated bags, one inside the other, what would likely be the effective temperature rating? 10-degree? 0-degree? negative-10-degree?

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My GF used two sleeping bags in Yosemite this winter, and thought they were over-the-top insanely warm. So, based on that one-time experience, I believe it would be closer to a -10-degree bags. – theJollySin Dec 18 '13 at 20:36

Warning This is pure conjecture! Test this information with your gear in a safe place before using!

That being said, the basic warmth of a sleeping bag is determined by the amount of loft the fill provides. Quality of fill has a huge impact on how much loft is necessary to provide a given warmth, along with construction (i.e. baffles vs sewn through, hood quality, down distribution).

Putting all that aside, a good 20F down bag will have 4-5" of loft. So theoretically one inside the other would have 8-10". Again, theoretically that would give you ~ -10F (or perhaps even lower!).

This is assuming you don't have any down compression happening when you stuff one inside the other (i.e. you choose two bags with girths such that they don't compress each other). Since that's not likely possible, I would imagine real world performance would be quite a bit lower (maybe -5F?), and I would certainly want to test it with backups available before striking out into the wild!

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I guess two extra layers of fabric in the middle would help compress the down as you say, but would also help keeping in warmth a bit. – nsandersen Dec 14 '13 at 11:54
If the bags are really identical, as per the OP, then I suspect the inner bag will get pretty compressed and not insulated to its full potential. For a good selection of firsthand experiences layering bags/quilts, check out the Backpacking Light forums:… – ppl Dec 15 '13 at 0:55
@ppl - yes indeed, I didn't see the part about two identical bags. I've seen weird results with compressed down though - some things suggest that (in jackets at least) volume of down is almost as important as loft... so it just goes to show you actually have to test your own setup! – Ryley Dec 15 '13 at 4:23

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