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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
    
While you may be asking out of curiosity or may intend to do this in a scenario where weight isn't an issue, it's much better in terms of weight to just buy one sleeping bag for that intended purpose. It also leaves no question as the rating on the bag is "what you see is what you get" and doesn't leave you guessing. If I were to go out into the wilderness I'd want one sleeping bag that is rated to the lowest possible expected tempurature. – tsturzl Mar 2 at 21:04
    
That said something I do, and noticed many others doing: I have a fleece sleeping bag that is good for maybe 50F, or really warm nights. I sometimes use this fleece bag as a liner to add warmth. I've been able to sleep in 10F cooler nights than the comfort rating on my sleeping bag using the fleece bag, but that's subject to my actual testing, not the combined ratings. – tsturzl Mar 2 at 21:07

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: backpackinglight.com/cgi-bin/backpackinglight/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

In terms of clothing insulation there is the concept of the CLO. The key equation is

T = (31 − 0.155·P·R)°C

where P is 48 W/m² while sleeping, R is the number of CLOs, and T is the temperature in Celsius.

To stay warm while sleeping at 20F you need 5 CLOs (yes there is huge variation between people). If you double the bags and ignore the air gap between the bags (which will add insulation) and any compression of the insulation (which will reduce insulation), then you can just add the CLOs (just like you do with clothing). A bag with 10 CLOs will keep you "warm" while sleeping to -46.12 F.

One should obviously be careful before heading off into -46F sleeping conditions. There can be big variations depending on how warm you sleep (i.e., the P value in the equation) and if you are using a tent or snow cave. There is also the concern of the bags compressing each other and reducing the insulation properties.

While -46F seems crazy, if we compare weights, the North Face Inferno 15F weighs 1049 g with 550 g of fill while the -40F Inferno weighs 1758 g with 1249 g of identical fill. In other words a -40 degree bag has similar amounts of fill as two 15 degree bags.

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-46F seems like a steep example. Most mountaineering bags only go down to 0F. – tsturzl Mar 2 at 21:11
    
@tsturzl and two 20 degree bags would way a ton. I added an example with fill weights of a 15 degree bag and a -40 degree bag. Hope it helps. – StrongBad Mar 2 at 22:42

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