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I have a bag liner that claims to add 15 degrees to the "range" of a sleeping bag. Is it reasonable to assume that it would work the same for all bags? For instance, would it extend a 40 degree bag to 25, and a 0 degree bag to -15?

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What’s its weight? Unless it’s several hundred grams I doubt the insulation value. 15 degrees of difference is a lot. –  zoul Feb 26 '13 at 7:34
    
On non-backpacking trips I have taken to filling my sleeping bag with all the clothes I brought, coats, hats, socks, you name it. We found filling the sleeping bag with cloth is more effective than liners. (Of course, if you Are backpacking, all you'll have is a coat and some extra socks.) –  theJollySin Feb 26 '13 at 19:38
    
I second everyone that questions the magic warming power of liners (especially the thin silk-type ones). Waterproof liners (aka vapour barriers) have a place in winter camping, but beyond that, insulated liners would be the only sure-fire way I can see to increase warmth! –  Ryley Feb 27 '13 at 17:27
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2 Answers

I'd say yes, it would add roughly the same temperature boost to all bags. This is of course assuming other conditions are the same, like the surface you are on, stillness of the air, etc.

Short of the material actually changing physically (like down getting more fluffy), one piece of insulating material will have the same temperture drop accross it for the same heat power going thru it, regardless of the absolute temperature. Put another way, if something exhibits a 10° drop when you put 50 watts thru it, it will exhibit that same 10° drop whether that is from 10 to 20 degrees or 50 to 60 degrees. Again, this is assuming the material is not changed by the temperature difference, like it melts, clumps, gets more fluffy, etc.

Of course things will vary, but as a first rough approximation I'd say the degrees added "warmth" will be about the same accross different bags inside.

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I think this makes sense. Thermodynamics is the same backpacking or not. However I wish we had a way to confirm this by testing. The real question to me though is does it truly provide the indicated 15° boost. My guess is that figure is more marketing than real R-value. –  manoftheson Feb 27 '13 at 6:52
    
It seems like it would depend on some other factors. For example, if the liner is porous and the sleeping bag is porous, then the liner will have less insulation in the wind. If the line is thin, then it will have less insulation on the snow (on the bottom), at least in the case of no or little insulating pad, than it would on dry ground because of the greater temperature differential and, as you mentioned, it changes physically under body weight. –  xpda Feb 27 '13 at 7:53
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It's not reasonable to assume that it would add 15 degrees to all bags. Every bag is different, and the ground (snow? type of pad insulation?) and amount of air flow makes a difference.

Incidentally, some bag liners are waterproof and not breathable. This can cause condensation and make it feel colder inside than it would without the liner.

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I was afraid of this, which then leaves the question of how to test it without putting myself at risk ;) –  Russell Steen Feb 26 '13 at 5:37
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You can probably add more than 15 degrees by wearing a bunch of fleece to bed (as long as it's dry), in case the liner doesn't work well. –  xpda Feb 26 '13 at 5:49
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I don't think it's reasonable to trust that a liner would actually add the claimed 15° (or other number) to any sleeping bag no matter the bag's temperature rating, even though I know they do add some insulation. I'd maybe count on getting about half of what is put on the label. Liners to me are more about preserving the insulation in the bag itself so that it stays cleaner longer and thus maintains its insulating capacity longer. –  manoftheson Feb 27 '13 at 7:02
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