I suppose this was what you were talking about?
I decided to take this combination and post it for future reference for other in the article section here.
This came from "Peter_pan".
A lot depends on how well they go together... if one restricts the other from full loft then you won't get a lot of gain...
First estimate, take the bag rating subtract it from 70, divide that number by two and then subtract it from the first bag rate for a combined value ex
Two 40 degree bags... 70-40 =30 divided by 2 =15 subtracted from other bag 40-15=25 for a first cut estimate of the combined bags... it may actually be better but probably not worse, so this works as a good conservative estimator, IMHO.
Fixed his English errors and crunked this out, sciency, ain't it?
Pan's Insulatory Factor:So in this case: 15 - (70 - 40)/ 2 = 0
x - (70 - y)/2 = z
x = first bag
y = second bag
z = rating of doubled bags
So according to this theory (which seems about right to me) doubled up you've got a 0* system