Anytime you're sleeping on a bag, you're compressing the bottom, and the R value goes to crap.
That's why Big Agnes makes bags with no insulation on the bottom, but has a sleeve to fit their insulated pad in there.
That's also why a hammock is so cold, of you don't have a bottom quilt hanging just beneath the fabric on the bottom.
I have a cot for sleeping in elk camp, and it has insulation on the underside, where me sleeping on it won't compress it.
When sleeping on the ground, I have a Thermarest or Big Agnes, and a space blanket under that.
The new military three bag system (patrol bag, inside of a heavy bag, inside of a Goretex bivy) is pretty good, although very heavy, IF you have an insulated mat and a space blanket to sleep on.
Sleeping cold sucks.
