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.
That's the whole idea behind backpacking quilts. Since the bottom insulation is worthless, they leave it off along with the zipper and bottom of the bag. You get the same warmth but without the weight and bulk. A good 0 degree quilt will weigh less than 3lb.
