To the O.P. .... another point to consider in your sleeping system, if you are to sleep in cold weather, is the sleeping mattress.
Loft and air trapped is what isolates your body warmth form the exterior. Lying down, your body weight compresses all the loft and you will lose all your body temperature at those compression points.
The typical cell foam or finger thin ultra light air mattress will void the comfort temperature of your sleeping bag. I would suggst looking at something like the EXPED Down Mat.
I use an air mattress with an R4 rating. That's not enough when it's real cold but a closed cell pad either under or on top of it takes care of anything I've slept in.
