I use a Big Agnes sleep system bag. No insulation on bottom. It has a sleeve that a pad slips into. I have a tapered Thermarest Pro Plus pad. Works well.

For a bit of added insulation I bought a roll of Reflectix. $16 w/shipping. I cut a piece to fit in the pad sleeve to go under the Thermarest, doesn't weigh enough to worry about:

Reflectix

I was big enough to cut to size and used the "drop" for a few pot cozies and cover for the critters travel crate.

It hasn't been cold enough in Georgia to actually test it. But the theory seems good!!