Just a straight air mattress insulates less than a foam one. I use a Big Agnes sleeping bag with the Air Core pad (just a small air mattress w/o foam) till about 32ºF then add a 1/4" foam pad underneath it till about 10ºF. That's about as cold as this bag will go.
1 pointer is right, you need an insulating material of some kind between you and the groung. I use a 1 1/2" self infalting pad w/a closed cell pad under that. Nothing worse that a cold miserable night's sleep.