I live in Southern Ohio and don't need an underquilt from June through the end of August.
I don't use a sleeping pad either. I use a 40 degree sleeping bag and leave it mostly unzipped.
You can use an inflatable sleeping pad just don't blow it up all the way. Maybe halfway and you'd be good to go. Just slide it in your bag and you should be good to go.
Chasing the sun,
Turtle