>>2930601
Most heat is lost at holes/drafts.
Then if properly sealed most heat is lost at the roof level.since warm air rises.
Then windows.
Then walls.
Then floor.

You will need to do some investigation to understand what the house is made of. An "older house" means a lot of different things depending on region. Since you mentioned AC, I'm guessing you live in America, which means the house is probably stick frame.

Is the house one, or more levels?

Check what the insulation is like at the ceiling, walls, and floor and we will better be able to offer advice.