Need to divide a room if it's half over a porch deck?

[GeneDavis]

I've an upstairs room that is half over a porch and the porch needs a lower ceiling than the adjacent inside room on floor one.  Do I need to divide the upstairs room so as to have a ceiling below with the air gap layer I need to lower the ceiling?  I already divided the porch deck into that which is under the floor of the room above and that which has only roof over.