Converting Porch to Room

I am not a builder and I am trying to help my husband figure out the best way to do something for our renters.

I have a small cabin on my property with an 8x32 porch across the front. The roof over the porch is metal and is fastened directly to the joists. Of course this poses a condensation issue as we are in Alabama. We are converting about 12 feet of the porch into a bedroom, but aren't exactly sure the best way to insulate/vapor barrier the roof without taking it off. The roof is sloped and we are planning to make the ceiling on the room slope with it. Do you guys have any suggestions or thoughts on the best way to approach this from a moisture perspective. We do not want to run into an issue down the road. Thanks so much!

My recommendation would be to install the roof over plywood decking and if desired, spray the underside of the roof deck with closed cell urethane foam.