I have a 1.5 story, 100-year-old bungalow in Atlanta, GA with the half-story built into the attic space. The attic is insulated at the ceiling plane (blown cellulose) and knee walls (fiberglass batts + rigid foam), but it is not properly vented: there is a ridge vent but no soffit, gable, or other intake vents (a choice from a prior owner at some point).

I need to replace my asphalt roof, and likely the sheathing as well. Should I leave the ridge vent, or close it? If I leave it, how strongly should I consider adding soffit vents (there are no usable gables)? Also: I believe a vapor diffusion port would not be allowable with the insulation at the ceiling plane, but might it actually be an option? I want to do a good job handling attic moisture; my ceiling is quite airtight but I have a damp basement and modest indoor humidity due to infiltration (work in progress).

Last — in this scenario, what roofing materials choices, if any, would you prioritize to make a dent in the summer attic temp? Is there ever a place for rigid foam above the sheathing when the insulation is at the ceiling plane? I understand if might be appropriate to install sheathing with radiant barrier on the underside in this case–but would it be worth it?