Hello – I am seeking input on insulating for a project.  I am converting an existing screened porch to a sunroom and have been searching for tips.  I am in Eastern NC (hot and humid summers), and the floor of the porch/sunroom is 4 feet above ground.  I plan to lay roofing fabric on top of the existing boards (in good shape), then concrete backer board and then tile for the interior floor.  I will have a mini-split system to heat/cool the interior as well.  Below the floor is where I have questions. 

I plan to put 8″ R30 batts between existing joists (unfaced).  However, I thought I would also use spray foam to seal the inside perimeter of the frame, screw in rigid foam board, tape/seal joints.  The exterior of the frame will have cement board siding (and sealed underneath).  

I have doubts about using the foam board.  I have read and seen some videos that recommend it to keep the joists dry and insulated, and I have seen others that say absolutely not and to use plywood or just a wire screen underneath.  I don’t want to trap any moisture that could leak into that space and have mold issues over time.  

I appreciate any and all input here.  Thank you.  🙂