I’m in CZ 3 in southern California.  Retrofitting a flat roof (currently open top and bottom) that is board sheathed with balloon framed walls.  

I’m planning on adding two 1″ layers of polyiso on top of the board sheathing followed by 1/2″ plywood and either modified bitumen or TPO. 

The parapet wall has detailing that I don’t want to remove on the outside.  I know I need good air sealing so I have a few questions. 

1) Articles I’ve read here recommend a vapor barrier under the insulation.  Mine needs to be an air barrier as well.  A peel and stick would work but then I have no drying potential to the inside.  Any vapor retarders that would fit the bill?   

2) In regards to the poly-iso, if I don’t use a foil backed that should help drying to the inside but may make taping more difficult.  Any thoughts? 

3) I’ve considered closed cell spray foam to seal the parapet and potentially the whole ceiling but there are few contractors in this area that do SPF.   

4)  I’m guessing that I need to open the back side of the parapet wall and seal from above with either a peel and stick or cut-cobble poly-iso (or both?).    

5) Would you insulate the back side of the parapet wall with poly-iso?  If I did would that decrease my need to block and air seal inside of the parapet wall? 

This question builds off of a prior one I asked: 
https://www.greenbuildingadvisor.com/question/parapet-wall-air-sealing-detail

Thanks in advance for your comments.  This site has been an incredible resource.  

Matt