Install over radiant barrier

I currently have an existing wood shingle roof with a cathedral ceiling. The shingles are installed from what I can tell over 1/2" fesco board over 1x2's over building paper over 1x t&g decking which spans between wood ceiling joists. A roofing contractor I am looking at using has proposed taking off the wood singles only down to the fesco board, leaving the fesco board in place and building on top of that with: metal shingles over building paper over radiant barrier over 2x2's infilled with 1" of rigid insulation, leaving a 1/2" air space (with a ridge vent and an eave vent at the bottom) over the existing fesco board. My questions: - Doesn't the air space want to be directly below the metal roof above the OSB? Or is it ok to have it below the OSB? - Does the radiant barrier go foil side down or up? - What is the point of leaving the existing fesco board in place and battens in place? My goal is increase the R-value of the assembly but it doesn't seem to me that the fesco is worth the R-value to leave in place. Intuitively it seems like you would want to remove all down to the deck and build up from there. I may then want to start witth 2x4's to get 3" of rigid for a better R value with a 1/2" air space. - I goal would be to achieve an R-30. Thoughts? Thanks!!