I live in South Carolina in climate zone 3 and doing a home remodel.  I’m trying to maximize R-value while adding a radiant barrier to a cathedral ceiling constructed using existing 2×8’s.  One side of this cathedral ceiling is vented from the soffit to the roof vent and the other half is inside existing attic space.  

Currently, the vented side of this cathedral ceiling is insulated/constructed as follows from the roof down:

1.  Metal Roof (silver colored aluminum)
2.  Grace ice and water underlayment
3.  3/4” plywood roof sheathing
4.  1.5” air gap created with styrofoam rafter vents
5.  Faced r-19 insulation (pink stuff) run between 2×8’s
6.  Sheetrock

Total of R-19

The other half of the cathedral ceiling that sits in the attic space is insulated/constructed as follows from the roof down:

1.  Metal roof (silver colored aluminum)
2.  Grace ice and water underlayment
3.  3/4” plywood roof sheathing
4.  3’ of air space between rafters and top of insulation listed in #5 below 
5.  R-19 rolled insulation run perpendicular to #6 below 
6.  R-30 faced batts run between 2×8’s
7.  Sheetrock

Total of R-49

I’m looking for verification that I would be able to do the following on the vented side of the cathedral ceiling without causing any moisture issues or other unforeseen problems:

1.  Metal roof (silver colored aluminum)
2.  Grace ice and water underlayment
3.  3/4” plywood roof sheathing
4.  1.5” air gap created by nailing 1×2 spacer to rafters
5.  1.5” Rmax Sika Thermasheath nailed to 1×2 spacer and air sealed with spray foam (creating R-9.6).  Thermasheath has a foil face that would face the roof sheathing in the 1.5” gap and act as a radiant barrier.  
5.  4” closed cell spray foam (creating R-26)
6.  Sheetrock

Total of +/- R35

This construction would create an air tight seal between the conditioned space and the air gap.  The air gap between the Thermasheath and the roof sheathing would dry the plywood (if it ever got wet) and allow any moisture to be removed through the rafter vent by convection.  

Please let me know if you foresee any issues with this construction and if you agree that it will work.  Thanks for your help!