I am building a new house in upstate South Carolina.  The builder has recommended using OSB with a foil backing for the roofing substrate with 30 inch blown in insulation.   From what I have read the foil backed OSB material is worth incorporating but I am not sure if there is not a more effective insulation configuration (e.g. batt or foam) that would offer better performance at near the same cost.  I do know foam insulation is more expensive and maybe overkill.  What are your recommendations?