Would the R-Value of the insulation be devalued if spray foam was already sprayed onto the underside of the roof and the decking as the radiant barrier is already attached? Besides the wasted money of the radiant barrier, what other negatives are there?
Answer:
The R-Value of the system that would have been created if they had an air space between the spray foam and the radiant barrier would be greater than it is now — whether that would serve you with lower energy costs I cannot say. If you have HVAC ducts running in the attic space, putting a radiant barrier over them would be beneficial.