Which is better spray foam or radiant barrier in the attic space?
My preference is the radiant barrier. Radiant barrier sheathing may cost you a few dollars more than regular sheathing but foam can cost you many thousands of dollars and that makes any payback a long term issue. You cannot do both foam and radiant barrier because you need a minimum of ¾” air space in front of the foil for it to work. Any material that comes in contact with the foil completely negates all the benefits of a radiant barrier. I will also point out that most shingle manufacturers state their shingle warranties are void or reduced if applied to a roof with spray foam in the underside.