Some of the double foil faced insulations are claiming claiming very high R ratings, but you have to read the fine print.
To achieve their high ratings they are installing their double foil faced bubble/¼" fiberglass in the center of a 2x4 cavity securely stapling it on all sides and then covering both sides with ¾" OSB/plywood. (When was the last time you saw a home built with ¾" roof decking ? I had to pay extra to get ½ decking ! And ¾" on the inside ?)
It is the dead air above between the sheathing and their insulation that is actually providing the insulation. The only thing better than dead air is no air - a vacuum (Google Dewar's flask).