I'm in the process of air sealing my house before adding to the existing insulation. Among other sources of information, I've found the book Insulate and Weatherize by Bruce Harley to be an excellent reference. In a few spots, he recommends using rigid foam board to effectively air seal larger openings including subfloor joist spaces that are completely open to unconditioned air in the attic over an attached garage. The attached photo shows an example of what I'm trying to block in my attic.
I'm concerned, though, that almost everything I've read about rigid foam board insulation says clearly that it must be covered by a fire barrier such as 1/2" drywall. If I were to install this in the joist space as an air barrier, would I also have to install drywall on both sides of the foam board in order to meet the fire barrier requirement?
Any other recommendations on how to properly seal off the subfloor joist space from the unconditioned attic air to help with my air sealing efforts?