As an Insulator for 22 years I agree with JGANDEE. Except in your area. I would use 1 1/2" to 2 1/2" FOIL FACED FOAM such as Thermax for an exposure rating to fire, with all joints taped with FSK or a foil tape. Anything else needs to be covered with drywall for a fire rating.(check your local codes for your exact needs in foam) Good luck, Its not too difficult of a job.