Best way to use extra insulation boards – GreenBuildingAdvisor

Good afternoon,

Hope everyone is doing well. I have 15 1inch r tech foam boards left over from a project. 1 side is foil faced. I have 2 car detached garage that has 0 insulation and Sheetrock. Garage is 20×24 2 doors, with a cathedral ceiling. There are rafters so I have an option to build an attic on top. Not sure how to best use the insulation. Would you put it in walls and cut it up? Should I lay it on top of rafters with foil face facing down? The garage is unvented. Would you put the foam boards against the underside of the roof? Any help is appreciated, I’m in westchester county NY for zone reference.