Best way to use extra insulation boards

A homeowner in Westchester County, NY posted a question on a building forum about what to do with 15 leftover 1-inch rigid foam boards (R-Tech, foil-faced on one side) from another project. The boards are headed for a detached two-car garage that currently has drywall but no insulation. The garage has a cathedral ceiling with exposed rafters and is unvented, meaning there's no airflow path built into the roof structure.

The homeowner is weighing a few options: cutting the boards down to fit inside the stud walls, laying them across the top of the rafters with the foil facing down, or attaching them directly to the underside of the roof deck. No answers had been posted yet, so there's no expert guidance to pass along here.

For anyone in a similar spot, the general issue is a real one — where you place rigid foam in an unvented roof affects whether moisture can get trapped and whether the foil facing does anything useful, since foil only works as a radiant barrier when it faces an air gap. If you're using leftover foam boards on a garage or similar structure, this is the kind of detail worth getting right before installation, since insulation placed the wrong way in an unvented assembly can cause condensation problems down the road rather than saving you energy.