Which side up on installation of Fesco Foam Insulation?
The J-M Fesco data sheet shows the black TopLoc layer on top, with a glass reinforced facer on the opposite side.

The data sheet I saw didn't give many installation details, but I doubt a few fasteners with large washers would have a large impact on your thermal performance as long as they only cover a small percentage of the total surface area. Focus more on getting tight connections between the sheets and eliminating air leaks.