I am reroofing my house. I am taking the opportunity to pull up the decking above the cathedral ceiling and adding air baffles to help cool the roof and reduce ice dams.
In order to accomplish this, I am using 1 inch rigid foam board pushed down two inches with spacers between board and roof deck. I am using the foam board to regain some of the reduced r value from compressing the fiberglass insulation, but also for the rigidity to help compress the fiberglass.
Should I air seal all the seams between sheets of foam board, or leave them open to allow moisture in the fiberglass a path to escape? The fiberglass is foil backed with the foil against the ceiling. Ceiling is tongue and groove wood, so probably pretty permeable.