The situation: Old house (115 years) with old style 2X4 (actual 2"X4"!) rafters running to ridge/peak, 1x8 sheathing for old shakes on top of rafters, with new plywood sheathing on top of the 1X8 for the new composite shingles. No soffits or soffit vents. We need to insulate these rafters ahead of sheet rocking the underside of the rafters.
The question: To get the best r-value, can I use a 1" layer of rigid foam under (actually over) the r-15 fiberglass? That would effectively seal the space and eliminate needed ventilation. It would also provide the needed moisture barrier--I think.