Most warranties require roofs to be ventilated.
That's not difficult with an attic, but exposed-rafter cathedral ceilings pose a problem. ISO-VENT was designed to solve this problem. It combines insulation and vent channels into one product. The polyisocyanurate panels are flat on the bottom, but have ridges on the top. The ridges are 16 inches on center and create a one-inch gap for air flow.