The ventilation needs to be between the insulation and the roof sheathing (plywood). You need soffit vents and a ridge vent and plastic "vent keepers" between the insulation and the sheathing to keep the insulation from pressing right up to the plywood and blocking where the air should flow. Steve is VERY correct in suggesting that you wire for a ceiling fan (or two depending on the size of the room) as these are pretty much a requirement to stay comfy in a room with tall ceilings. Don't buy the cheap fans they are junk. Get a powerful one with 5 blades and aggresive pitched blades to really move some air.