Hi this is Glen
We are a roofing company in climate zone 6. Many times as part of a reroof project we are involved in improving insulation and ventilation on older homes that have Ice dam and condensation problems. primarily cathedral ceilings. If we add additional Insulation on top of the roof deck the code requires R-25 My question is common sense tells me whatever amount we install on top of the roof deck should be a improvement over the existing system.