Closed Cell Spray Foam Depth. Help me understand R value and diminishing returns

I’m insulating an old roof deck. Attic conversion. And I’ve come across conflicting advice regarding closed cell spray foam installation. It boils down to ideal R values vs the diminishing… I’m insulating an old roof deck. Attic conversion. And I’ve come across conflicting advice regarding closed cell spray foam installation. It boils down to ideal R values vs the diminishing…Read MoreGreenBuildingAdvisor