Hello, I think I know what your up against as I am facing the same problem right now. I have 2x6 rafters (they go from the outside wall up to the ridge vent) as when someone hand frames your roof. I also want to put R-19 in between my rafters but need room for the 1.5" foam channels. I plan on, haven't done yet, nailing 2x2's on the rafters to fur them out an additonal 1.5". This will give me a total of 7" total. This will be enough room for the 5.5" of R-19 insulation and the 1.5" foam channels for ventilation. Does this answer your question?