I want to insulate a 24x24 detached garage in zone 6, for occasional use as a workshop. The roof is framed with open trusses made of 2x4 material, and I don't want to give up the storage area that the truss space provides. So I don't want to build a ceiling and insulate above it, rather, I want to cut rigid foam board and place it between the rafters against the underside of the roof, and then cover it with drywall. Someday, perhaps, when it's time to re-roof, I can add foam board on top of the roof as well, but for now I just want to get the best r-value practical from the small space available.

I am puzzled by different things I've read about the possible condensation issues. I have a supply of used 2.5-inch thick fiberglass-faced polyiso board. I had been thinking of putting this layer agaist the underside of the roof, adding another inch of some kind to fill the 3.5" depth of the rafters, then a layer of foil-faced board covering the rafters, and finally the drywall, painted with a vapor barrier paint.

Some things I've read say there should be no vapor barrier on the inside, so no foil-faced and no paint. Other things say if you achieve an air-impermeable barrier, all is well. One article seemed to suggest I might want to change the order of the layers, by putting the foil faced up against the underside of the roof, and not use a vapor barrier below that.

Any thoughts?