I’m currently building my own house with an unvented cathedral roof in climate zone 4A. So far, I have 2 taped layers of polyiso (total of R-15) sandwiched between 2 layers of OSB above 2X12 rafters which will be insulated with rock wool. Most of the roof is a 12:12 gable but there is a small (8’x16′) shed roof with a 2:12 pitch which abuts one of the gable ends.

I’m going to use asphalt shingles and the installation instructions insist on ice and water membrane for 2:12 roofs but I’m concerned about trapping moisture in the upper layer of OSB between the polyiso and the membrane. Any suggestions of how best to proceed?