I have questions about roof cavity insulation, have input from local experts, though welcome hearing from those who are practicing in cold climate and hopefully near/at the cutting edge of Wx knowledge and building science.
I am specifying flat roof insulation for projects that include 1) roof cavity (separate rafters and ceiling joists, "roof crawl space") and, 2) rafters with deck and ceiling finish attached top and bottom. Building with roof cavity has solid brick walls with retrofitted insulated stud walls, and building with single layer roof has furring and insulation on brick/block walls. Both buildings have received "cool roof" replacement (specific product not identified). In both buildings the roof is essentially unvented with roof deck/roofing the air pressure boundary. Roofs are entirely exposed to sky. The current local weatherization standard calls for dense pack cellulose for inaccessible framed roof cavities. This is at odds with Standard Work Specifications for Multi-Family Homes "Insulating Inaccessible Attic."
Let me know of WUFI analysis of dense pack cellulose (or other fiber) for unvented roof cavities, and if roofs have been opened for inspection several years following this practice in cold/humid climate like N Illinois. Roofings vary in permeability and either the insulated surface (ceiling) or roof may be the air pressure boundary. GBA has strings that reveal the consternation around managing condensation within an insulation strategy, addressing hybrid insulation and Lstiburek’s “Don’t be Dense.” I have requested Building America perform "proof of concept" studies to clarify best retrofit installation practices. See work I've done in distant past > How to House Doctor Roof Cavity Air Seal
I was hoping someone in your neck of the woods would have replied & I can't seem to find the SWS so let me just offer a few quick thoughts - SWS are not the only way to skin a cat & if best practice or other trusted information says that is not a good idea, then go with what works.
Being in a cold climate both the roof decking & the drywall should be an air barrier (assuming I am reading your question correctly - entire space dense packed with insulation above the sheathing) or you are looking for water problems. Permeability has nothing to do with air tightness, that is another story all together.
Just as a quick thought, if you want to post a pdf for people to read, it helps if it is flipped in the right orientation