Great news! The building department accepted my proposal to create an unvented attic. This means we can insulate and drywall directly under the roof deck which will allow me to develop the attic space into a loft. Typically, attics are insulated at the ceiling level and outside air is vented through the attic to prevent the build up of heat and moisture in the space. The attic ventilation is a building code requirement.
My proposal is to install four inches of closed cell spray foam at the underside of the roof deck thereby eliminating the ventilation space entirely. The closed cell spray foam reduces the heat and moisture transfer to the extent that the ventilation space is unnecessary. (Sometimes I think closed cell spray foam must be magic.) Then we will add another six inches of fibreglass batt insulation followed by a layer of drywall. The total R-value of the assembly is 44, which is well above the code requirement of R-31.
Here is the drawing I submitted to the building department.