Hi,
I have a 1970 house with an original flat roof that has been covered with a pitched roof. The original roof has 2x8 joists spaced at 24" covered with 1/2" sheathing and topped with tar/gravel (about 60% of the tar/gravel has been removed). The original roof/joist cavities have 2" of rock wool batting (R7.4), poorly installed. I can open 12"-16" slots perpendicular to the joists every 10 feet (40 ft house length) to provide access to the joist cavities for insulation installation and to allow for ventilation into the new roof. 

I would like to blow fiberglass (as opposed to cellulose due to weight on the underlying sheetrock) into the joist cavities to fill the 5+" void between the existing rock wool and the top sheathing. The blower hose would be run through the slots cut out of the sheathing and pushed back into the joist cavity, pulling the hose back as the cavity fills with insulation, repeating this for each joist cavity and each access slot. (Either blown in or batt insulation would then be added above the original roof sheathing to increase total R value.)

My questions: 1) does this make sense, 2) will three 1-foot wide slots cut perpendicular to the ceiling joists spaced about 10 feet apart be adequate to avoid any moisture buildup in the joist cavities that might migrate from the living space below (allowing that the Salt Lake City climate is pretty dry), 3) can I try to blow the insulation fairly densely to ensure thorough fill, since it is not possible to observe how well the insulation is blowing into the cavities, 4) any other potential problems.

Thanks!