well first make sure there is ventaltion above the insulation.

then.... lots of work and cost but.... rip out drywall, install baffles that leave an airgap under the roof sheet then have someone come in and spray foam it. You may want to reframe the ceiling/walls to get 8" of foam. very expensive but will save you money in the long run

You can always do something.

Is the roof flat or pitched? - a flat roof could be redone with a coating of closed cell poly-urethane insulation before the new roof. A pitched roof could be furred out and re-roofed with insulation in between. this adds considerable weight to a roof and should be discussed with an engineer first.

How tall are your second floor ceilings? - do you have enough height to remove the ceiling sheetrock, fur down by a 2x6 and re-insulate the entire ceiling?

All options will cost money.