There is no access into the small sidewall areas and thus must be accessed by drilling holes in the sidewalls in this case the roof and using dense pack of poured foam methods to properly insulate.
Insulating a gambrel roof room.
Interview with the contractor at the end.
If you do not want any extra living space in your attic then you can install a low height roof but you will also have a small space to install the insulation and the ventilation system.
One of the best ways to protect your mansard roof from the problems of ventilation is to use a vapor barrier on the inside.
How to vent a gambrel roof using reflectix.
Step 1 add a vapor barrier.
To insulate the roof you will need to use a combination of insulation to fully protect the roof.
This is why careful planning is very important.