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.
This should be a must see video for venting a gambrel r.
One of the best ways to protect your mansard roof from the problems of ventilation is to use a vapor barrier on the inside.
If you have an existing house with this style then dense pack cellulose or pour in place open cell foam are your only options.
This is why careful planning is very important.
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.