What's the efficiency of extruded polystyrene foam for roof insulation?
An easy method for simulation is offered on the site www.ubakus.com.
It is worth to play around with width and materials before planning or construction work begins.
Parameters like dew points,  moisture and drying time can be seen in seconds, thus disclosing any critical layer combination that could produce mold and damage to the building.
And of course the U or R value are calculated, which might be important for subsidies. F.e. some EU countries demand an U value for  public subsidies.
Best way is to open a roof example and change/add layers. Each layer's width can be decreased/increased  and the layer can be switched on/off by a click to instantly see any changes.
The outside temperature and humidity levels (winter average) should be adapted.