SISALATION OR SARKING
The addition of a non reflective sisalation or sarking membrane installed beneath a tiled would would make no impact on improving the R Value up or down. The only effect would the roof space ventilation would need to be changed from a ventilated cavity to a non ventilated cavity - as per following example.
The R-Value up or down would only alter if a reflective sisalation or sarking membrane was installed beneath a tiled roof. The overall on the R-Value up or down remains the same regardless of whether a foil / anti-glare or double sided foil was installed - as per following example.
VENTED OR NON VENTED ROOF CAVITY
A whirly bird or a gable vent will allow moisture and hot air to escape / be removed from a roof space, it does not allow air to circulate or be ventilated across the roof cavity, which means the roof cavity would remain as unvented based on minimal whirly birds.
A ventilated roof requires proper air flow which allows for constant air flow / ventilation across the roof cavity. An example would be a number of ridge vents and soffit vents. The following design example (if detailed on the plans) can be modelled as either Ventilated or Highly Ventilated.