I posted this same thing under insulation-I"m not sure where it should go-because it's kind of a combined issue in my mind

We have a flat roof approx 1550 sq ft in CT which was just redone with new decking and cool roof seemless membrane. The joists running from front to back are 16in oc with 8 in of airspace between decking and ceiling's that are 7 1/2 ft.
What is the preferred way to insulate so we don't get condensation? We put up 3 1/2 batts R13 that leaves a good amount of airflow..should we also use the durovents end to end or will just leaving the 4 1/2 airspace be sufficient. We have soffit vents on front and back sides of the house. I'm hesitant about spraying all that open cell insulation because of the cost to only find it out it doesn't work. I haven't drywalled yet because I want to get it right before wasting money. We are trying to restore the house to it's mid century modern intent so it's constant work in progress
Also can using a kero heater cause condensation in the airspace? I maintain it will-dinner is riding on this bet.