In climate zone 4 I have an old house with a small half of a hip roof off the side of the main house. This roof area is above what will be a finished bedroom. I have access to the underside of the roof and there is sufficient depth to meet the required R value. What I am trying to determine is should I use a combination of closed cell and open cell spray foam applied to the underside of the roof sheathing  or would building up enough open cell be adequate to take care of any vapor barrier needs. The ceiling immediately above the bedroom will be a flat ceiling composed of sheet rock attached to 2 x 6 ceiling joists with no access or vents to this uninsulated space above the finished bedroom ceiling, and no soffit or ridge vents in the roof. I understand that I need to air seal the sheet rock to the ceiling joists so that no warm air from the heated bedroom escapes  into the uninsulated space above the finished bedroom ceiling which will then cause condensation.  I would greatly appreciate your thoughts.
Thank you, Kevin