I like to enclose the whole frame within the layer of foam to avoid the problems mentioned above, air leakage and such. The method Brad, timberwrestler mentions is one I favor as well, in this case I would dedicate the roof to attic space not living space. Insulate the ceiling instead. All these approaches should be considered before things are set on stone. It is kind of late to make decisions after a certain point, at which time compromises are made.
One other way to look at it is that the purlins are not raised up but the principal rafters are set into the building. This gives more relish on the ends of the tie beams where the rafter sits upon the tie.
