At this job we sprayed Icynene Classic Spray Foam Insulation to the underside of the roofdeck. This allowed the customer to bring all of there hot air heat runs inside the building envelope. If this space was empty and the customer did not need it heated we would spray down to the floor. Bringing the heat runs into the envelope helps with the over all efficiency of the heating system.
This is an excellent Insulation System. At the thicknesses used here we are stopping around 95% of the heat flow through the cavity. The only way to make this roof any more efficient would be to add rigid board to either the inside or outside. That would help to cut the heat transfer through the wood studs. However, wood is a very poor conductor of heat. I would consider adding blue board as a premium package and is not needed to build a very efficient house.