My son's house is in central Alabama. It has a pyramid style roof with a gable roof over the attached garage. The unfinished bonus room is over the garage and has a cathedral style ceiling. the rafters are 2x6. It's 68" from the top of the knee wall on each side to the collar ties. The collar ties are 8' above the floor. the floor is on 2x12's and is insulated over the garage. He wants to finish the bonus room and we don't know what to do about insulating the ceiling from the knee wall to the collar ties. There is no ridge vent on the roof over the bonus room and the rest of the roof is vented with two of the old fashioned turbines and there are sofitt vents all around the house. I can furr down with a 2x on the rafters to make them 7" deep. I am considering doing this and using some R30 batts slightly compressed with a 1/2" foam panel at each end of the batts to stop any air flow through the batts. The knee walls will also get batts and 1/2" drywall over all. Will this work since there is no ridge vent over the bonus room? The small area above the collar ties will be open to the pyramid portion of the roof as will the area behind the knee walls. I don't want to have to deal with trying to put up some sort of additional vent on the 12-12 pitch pyramid roof and spray foam insulation is too $$$.