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Central TX cathedral ceiling from botton to top:
dry wall, 2 X 8 rafters, perpendicular 1 X 4’s spaced for former shake shingles now with plywood desking on top with shingle roof. Present insulation is black rock wool (?). No thermal barrier. Added eave vents and even put fans in adjacent attic ridge to pull air through like a car radiator.
Options:blow in better insulation with or without 1-2 inch air channel under decking? Take down ceiling drywall & insert rigid insulation within avail rafter space? With next roof job build up rafters on top of plywood decking to allow more fiberglass insulation?
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Brian,
If you have the option, remove the drywall and old insulation. Have Icynene foam sprayed directly on the underside of the roof deck. No ventilation would be needed.