I am buying a home with a detached garage that the original owner insulated. The garage is probably 25 years old. At the time of construction, 1" expanded foam was nailed directly to the rafters to insulate the ceiling. Over time, the foam has shrunk (and deteriorated to some degree) leaving 1/2 - 1" gaps around all the seams and is not sufficient any longer. The attic is vented properly.
I would like to re-insulate the ceiling with standard blown-in fiberglass or cellulose. I would also like to sheetrock the ceiling to provide a better finish.
Can I install the sheetrock over the existing styrofoam and then blow in the insulation in the attic?