Well, crap! Got to looking at seams in ceiling drywall where it has started to droop and found out that the drywall panels and rafters don't match-up very well and some of the drywall screws don't hit wood. Not that big of a deal since some 3-4" wide trim board along the seams will solve the problem (it's a garage, it doesn't have to be pretty). While measuring for trim board, discovered that rafters are on 24" centers, not 16" as in the rest of the house. My styrofoam panels for 16" centers aren't wide enough.
