A friend recently bought a house and the unfinished (not taped and mudded) drywall is bowed down between the trusses across the whole ceiling. I looked in the attic and there is standard %30 batt insulation between the trusses and I don't know why the drywall is bowing down. What is the easiest/least expensive way to fix this? He doesn't want to tear all the existing drywall out if it can be helped. Thanks.