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Q. A house we recently completed has a lot of nail popping—in about 30 places—in the drywall on the cathedral ceiling. Most of the popping has taken place near where the ceiling meets the walls. The cathedral ceiling is framed with 2x10 rafters with one-inch, foilfaced foam insulation between the rafters and the 1/2-inch drywall. The drywall was attached with screws, but I have reason to believe some of them were driven home with a hammer. What is causing the nail popping and how can I prevent it in the future?

A. The nail popping is caused by the same thing that always causes nail popping—wood shrinkage. I assume that you used 2 1/2-inch screws to give an inch penetration into the rafters. As the rafters dried, the point of the screw would remain in place and the wood would shrink along the screw about 1/16 inch. This is not normally a problem as long as nothing presses against the drywall to cause the screw to pop the taping compound. However, the wall drywall is usually installed butted against the ceiling drywall. As the slight, lengthwise shrinkage of the studs causes the ceiling to drop slightly, the wall drywall, which does not shrink, presses tightly against the ceiling, forcing the ceiling drywall nails to pop. A screw that was driven with a hammer would tend to force the wood fibers inward, and any tendency for the fibers to straighten themselves would also tend to force the screw outward. Driving drywall screws with anything but a screw gun with a positive clutch to provide proper head indentation invites problems. What can be done to minimize the problems? First, use the driest lumber available. Second, drive the screws properly. A third possibility is to finish the top of the wall drywall with a vinyl trim piece such as USG P-1, which fits over the top edge of the panel and provides a control joint between the wall and the ceiling, eliminating or reducing the pressure applied to the ceiling finish by the wall drywall. The use of these moldings is discussed in the USG Gypsum Construction Handbook, which should be available from your drywall distributor.