I went to quote an interior job and found something I had never seen before. All the painted drywall on exterior walls only had floor to ceiling lines every 16 inches. It was obvious to me these were the studs. Also on these lines were screw head size dots every 15 inches or so which I assume are the drywall screws. The client had in some places wiped with a damp cloth and the discoloration comes off like a layer of soot. I was wondering if anyone has seen this before and may know what it is caused by.
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Yup, its called "Ghosting" Here is something I copied(can't remember from where) when I ran into the same problem a few years back. hope it helps.
Ghosting is a phenomenon that is generally seen during the winter months. Ghosting can be identified as dark lines on the drywall of ceilings and walls that outline the wall studs or the ceiling joists inside the living space. Even though insulation is installed between the joists themselves are exposed to the cold winter temperatures in the attic. Since the wood joists are not effective insulators, they act as thermal bridges. Consequently, the temperature at the underside of each joist (attached to the drywall) is lower than the adjacent sections of the drywall that are covered with insulation bats. The lower temperature along the bottom of the joists encourages the formation of microscopic condensation. Over time, the moisture will trap dust particles hat show up as shadow lines that outline the wood joists or studs.
To prevent ghosting, install insulation bats over the exposed ceiling joists. Ideally, the insulation should fill the spaces between the joists and cover the tops of the joists as well. The same holds true for the stud spaces; however, the fix would be more difficult.
There are 2 types of scuba divers. Those that pee in their wetsuits and those that lie and say they don't.