What Is Ghosting?
To a builder or painter, "ghosting" is the term used to describe unexplainable dark streaks or patches on wall, ceilings, and carpets. Generally, this is caused by the slow buildup of dust, soot, and occasionally even mold. There are several causes behind it.
A combination of moisture, cold spots, static electricity, air currents, and dust, soot, or other particulates that drift with the air can all lead to a buildup of ghost marks. As a homeowner, these may effect the cosmetic appeal of a room or multiple areas within your home and lead to you wanting to repaint.
As a potential home buyer, they're something that is worth a second look so that your home inspector can identify the exact cause behind the ghosting in your house.
Cold spots on ceilings and walls will grow damp from condensation in combination with air-borne dust, which will cling to the damp areas. Ghosts indicate studs, joists, wall headers, and even individual Sheetrock screws in this situation because these items are naturally poor insulators. Ghosts can also reveal insulation gaps that could be caused by dampness, miss, or poorly fitted materials. These cold channels can be fixed with careful planning.
Generally, those kinds of ghosts aren't that big of a deal, although they are something you may want to address eventually. But, it's when ghosts are caused by too much moisture--perhaps from the overuse of humidifiers, drying clothing frequently indoors, or leaving a dirt floor or stone foundation unsealed--that you should worry. Moisture builds up and can lead to ghosting when an area is improperly vented and/or humidity it high overall.
As you know, moisture leads to mold and mildew buildup, which can show itself in the form of ghosts. In this situation, it's important that your home inspector looks at the ghosts to determine if mold and mildew are too blame. Of course, too much moisture isn't always the problem. Too little moisture, meaning dry air, can also cause ghosts because it leads to a build up of static electricity. This charges the dust and other particles in the area, making them more likely to cling to walls, ceilings, and all other things in the room.
In any of these cases, simply painting over the ghost marks will not work. The walls and ceilings need to be thoroughly cleaned to ensure the paint will properly adhere. If moisture is a cause, it also needs to be dried out. Of course, in any situation, painting is only a temporary, visual fix.
The ghosts will reappear overtime if the root cause is not addressed. It's important that you have your home inspector figure out the root cause so that you know whether it's something serious, like a potential mold or mildew problem, or something that's more on the irritating side of the scale, like extra static.