Question: I painted the inside of my home six months ago, and already it needs to be repainted. Portions of the walls have become visibly darkened. Strangely, this occurs wherever there are framing members behind the drywall. I can see where all the wall studs and ceiling joists are. Not only that, I can see dark spots where all the drywall nails are. What could be causing this, and what can I do about it?
Answer: What you are seeing is a phenomenon commonly called "ghosting." When ghosting occurs, extremely fine soot in the air collects on the walls and ceilings where the temperature is cooler and there is higher electrostatic attraction.
If you paint your home again, the ghosting will reappear unless you eliminate the source of the soot. Soot typically comes from fireplaces, furnaces and other fuel-burning fixtures that are not properly vented. The fixtures may be defective, but low air pressure in energy-efficient homes is another common cause. Some buildings are so well sealed that the use of exhaust fans can cause low air pressure. This can cause poor venting of combustion exhaust from fuel-burning fixtures.
If you live in a high-efficiency home, you may need to make it less air-tight by providing some exterior ventilation. For an evaluation of your specific situation, consult an indoor air quality specialist. Your gas company may even have consultants who provide this kind of service and advice.