My friends house is six years old. He has noticed a black residue (kind of looks like black smoke) on the walls in his house. The residue is where the wall meets the ceiling and occurs in a pattern - every 16 inches ( probably where the 2X4s are). What is it and how do you deal with it? Will a blocking primer be sufficient to paint over it or is there a problem with the structure i.e. not built well?
Answer:
If there is less than adequate insulation, and more importantly continuous vapour barrier on an exterior wall or ceiling, the ceiling shows dark shadows. These are the result of solid particles in the atmosphere having stuck to that part of the plastered surface that remained moist the longest after condensation soaked it from behind. The staining always follows strapping or other structural components behind the plasterboard because wood holds moisture better than insulation. This process normally takes years to show up as shadows. Rectification entails painting the stains with a stain deadener, prime painting the area and then giving it a finish coat.
