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Mildew Ghosting on Ceiling

Q. My in-laws, who live in Peoria, IL, are having a mildew problem that they can't figure out. When visiting them over the holidays, they pointed out gray stripes on their kitchen and living room ceilings which seem to coincide with the rafters in the attic. The stripes appear to be mildew growing on/in the painted drywall ceiling. There are also some occasional large darker gray spots within those gray stripes that we presume coincide with the location of drywall screws. So it appears that moisture is bleeding through from the attic wherever there is a rafter???

The attic is insulated. They use it for storage. I do not know if they have ridge vents or any other sort of attic ventilation. While there, I noticed that they had multiple humidifiers running. A humidity gauge they had sitting in one of the humidified rooms read something like 75 or 80%. I recommended that they get that down below 50%. Could that be causing the mildew problem, perhaps? If so, why would the mildew only show up where the drywall on the ceiling touches the rafters?

A. The ghosting through the ceiling paint matches the ceiling joists and the drywall nails and screws. The wood joists and the steel nails are colder than the drywall on the ceiling and therefore cause condensation from the high humidity. This condensation happens on the coldest surfaces which is on the nails and ceiling joists. The ghosting is from moisture inside the house and not likely from water leaks outside the house.

Drop the interior relative humidity below 50% ASAP for indoor health. Add insulation to the attic to an R-40 to cover the joists and nails to better maintain the steel nails and wood ceiling joists. As these building materials remain warmer fro better insulation the spotting and ghosting will diminish.

The ceiling must be repainted for the ghosting appearance to disappear. You must thoroughly seal the dark spots on the ceiling with a stain blocker and repaint the ceiling.

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