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Question DetailsAsked on 11/26/2013

What are black marks on wall and ceiling in rooms?

Usally happens in winter when baseboard heat is on. I tested for mold & not it. Shows up on ceilings & sometimes wall, especially in catherdral areas. Seen it it other peoples houses & no one knows reason or solution. I paint over but comes back in usually 2 seasons. Double checked back side of sheetorck for mold & nothing. Ventilation is good. Soffets are clear & good. No mositure in house. Anyone know what this is or how to permanently remediate? Thanks

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1 Answer


Sounds like you are talking about Ghosting - which is where the more conductive studs and headers in the wall (or joists in ceilings) make for cold spots on the walls, which then condenses small amounts of moisture, which then traps dust, which then traps airborne oils and other cantaminants, and can eventually support mildew or mold growth. Because the studs and joists conduct the heat better, ghosting tend to show up as linear dark streaks. In extreme outdoor cold or with very low house temperatures you can even get frost on the inside surface of the wall, starting at studs, headers, and door/window frames.

The reason you have condensation is because the warm house air usually holds normal moisture from bathing, cooking, and respiration moisture fine (usually around 40-55% humidity), but at the cold spots on the walls the cold air and surface cannot hold as much moisture so the relative humidity at that location goes up to 100%, causing condensation. This is the same thing that makes the inside of your car windshield frost up after you get in the car - the car air as it heats up can hold the moisture from skin evaporation and especially from breathing, but the moisture condenses on the windshield and windows, and because they have essentially zero insulating value, not only condenses but freezes to frost until the inside temperature rises significantly.

If ghosting is happening, it is a sure bet you have condensation (or frost) on metal door and window parts like frames or handles or metal doors, and likely at the bottom of windows and exterior doors also.

Ghosting is common with forced air systems because they circulate more dust through the house and also tend to leave walls much colder because the air enters the room at only one or two spots, whereas basebaord heating systems tend to distribute the heat more along the walls and actually heat the walls with the embedded pipes as well, which is why they are more common in very cold climates. Ghosting can happen with any other heating system, especially in cold climates, and is probably most pronounced in the cold corners of woodstove heated rooms, where the heat distribution is extremely uneven and circulation tends to be low, and in highly insulated airtight modern construction, where air changes per hour are low so dust tends to stay trapped in the room air more.

As you noted, cathedral ceilings and skylight wells are two of the most common locations, because warmer moist air rises up in that area but with little circulation to remove moisture, providing more moisture to condense on and "prime" the surfaces.

Solutions -

1) reducing household airborne moisture sources like house plants, unvented cooking on the stove (especially boiling and broiling), unvented baths and showers, unvented dishwasher, etc.

2) increasing ventilation in trouble areas, like placing circulating fans in high ceilings

3) improve dust capture in air handling system (more effective filtration)

4) increase indoor air temperature to increase moisture holding capacity

5) increase wall/ceiling insulation, if not fully insulated

6) eliminate dust sources like interior venter dryer, eliminate or increase frequency of vacuuming shedding rugs or carpets, etc

7) more frequent wall washing to remove the ghosting

You can find lots more explanation and recommended solutions by googling this search phrase and reading some of the articles that come up - eliminating ghosting on walls

Answered 6 years ago by LCD

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