"Darkness" at joist in ceiling

Inspected a house today built in 1991. In addition to dark spots at the drywall nails and/or screws in the ceiling, there were also dark outlines of the ceiling joists in many areas of the house. In other words, you could easily see where most of the joists were located because those areas were darker than the rest of the white ceiling.

The roof had ridge vents as well as soffit vents and there were no indications in the attic, on the sheathing or rafters, of any excessive moisture.

Any ideas on what may have caused this?

I’ve attached pictures, but they don’t really show it well.




There must have been insulation between the joists. The insulation on the ceiling drywall keeps it warmer than the low R value wood joists. The wood is just that much cooler than the drywall to cause a little microclimate of coolness and higher RH, leading to deposition of fine dust on the cooler drywall surface. This is especially noticeable in older uninsulated wood lath/plaster ceilngs. You can see where the wood with at least some insulation value is (lighter) and where the cool plaster and plaster keys are as they’re significantly darker.

It’s called Ghosting and has been mentioned here on NACHI a few times .

Go to and read much information.


… Cookie

Thanks Roy and Brian, much appreciated!


The tops of the joists may also be exposed above the level of insulation. Thus the joists become colder creating the ghosting. Candle soot and smoking and poor housekeeping may highlight the problem.

Additional insulation if required to cover the tops of the joists would be a good idea.

I see this on an infrequent but regular basis. I saw it in a master bedroom last week. It was the only room in the house with candles on every flat surface. Ghosting it is.

This is often what it looks like thermally.

The difference in temperature across the ceiling causes a difference in molecular activity resulting in pollutants in the air to become statically charged and plate out on the ceiling.