It’s called “thermal tracking” and is caused by a difference in temperature over a surface created by what is happening behind it. The hazy lines develop slowly and are usually dark, but sometimes all of the area except at the lines darken, like in the photo above of the ceiling of a screen porch.

    In the case of this screen porch, the attic above it is not insulated, but the bottom chords of the wood trusses and bridging used to support the drywall provide some insulation value. The areas that do not have wood framing above them will cool down further as outdoor temperatures drop overnight, and those areas will reach the dew point first and be more moist in the morning. This, in turn, allows more dust in the air to attach to those areas. When enough dust accumulates and if the moisture is sufficient, a minor mold growth can develop. Air movement patterns over the surface can also affect the rate of deposit of the dust.

    Dark ghost lines on interior ceilings near a fireplace are what we see more often. Here the temperature differential is reversed, with the attic insulation above the ceiling providing more temperature stability and the areas touching the bottom chords of trusses being less insulated. The truss lines become more attractive to soot from a fireplace if it is not venting well, along with particles emitted by an unvented natural gas fireplace. Candles are one more source of soot that can build up into ghost lines over time.

    The fix for ghost lines, after cleaning and repainting the area, is to install additional  insulation to resolve the temperature differential and/or reduce the amount of the soot or dust in the area. Additional air circulation with ceiling fans can sometimes also help, although it may increase the flow of dust over the surface.

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Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• Why is the grout cracking and coming loose at my floor tile?

• What are the building code requirements for notching and boring holes in a wall stud? 

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code? 

• Why is my garage ceiling sagging? 

• How can I identify what kind of wood flooring I am looking at?

• Why does my concrete floor slab sweat and get slippery?

• What is the minimum ceiling height for rooms in a house? 

• Why are there score line grooves in the concrete floor of the garage?

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

    Visit our STRUCTURE AND ROOMS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.