How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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What causes dark or light ghost lines on ceilings and walls?
Thursday, June 21, 2018
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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