I Was emailed a pic of Sheetrock in a bedroom that had black stripes at each tape seam on the walls and ceiling. What do you think it is ?
At “each stud” or at “each tape seam”??? Makes a difference.
Most likely “ghosting”.
Sorry, each stud.
Yes like that. I can’t post pics from my iPhone. I just thought it was odd being at each stud.
I think Troy has the literal definition. “ghosting” is the common name.
Are there/were there candles in this bathroom? Perhaps a wood fueled heater nearby? Heavy smokers?
Yep, I agree with Troy – thermal bridging causing condensation to trap dust.