A.Don Jackson, managing editor of JLC, responds: Steel studs are excellent conductors of heat (in other words, lousy insulators), so the surface of the drywall directly over the stud flange stays cooler than the surface of the drywall between the studs. As air moves across the surface of the drywall (chances are there’s forced-air heat in the house), dust and soot particles will stick to the cooler surface of the wall over the stud. Condensation may also form on the cooler surface, attracting dust and soot and even supporting mildew growth. The result will be dark streaks on the wall at the studs. The screw heads may also appear as dark spots.

If the stains seem to be sooty or oily, it’s time to have the furnace checked to see whether the burners are operating properly. Check also to see whether the insulation on the inside of the air handler housing is breaking down, or whether there’s a small crack in the heat exchanger. Oily stains might indicate that unburned atomized fuel oil is...

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