Thermal tracking on walls appears as dark soot stains. The air in the house must be full of moisture and warmer than the wall to produce the stains. Once the warm, damp air touches a cooler spot on the wall, some of the air's moisture coats the wall's surface with condensation. Debris in the house air, particularly soot, sticks to the damp wall surfaces, leaving thermal tracking stains. Air circulation problems, poor insulation and other structural problems can cause wall condensation, but the stains are actually caused by indoor appliances or activities.
Fireplaces and Heating Appliances
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Gas fireplaces and heating appliances are a cause of thermal tracking on walls. If a gas fireplace isn't working properly or not maintained, the fireplace may release excessive ash and soot particles into the air.
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Both gas and oil heating appliances are a possible cause of thermal tracking wall stains. While an oil-heating appliance sometimes produces soot during normal operation, a malfunctioning unit might produce excess soot. Gas heating units don't usually produce soot or ash unless the internal gas ventilation or combustion airflow is restricted.
Burning Items Indoors and Dust
Cigarette smoking and burning incense or candles in a home contribute to thermal tracking wall stains. These actions release soot particles into the air that adhere to damp areas on the wall. Scented candles contain extra oils for fragrance purposes and may release more soot than an unscented candle. Other actions, such as burning paper, food or wood, release soot into the air and contribute to thermal tracking stains. Skin cells and fabric fibers -- common parts of house dust -- may adhere to damp wall spots.
Patterns
Thermal tracking stains commonly appear as vertical lines or streaks on the wall. Structural framing members such as studs are usually the coolest part of the wall, so stains may mark the location of the framing members. A telltale sign is if the streaks are16 to 24 inches apart; conventional wall frames are spaced at these measurements. Stains going up walls over heat sources such as baseboard heaters might appear if the house is unclean or has animals or cigarette smokers. These stains may appear even if the home doesn't have a moisture problem, reports the website Inspectapedia.
Similar Stains
Some stains look like the beginning of thermal tracking patterns but signal different problems. Possible thermal tracking stains near the bottom of the wall may be dirt from household pets or household members. Early house mold problems might be mistaken for thermal tracking stains. If the cause of stain is unclear, an inspector may take samples of the stain to determine the problem.