Thermal Tracking Stains on Walls

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Thermal tracking, also called “ghosting”, on walls can appear as dark streaked stains. The air in the home carries moisture and dust particles and as the warm, damp air touches a cold spot on the wall, condensation forms. Fine dust particles a

Thermal tracking, also called “ghosting”, on walls can appear as dark streaked stains. The air in the home carries moisture and dust particles and as the warm, damp air touches a cold spot on the wall, condensation forms. Fine dust particles adhere to the damp wall surfaces, leaving thermal tracking stains. Air circulation problems, inadequate insulation and other structural problems can cause wall condensation, but the stains are caused by dirt inside ductwork, dirty heating units, and improper combustion of appliances.  Often these stains are mistaken for toxic black mold.

Thermal tracking stains commonly appear as vertical lines or streaks on the wall. Structural framing members such as studs or beams are the coldest part of the wall, so stains may mark the location of the framing members, especially in homes or additions older than 10 years. Dark streaks are usually 16 to 24 inches apart to line up with wall studs or ceiling joists and rafters. Stains such as these may even be present even in homes without a moisture problem.

Thermal tracking is a sign that energy is being wasted in the heating or cooling season. Identifying the source of the problem can save money and improve efficiency.


Thermal bridging is when the framing members transfer heat through the wall. (During summer the wall studs show up warmer than the wall cavities)

Cold air enters at the top of the wall and flows between the ceiling rafters.

Painting or cleaning the walls and ceilings can remove the stains, but it does not correct the problem.


One of the most common causes for thermal tracking is insufficient or missing insulation. This can also occur in homes with blown-in loose-fill insulation inside the wall cavities. As the insulation settles, cold spots can form near the top of the wall where thermal tracking can develop. Cathedral ceilings are another likely area to find thermal tracking stains as the insulation installed may not prevent the surface from cooling.

Older homes are notorious for thermal tracking as the walls do not have the correct amount of insulation and there is typically more dirt accumulation inside the home.

Natural Convection

Air movement upwards in the wall cavity can draw cooler air from the basement or crawl space.  Any openings in the wall cavity inside the basement can create a natural draft inside the wall. As the section of wall cools, dust particles in the air will collect on the surface of the wall as moisture in the air condenses on the wall. There may not be enough moisture to cause a mold problem, but the moisture will accelerate the thermal tracking.

Dark gray or black, rectangular stains on interior or exterior walls: may occur where insulation has been omitted. These stains typically appear on colder exterior walls; however they may appear on interior walls. Stains on uninsulated interior walls may appear darker especially if there is a clear path for cold or moist air to flow inside the wall cavity. The empty wall cavity creates a chimney effect where the air travels up through the wall until it finds an exit, such as a soffit or attic space.


Many people have noticed dark streaks above fireplaces due to smoke from fires, but gas fireplaces can cause of thermal tracking on walls. If a gas fireplace isn't working properly or not maintained, the fireplace may release fine soot particles into the air. If you notice thermal tracking above a gas fireplace, you should shut it off immediately and have it inspected by a qualified technician.

Baseboard Heaters

Baseboard heat can cause of thermal tracking wall stains. Dust particles are carried with the warm air as it rises and the particles are deposited on the cooler surface of the wall. This can happen even when the walls are properly insulated. The best fix for this is to thoroughly clean the fins on the baseboard heaters. See my article about cleaning fin-tube baseboard radiators:

Air Distribution

In commercial building or schools, you may have noticed dark streaks on the ceiling around diffusers.  The same thing can happen in your home especially if you have dirty ductwork or you do not change your filters regularly. While this is not considered thermal tracking, it can be confused with it.

Dirt deposits around ceiling diffusers.

Even with filter changes, a small amount of debris is carried along with the warm or cool air and thermal streaking can occur around ceiling diffusers or above floor registers.

Thermal tracking seen above floor register. (Note the clean areas where pictures have been removed)

Indoor Combustion

Cigarette smoking or lighting candles can contribute to thermal tracking wall stains. These actions release soot particles into the air that adhere to damp areas on the wall.  Cooking does not usually contribute to thermal tracking problems, unless smoke is produced frequently.

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 pets or wall to wall carpeting. Mold problems might be mistaken for thermal tracking stains, especially in the early stage; however mold colonies often have more rounded edges. If the cause of stain is unclear, you should hire an inspector with specialized tools to investigate the problem. Some causes for thermal tracking can lead to mold or moisture problems inside wall and ceiling cavities if left uncorrected.


Roberta Baxter
Posted on Apr 8, 2012
Posted on Apr 6, 2012