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This article describes how to identify and diagnose ceiling stains in buildings focusing on the cause of various interior wall and ceiling stains and explains how to recognize thermal tracking, thermal bridging stains, building air leaks, and building insulation defects. Often these stains are mistaken for toxic indoor mold. We also include a description and photographs of normal dirt or debris deposition that occurs around heating or cooling air supply registers.

Our page top photo shows two common thermal tracking or "ghosting" patterns. The narrow dark stains on the ceiling show the location of ceiling joists, and the wide black sooty stain in the upper left of the photo shows where ceiling insulation is less or has been omitted entirely. Readers should see HEAT LOSS in buildings. Also see AIR SEALING STRATEGIES and AIR LEAK DETECTION TOOLS as well as AIR LEAK MINIMIZATION. For photos of snow melt on rooftops that also show points of building heat transfer and loss, see STAIN DIAGNOSIS on BUILDING EXTERIORS.

When investigating a building for a mold problem, you can save mold test costs by learning how to recognize Stuff that is Not Mold or is only Harmless Mold but may be mistaken for more serious contamination.

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Diagnostic Guide to Ceiling Stains: Building Air Leaks & Thermal Tracking Marks at Interior Ceilings

Long, narrow, parallel black or gray ceiling stains in buildings may be due to thermal tracking as we explain here.

Definition of thermal tracking or "ghosting": sooty or dark lines or stains appearing in patterns on building ceilings or walls on the inside of buildings, especially in older homes whose interiors have not been re-painted or cleaned in some time. Thermal tracking or ghosting stains usually map the location of cooler surfaces such as the locations of ceiling joists, wall studs,or areas of smaller amounts of or completely missing building insulation. Note those dark "stripes" extending along the ceiling? These ceiling stains probably mark the location of ceiling joists (where the in-room ceiling surface temperature was kept a bit cooler since these locations in the ceiling cavity are occupied by a wood joist rather than by insulation). Thermal tracking ceiling stains, usually black, gray, or dark brown in color, may appear to follow the lines of ceiling joists (and will typically appear roughly 16-inches on center or 24-inches on center depending on how the building was framed (photo at left).

Sooty or dark smudges or stains appearing near the ceiling on the inside of building exterior walls or at the ceiling-wall juncture, especially in older homes whose interiors have not been re-painted or cleaned in some time. Thermal tracking stains may appear at the top of the wall and extend onto the ceiling surface such as shown in this photograph.

Thermal tracking or "soot tracking" (also called thermal bridging or ghosting stains) such as shown in the photograph at the top of this page may be found wherever moisture condenses on cool building surfaces.

How to Diagnose Stain Patterns on Cathedral Ceilings

Cathedral Ceiling Stains: Alan Carson, a home inspection educator and researcher, points out that on cathedral ceilings, where we see dark streaks following exterior studs or roof joists, (on cathedral ceilings very often), this is often associated with condensation on the wall or ceiling surface, and dirt in the air sticking to the condensation. When the condensation dries, you just have the dirt following the stud or joists lines. In the upper left section of our photograph below you can see stains marking the location of ceiling joists.

Ceiling stains over lights: Alan Carson adds that we also see dirt marks on ceilings above light bulbs from chandeliers or other light fixtures using incandescent bulbs. For example, if there are five candelabra type bulbs on a chandelier, we will often see five dark rings on the ceiling above. These relate to airborne dirt being thrust against the ceiling as a result of the thermals that are created by the heat of the light bulb.

Questions & Answers About Indoor Ceiling & Wall Thermal Tracking Stains

Question: Are these stains around ceiling air supply registers due to thermal tracking?

These interior stain photographs are from my daughter's home in Tulsa, OK - could this indicate thermal tracking? It worries me, but her in-laws say it's just dirt.

She has a special-needs 2 year old who has allergies and frequent bouts of pneumonia.

Reply: No. Here is how to distinguish between thermal tracking stains and dust deposited from the heating or air conditioning ductwork

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem, in this case perhaps a problem with your heating system, air filters, or maybe dirty ductwork, or perhaps finally, an extra source of dust and debris in the building indoor air.

But the pattern of stains and debris in your photos looks as if it is dust and debris being deposited by air blowing out of HVAC supply registers - this is not thermal tracking. Your stain photo above shows that dirt and debris have been deposited on a textured paint ceiling in a dark fan-shaped pattern radiating out from the air supply register at the top of the wall. Additional debris stains at either side of and just below that same air register track where air probably leaks out around the register cover edges.

The differences between thermal tracking and indoor dust or stains that are coming from the HVAC system include:

• Thermal tracking stains generally will appear in stripes on exterior walls and on building ceilings where the cooler surface associated with the wall or ceiling framing cause a slightly higher moisture condensation level in those areas, causing in turn, building dust to adhere more rapidly and at greater levels than elsewhere on those walls or ceilings.
• Ductwork dust and debris stains tend to be located close to air supply registers and appear in a "fanning out" pattern around the register, on a nearby ceiling, wall, or floor. In your second stain photo (left) we see that the dirt and debris are visible right on the register grille itself.

So it looks as if your daughter's in-laws are probably quite correct - it's indoor dirt, or more technically, airborne dust and debris being blown out of the air supply registers.

I agree that there can be a relationship between dusty or dirty HVAC systems or the absence of a duct system filter and debris stains on indoor surfaces that show up as thermal tracking. If debris is being picked up and blown through the duct system, and if we also have conditions that invite thermal tracking (minimal insulation, cooler walls, higher indoor moisture, for example), the dust being delivered by the duct system might indeed become deposited in a thermal tracking pattern - the striped pattern that is described above.

But your photos do not show that characteristic thermal tracking pattern - the stains in your photos are not thermal tracking in your case.

What to do about dust, debris, or dirt stains around indoor air duct supply registers

You might want to start addressing the debris stains in your photos by checking for the presence of an air filter in the duct system; if no filter is installed at all, one is needed. If a filter is already installed it may be dirty and needs to be changed.

Typically air filters in residential warm air heat or air conditioning systems should be changed monthly.

Check the ductwork and air handler for high levels of dust and debris. If necessary the ductwork, and air handler may be cleaned.

Considering that the youngster is reported to have recurrent respiratory illness, ask the pediatrician or pulmonologist about the advisability of improving the indoor air quality by reducing the overall dust levels (housecleaning, getting rid of carpets, improved air filtration, etc.) and about the need to have the building inspected by an expert who might look for evidence of mold, rodents, insects, etc.

More indoor air quality improvement advice that might help is found at INDOOR AIR QUALITY IMPROVEMENT GUIDE and at AIR FILTERS for HVAC SYSTEMS. To improve the level of air filtration see AIR FILTERS, OPTIMUM INDOOR and also AIR FILTERING CONTINUOUS FAN OPERATION.

Frequently Asked Questions (FAQs) about indoor ceiling & wall thermal tracking stains

Question: can't I just paint over thermal tracking stains to eliminate the problem?

Can I eliminate the stains on my ceilings and walls due to possible thermal tracking staining? Can I just apply two coats of Kilz to correct the problem before I paint? - pistolpete1950@hotmail.com

Reply: a cosmetic cure for thermal tracking should also include identifying and fixing the cause of staining

Pistolpete:

Painting over thermal tracking stains or many other types of stains on a ceiling, using a sealant such as Kilz™ or Bin™ or Enamelac™ (we get good results from) lacquer based primer sealers will make for a cosmetic repair.

But if you don't address the original cause of the stain, such as missing insulation, high interior moisture, sources of dirt or soot, building leaks, etc., then the stains will return.

Question: diagnosing ceiling and wall soot stains after heavy snow and a power outage?

after a major snowstorm last week, and 2 week power outage, i see soot stains on the ceiling and walls, with many soot spots, near a ceiling AC return vent. The AC compressor and air handler are unplugged and not working since the fall, so no air should be moving through the vent. Is it possible that somehow air came out of the vent, carrying with it soot from the AC ducts? - Allan

Reply: possible sources of black staining or "soot staining" during and following a power outage

Allan,

It might be helpful if you'd send me some sharp photos of your stains (use the CONTACT link found on our pages). Without seeing the pattern and knowing nothing else about your home I can only speculate.

Sometimes we "see" a stain for the first time when something makes us notice it, but the stain might not really be new. We need to know the pattern, location, and probable cause of your ceiling and wall stains in order to best suggest a solution to the cause. Some examples that might be related to snow cover and a power outage could be

• Use of a fireplace or woodstove during a power outage might add soot or smoke particles that deposit on walls or ceilings
• Watch out: Soot produced by a heating system whose draft was interfered with by snow cover over a too short chimney or blocking combustion air intake - check that your heating equipment is operating safely. I know, power was off, but at power-back-on a malfunctioning heater can produce soot, potentially dangerous.
• If the stains are really ONLY at ceiling A/C air supply registers and those ducts and registers are used only for cooling, then yes, there can be air movement in and out of ceiling ducts and registers, by convection, during cold weather even when the A/C system and blower are turned off. We discuss this air movement in and out of ducts in more detail at Duct System Air Movement.

Questions & Answers regarding this article

Questions & Answers About the diagnosis, cure, or prevention of indoor ceiling & wall thermal tracking stains.

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• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.
• We welcome more thermal tracking, soot tracking, air bypass leaks, and similar photos of indoor stains as well as text suggestions to expand this detail and would be glad to credit contributors.

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