Thermal bridging soot stains on a ceiling (C) InspectAPedia.comCeiling Stains & Thermal Tracking
How to Diagnose Indoor Ceiling Stains & Building Air Leaks

  • CEILING STAIN DIAGNOSIS - CONTENTS: Definition of thermal tracking or ghosting stains appearing on building ceilings (or walls): cause, diagnosis, cure and prevention. What causes those long parallel dark stains on building interior ceilings? Photos & text identify thermal tracking, thermal bridging, air bypass, insulation defects and air movement in buildings. How to recognize poorly insulated building walls or ceilings and how to pinpoint building air leaks. A photo-guide to common indoor ceiling and wall stains and what they mean. Links to more photos of indoor ceiling or wall stains that are not toxic mold
  • POST a QUESTION or READ FAQs about the diagnosis, cure, or prevention of indoor ceiling & wall thermal tracking stains
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Causes of stains on ceilings:

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. 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

Photograph of thermal tracking on an indoor wall

[Click to enlarge any image]

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

Common sources of ceiling stains

Ceiling stains are most often due to one of the following:

The size, shape, location, pattern of any building stain form important clues in diagnosing its cause. King among the easy-to-diagnose leak patterns is thermal tracking or ghosting as that condition tends to produce regular or pattern stains.

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).

Photograph of thermal tracking on an indoor wall

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.

Warm moisture-laden air touches the cooler surface of a building wall or ceiling, giving up some of its moisture to the surface as condensation.

Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited (C) Daniel Friedman 02-12-16

As air moves through the building, typically up walls and across ceilings, debris in the air, particularly soot such as that left by burning candles (scented candles may be more of an IAQ issue), adheres more to the damp surfaces than to others, leaving black marks or "tracks."

In a conventionally-framed wood structure, wall and ceiling framing is typically on 16" or 24" centers.

The wall or ceiling interior surface will be cooler where the framing is located than will be the spaces which are not touched by framing and which, perhaps, are insulated. So if you see black streaks up the building wall in a regular 16" or 24" pattern, particularly on cooler exterior walls but potentially anywhere, it may be thermal tracking.

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.

Ceiling stains at recessed lights © Daniel Friedman

Black rectangular stains on ceilings: that are about the width between ceiling joists are likely to mark areas where insulation has been reduced or omitted entirely - as you can see in our page top photo.

In a cathedral ceiling in which older recessed lights have been installed, you may see a combination of patterns:

Ceiling stain where insulation was left out © Daniel Friedman

Ceiling stains where insulation has been omitted completely: may be very apparent such as this photograph taken in a garage.

The garage ceiling was insulated except along one side where you see the long dark rectangular stain. This side of the garage was also an outside wall. In a damp garage we might find mold growth on this area.

See GHOSTING DARK RECTANGULAR STAINS: CAUSES for more examples of ceiling stains caused by poor insulation and/or air bypass leaks.

Examples of Diagnosing Indoor Ceiling & Wall Thermal Tracking Stains

Dust and dirt stains on a ceiling around an air supply register (C) D Friedman

Reader 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.

Dirt stains at a wall air supply register (C) D Friedman

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

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.

Reader Question: are these dark ceiling stains probably due to thermal tracking ?

Ceiling stains suggesting thermal tracking - what to check next (C) 2013 Daniel Friedman & D.C.

Does this look like "thermal tracking" to you? Here are some pictures of our ceilings on the exterior walls of the house.

North and south sides both (straight rancher. Started showing up a few years back 3 or so. Could this be due to the ac dehumidifier pulling moisture out of the house and actually pulling so much that it pulls from the eaves thru the insulation, then through the sheetrock from our attic? - D.C. 1/10/2013


A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose the causes of indoor stains, and s/he might find other causes than what we can offer here, such as an ice dam leak problem, an indoor soot source, or other causes.

That said, your photos look like the dark areas might be due to thermal tracking possibly related to inadequate insulation in the ceiling at the house eaves -particularly if all of the stains on ceilings are at / along exterior walls - in two of your photos I could confirm that the stains are at what looks like an exterior wall, but I also see through a window or door what may be an outdoor porch.

Sometimes where a porch roof is attached to an existing building or building roof, the porch roof is at a lower slope than the roof on the main building. In that case, we may find inadequate ventilation under the roof eaves of the main building along with an accumulation of snow or ice at the point of change in roof pitch. Either or both of those conditions could contribute to moisture or leaks in that area.

In turn, leaks from ice damming or accumulated snow in that area can dampen the ceiling that in turn invites accumulation of dust or soot stains.

Check the insulation in the attic; Also check that you have not had moisture or leaks in those ceiling areas due to ice dam leaks. Keep us posted.

Reader 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


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

Cures for Thermal Tracking Stains on Ceilings & Walls

Reader 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? -

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


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.


Continue reading at GHOSTING DARK RECTANGULAR STAINS: CAUSES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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