Vertical black stripe stains on walls are most likely thermal tracking or ghosting (C) InspectApedia Causes of Dark Lines or Rectangular Stains on Indoor Walls or Ceilings: Ghosting Stains
Building Air Leaks & Heat Loss Points show up as thermal tracking stains

  • GHOSTING DARK STREAKS or LINES: CAUSES - CONTENTS: what causes those dark lines or rectangular stains on building interior walls & ceilings? Photos & text identify thermal tracking, thermal bridging, air bypass leaks, insulation defects and air movement in buildings.
  • POST a QUESTION or READ FAQs about indoor stains and thermal tracking or ghosting on walls and ceilings

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Indoor stains appearing in streaks or dark lines or in rectangular areas in buildings traced to black or dark thermal tracking or ghosting:

What caused dark stain lines on walls or ceilings usually spaced at regular intervals, or dark rectangular stains on walls or ceilings? A combination of thermal tracking or ghosting and insulation voids may explain these indoor stains.

This article series describes & diagnoses the cause of various interior wall and ceiling stains and explains how to recognize thermal tracking, (also called ghosting or ghosting stains or thermal bridging stains), building air leaks, and building insulation defects.

Often these stains are mistaken for toxic indoor mold.

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Why does Thermal Tracking or Ghosting Often Appear in Streaks or Lines?

Photograph of thermal tracking on an indoor wall

[Click to enlarge any image]

Article Contents

Thermal tracking may mark the location of building framing members: In a conventionally-framed wood structure, wall and ceiling framing is typically spaced on 16" or 24" centers, and thermal tracking will tend to cause dust or soot to adhere to the interior surfaces at these locations. You can see this phenomenon in our ceiling stain photo above and in the wall stain photo at the top of this page.

But thermal tracking or bridging stains may occur on different intervals depending on how the building was constructed, where air is moving, where air leaks are occurring, and where ever building surfaces are cooler or more moist.

Ghosting or thermal tracking spots in parallel lines: This interior black wall stain pattern is particularly easy to identify on walls where in less extreme cases of ghosting you may just see dark spots in relatively straight parallel lines marking the locations of studs and joists: those spots are the locations of drywall nail or screw heads: see WALL THERMAL TRACKING STAINS for some interesting examples of spots at drywall fasteners.

The reason thermal tracking tends to mark the location of building framing members is because the interior wall or ceiling surface will be cooler (during the heating season) where framing members (joists or studs) are located.

The insulating value of wood is pretty low (about R1 per inch) compared with fiberglass insulation or other insulating materials. These points of increased building heat loss, caused by the presence of solid ceiling joists or wall studs separating building insulation are also called points of thermal bridging - points where there is more building heat loss than through the building insulation itself.

The sections of an interior wall or ceiling which are touching wood framing (inside the ceiling or wall cavity where a ceiling joist or wall stud was placed) will conduct heat to the outdoors faster than the "in between" sections of wall where insulation has been placed. In sum, 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.

In sum, 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.

Interior stains help diagnose building conditions: Since thermal tracking, or soot marking, or "thermal bridging" as a few folks call it usually tells us something about a lack of building insulation or about air leaks in buildings, we can use these marks or stains to learn important facts about a building.

Causes of Dark Areas, or Rectangular Stains on Ceilings or Walls

Photograph of thermal tracking on an indoor wall

[Click to enlarge any image]

Sooty or dark smudges or stains appearing near the ceiling On the inside of building exterior walls, 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.

Note those dark "stripes" extending along the ceiling and into the room? 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).

See CEILING STAIN DIAGNOSIS for details of diagnosing stain patterns on building ceilings and on cathedral ceilings.

Dark rectangular stains on ceilings showing missing insulation and ghosting or thermophoresis stains (C) InspectApedia TD

The dark rectangular stains on the kitchen ceiling shown in this reader-contributed photograph probably mark areas of insulation voids in the ceiling in the Australian home shown above. Additional air leakage and convection currents occurring around recessed ceiling lights (pot lights) can increase the air movement into such ceilings and thus increase staining around the lights. [Thanks to Aussie reader T.D. for contributing this photo 1/3/2016]

Parallel dark lines that are not thermal tracking

Parallel dark stains on walls that are just shadows (C) reader photo

The dark parallel wall stains above are probably just shadows. Details about this picture are found at STAINS MISTAKEN for GHOSTING

This photo along with more photos of actual thermal tracking that was present in this home are also discussed at THERMAL TRACKING GHOSTING FAQs

Reader question: will these stains reappear after we clean and paint the stained surfaces?

I just read your article on Thermal Tracking, Bridging and Ghosting, and believe I may have this problem in my home. I've had several builders around to look, and none of them have ever seen anything like it. We've owned the house for 3 years now, and it seems to have started around winter last 2015 (I live in Australia so May/June). We don't have any open fireplaces or anything, just 2 reverse cycle air conditioners.

We recently cleaned out the filters on the living room air conditioner and it was filthy, so I was wondering if they may have been contributing to the problem. The house was poorly insulated, however I insulated the entire ceiling around June of 2015. I recently cleaned some areas of the house with sugar soap and warm water, and the marks appear to come off, or just smudge around the wall/ceiling. Are you able to tell me what you think the problem might be, and the solutions to fixing it.

It seems to be slowly spreading to other areas of the house also. We were going to repaint the ceiling and walls, but I just want to know what this is and if it will come back again after I've repainted it. - T.D. 2016/01/03

Reply: Yes stains will recur until we correct the causes of thermal tracking or ghosting

Yes the chimney effect caused by the holes cut into a ceiling to mount pot lights (recessed ceiling lights) will increase the air movement around and up through each light, increasing deposits of building dust and soot thereon. But the root causes of thermal tracking remain:

  1. Missing insulation - cooler areas will see more condensation of moisture on the wall or ceiling and thus will collect more airborne dust particles
  2. Air leaks into a ceiling or wall will often increase dust deposition around the points of air leakage
  3. Indoor soot and dust sources - including normal house dust sources - more dust sources = more rapid stain formation. We see this in buildings where people use scented candles, fireplaces, smoke, or where an oil burner is not properly maintained and is producing soot; pets, particularly big long-haired dogs who bring dust indoors also increase ghosting stain formation.
  4. The indoor humidity level - more humid = more rapid dust deposition

Watch out: I agree that you don't want to insulate over recessed lights if they are not rated for that application as doing so can cause overheating or even a fire; and changing all of the recessed lights to those rated for direct contact in an insulated ceiling (DCIC) so that they can be insulated over is a desirable improvement but not a cheap one.

But if we don't correct the causes of thermal tracking or ghosting it will return after cleaning and painting.

Reader follow-up:

I've been up in the roof today, and there are areas around the downlights that aren't insulated, but they've been left there so that we don't start a fire in the roof from the insulation coming into contact with a hot surface. I will have to look at some heat shields for the downlights so we can insulate the entire area.

Another interesting thing, which I think could be a definite problem is the exhaust fan duct from the kitchen goes into the ceiling but doesn't go any further, so any hot, fatty, humid air from Cooking goes into the ceiling but can't escape. Could this be coming back down through the holes for the downlights and sticking to the ceiling causing these marks? The downlights do have a black sticky residue around them that spreads when touched with a wet cloth.


Watch out: Venting a kitchen exhaust into a ceiling not to outside is unsafe and probably violates your local building codes. Over time the accumulation of grease can cause a building fire. I would stop using the fan entirely until it can be vented to the outdoors.

You typically need 3" of clear un insulated space around the ceiling pot lights but the light fixture should contain a label with the specific clearance distance for insulation; Those large dark rectangles on the ceiling suggest large cold areas of missing or incomplete insulation.

If it were my home I'd change over to DCIC rated lights that can be covered by insulation.

Insulation leaks & thermal bypass defects: We use thermal tracking marks on an area where insulation is visible to identify and correct air bypass leaks, thus saving energy or reducing home heating or air conditioning costs. Details about air leaks in buildings are discussed at AIR BYPASS LEAKS.

Building air leak testing is described at BLOWER DOORS & AIR INFILTRATION


Insulation adequacy: Clues that suggest Insulation adequacy can be picked up easily if the observer will use thermal tracking marks on larger areas of interior walls or ceilings to tell us areas of the building that are not insulated, or are not well insulated - areas where we should consider adding or improving insulation to save energy by reducing building heating or cooling costs.


Usually soot marks, thermal bridging, or thermal tracking stains appear, if at all, in the building interior locations listed just below discussed in the remaining sections of this article.

Article Series Contents


Continue reading at GHOSTING DARK STAINS on INSULATION, AIR BYPASS LEAKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see WALL THERMAL TRACKING STAINS - diagnosing ghosting stains on walls


Or see THERMAL TRACKING REMEDIES - curing ghosting stain problems in buildings

Or see AIR MOVEMENT in BUILDINGS - When, where, how & why air moves in buildings.

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