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This article 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. 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.

Thermal tracking is the deposition of house dust and debris onto walls and ceilings in patterns caused by a combination of air movement, interior moisture, and in some cases, a source of high levels of particles. Thermal tracking, sooting, bridging, and ghosting are synonyms for the same effect, one which often is exacerbated by defects in the building insulation and ventilation system.

However beware that on occasion the leaks and moisture in a building that contribute to thermal tracking may have created a mold problem somewhere else than in the black thermal tracking or soot marks you see on walls and ceilings. MOLD EXPERT, WHEN TO HIRE provides assistance in deciding if you should hire a mold investigator.

Because some clients have on occasion sent samples to our mold test lab that really should not have been collected, much less looked-at, we recommend that you review the photographs in these articles to see if the black stains you see are something other than mold. 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 - save your money.

© Copyright 2010 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

A Photo-Illustrated Diagnostic Guide to Thermal Tracking & Other Wall or Ceiling Stains: Moisture & Air Movement in Buildings

What is Thermal Tracking & Why does Thermal Tracking or Thermal Staining Occur Indoors?

What is thermal tracking or ghosting and what causes it? Thermal tracking or "soot tracking" such as shown in the photo at left and also 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. (See DEW POINT TABLE - CONDENSATION POINT GUIDE for details.) That makes some areas on an indoor surface slightly more damp than others. As air moves by natural convection through the building, it typically flows up walls and across ceilings. Airborne debris in the air, particularly soot such as that left by airborne house dust, by a heating system that needs service or by burning candles (scented candles may be more of an IAQ issue), or by cigarette smoking, adheres more to the damp surfaces than to others, leaving black marks or "tracks." Black thermal tracking stains may appear on interior walls and ceilings, not just on cool exterior building walls. It is also possible that an interior partition wall may be conducting heat out of the building through convective loop heat losses as well. (See Convective Loops & Thermal Bypass Leaks.)

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

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. 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. This interior black wall stain pattern is particularly easy to identify on walls - see Wall Thermal Tracking Stains.

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. 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.

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.

Air bypass leak identification: Thermal tracking may also mark the location of air bypass leaks - often a more significant source of building heat loss than thermal bridging at solid joists or studs. If a building top floor ceiling or exterior wall has many air leaks, even though the building is supposed to be well-insulated, the total of all of these air leaks may in fact add up to the same heat loss as leaving one or more windows or doors open in freezing weather. Air Bypass Leaks Marks on Insulation (such as our photograph at left) shows how to identify air bypass leaks by the dust marks left on building insulation. These black stains are from building dust deposition; in this case they are not mold. See MOISTURE PROBLEMS: CAUSE & CURE for an explanation of moisture sources and movement in buildings.

SAFETY WARNING about Thermal Tracking or Ghosting Black Stains Indoors

Air Leaks or Insulation as Causes of Thermal Tracking or Indoor Ghosting Marks

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 and at AIR LEAK DETECTION TOOLS.

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. (See INSULATION & VENTILATION INSPECTION & IMPROVEMENT for more information.)

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.

Humidity as a Factor in the Development of Indoor Stains on Building Surfaces

High indoor humidity: If indoor humidity is excessive (say regularly over 50 or 55% RH) we may be encouraging both staining from soot and house-dust deposition as well as mold growth in a building. Also we may be inviting excessive levels of dust mite activity which in turn increases the level of allergens in the building.

That is because higher humidity indoors provides more moisture to condense on cooler building surfaces whenever the temperature of a building surface reaches the dew point. Stated another way, if a building has low indoor humidity, the amount of moisture available to condense on cool surfaces is less, so the rate of thermal tracking or soot deposition on those surfaces is less - at least due to this factor.

See DEW POINT TABLE - CONDENSATION POINT GUIDE for details about the dew point and how to measure or calculate it for a building area or surface. Also see HUMIDITY CONTROL TO PREVENT MOLD.

Thermophoresis as a Factor in the Development of Indoor Stains on Building Surfaces

When condensation occurs on a building surface at any level that makes that area more damp (and thus sticker) than its neighbors, the temperature and humidity-related particle deposition described above is almost certainly occurring in any building, and will be more noticeable in a building with high levels of airborne particulates and dust. 

But as IAQ consultant Steven Temes, an industrial hygienist and microbial consultant points out, ultrafine carbonaceous particles (such as candle soot) also accumulate on the colder surfaces due to a little known physical phenomenon called thermophoresis.  This has to do with the driving force of the particle motion being its kinetic energy (when not overwhelmed by air currents) and the movement of the particle toward the cold surface, where it "plates out", adhering to the surface.

If the particle has an electrical charge (such as in a home where ozone generators or negative ion generators are in use) this plating-out process will be significantly increased. If you are using an ozone generator as an "air purifier" indoors, be sure to review OZONE AIR PURIFIER WARNINGS and also OZONE HAZARDS.

Indoor Air Particle Level as a Factor in the Development of Indoor Stains on Building Surfaces

Even if the building humidity levels are low, high levels of indoor dust and debris can still lead to indoor stains and thermal tracking marks. Significant sources of soot, such as improperly operating heating equipment, use of candles, fireplaces, woodstoves (if they release smoke into the building interior), some pets such as large dogs, or simply generally poor housekeeping can all increase the indoor air particle level and lead to soot and stain marks.

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

Air Movement Patterns as Factors in the Development of Indoor Stains on Building Surfaces

Specific and non-uniform moment of building air can lead to uneven soot or house dust deposition on building surfaces, and thus will cause stains and dark areas that are non-uniform . Surfaces across which more building air moves are exposed to a greater volume of air and thus a greater volume of dust particles.

Multiple Complex Factors Determine Where Stains Appear in Buildings

Conflicting variables are at work in the deposition of soot and debris on building surfaces. Here are some examples:

• More dust where more air moves: Building surfaces exposed to more air movement are exposed to more airborne dust and debris - so may become stained.
  
  This phenomenon is most apparent where house dust and debris are deposited on and around air return registers in buildings. Where the air filtration system is not effective or the duct system is dirty, dust and debris are also deposited on and around warm or cool air supply registers. (This dust is most often harmless house dust comprised mostly of skin cells and fabric fibers.)
• Less dust where more air moves: Building surfaces exposed to more air movement might, however, also be kept warmer and more dry due to that same movement, so the same areas may become less stained than their neighbors.
  
  This phenomenon is most apparent in a building closet, or on a wall behind a picture or behind drapes, because less air moves over those surfaces, leaving the surface cooler, exposing the surface to higher moisture condensation, and thus causing more house dust (or soot) deposition than in other areas.

Visual Inspection by an Expert can Usually Sort Out the Causes of Building Stains

Conceptually, the deposition of dust, soot, or debris on building surfaces out of moving air in buildings (as opposed to caused by animals or people touching surfaces) is a complex linear equation that is weighing different and conflicting factors.

Luckily, the visual inspection of the stained areas, combined with inspection of the building for moisture problems, insulation and ventilation problems, or for soot and debris sources, can normally identify the dominant effect and can with confidence conclude the cause and thus suggest the cure for these stains.

In the following sections of this article we describe the ways that stains appear in buildings, what causes these stains, what they mean for building condition and safety, and how they can be cleaned, remedied, and prevented in the future.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.
• Steven Temes, is an industrial hygienist and certified microbial consultant specializing in is investigating the causes of building-related health effects. Mr. Temes can be reached at AirWays Environmental Services, P.O Box 8576, Red Bank, NJ 07701, 732-842-5564 (office) or 732-673-9914 cell, or by email to AIRWAYSENV@cs.com

Building Air Leaks & Thermal Tracking Marks at Interior Ceilings

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 Thermal Tracking Marks for details of diagnosing stain patterns on building ceilings and on cathedral ceilings.

Air Leaks Causing Thermal Tracking Stains on Building Walls

Thermal tracking stains may appear on the building walls themselves, in a pattern of vertical "stripes" which may be roughly 16" or 24" on center, marking the (cooler) location of wall studs or ceiling joists. Thermal tracking stains may appear as dark black/brown wall stains appearing close to floor level, above heating baseboards mounted on exterior walls are probably due to thermal tracking. See Wall Thermal Tracking Stains for details of diagnosing stain patterns on building walls and above heat sources.

Heaters can cause other dirt or soot stains on walls: But beware, a baseboard heater itself causes an air convection current up through the heating baseboards and in a dirty home or in a home occupied by pets or smokers similar stains may appear on other building interior walls (such as in this photograph) even if the wall is well insulated. If staining appears vertical and at intervals thermal tracking is probably occurring.

How to Recognize Air Bypass Leaks and Thermal Bridging Marks on Fiberglass Insulation

We have frequently observed evidence of air bypass leaks and thermal tracking as dust deposition on attic insulation, a condition some owners have mistaken for mold (and wasted money on mold testing and remediation). In this photo we've removed a "nearly new" insulation batt that was installed in a cantilevered floor which was leaking air up through the overhanging floor and into the building wall cavity. In just a month of exposure the insulation is already showing dust tracking at its lower right corner, marking the air leak path in this building. See Air Bypass Leaks Marks on Insulation for details of diagnosing stain patterns on building or on building surfaces where insulation is missing or poorly installed.

How to Use Indoor Stains Like Thermal Tracking to Diagnose Building Air Leaks, Insulation Defects, and Indoor Air Quality Problems

Significance of Other Indoor Stains and Marks for Building Framing Location or Defects in Heating & Cooling System Operation

Since thermal tracking, or soot marking, or "thermal bridging" always tell us about missing or openings in building insulation or about air leaks, we can learn other facts about a building such as those listed here:

• Location & spacing of building framing members. Indoor stains can help find the location and spacing of wall studs or ceiling joists: use thermal tracking marks on building interior surfaces to spot the location of wall studs or ceiling joists when we need to find those members (for placing a picture nail, for example). Observing the spacing between these stains can tell us how a building has been framed.
• Identify unsafe heating equipment or poorly-functioning fireplaces: use thermal tracking marks on interior building walls and ceilings, particularly where they vary from room to room, to diagnose poorly-vented chimneys such as fireplaces that do not work well (also look for soot stains at the hearth face). Such marks may also indicate poor and even unsafe operation of central heating systems since a poorly maintained oil burner will often leak soot into a building interior, and a dangerously un-vented gas appliance may also produce both soot and potentially fatal carbon monoxide gas.

How "Read" Indoor Stains to Identify Indoor Air Quality Concerns

A careful examination of the location of indoor stains permits the observer to use thermal tracking or soot marks on building walls or ceilings as an indicator of possible excessive (seasonal) interior moisture or other potential indoor air quality concerns.

Dark stains on building interior walls may appear in other patterns and could be from other causes - we provide photographs, description, diagnosis, and advice for many of these indoor stains in this article.

How to Diagnose Stains and Debris at Ceiling and Wall Heating or Cooling Air Supply Registers

Dirty HVAC duct work, lack of air filter maintenance, or furnace malfunctions can all be indicated by black stains that appear around warm or cool air delivery registers in building ceilings or walls. (This is usually a deposition of house dust, rarely is it mold, and rarely is it related to thermal tracking. An investigation of what was at first called Killer House Dust from an HVAC system showed that the dust of concern turned out to be cotton and other carpet fibers having nothing to do with the Heating or Cooling equipment was discussed at our Fear of Mold WebLog or "Blog" where we periodically post results of interesting forensic investigations.

How to Recognize Pet Stains on Building Walls

How to Recognize Wall Stains from Human Occupants

How to Recognize Wall or Ceiling Stains due to use of Candles, Woodstoves, or Fireplaces

Thermal tracking on buildings combined with this evidence can also be a clue useful in diagnosing indoor air quality complaints. In these photographs, all of the stains visible appear due to use of candles. Note the absence of framing-interval staining (16" or 24" on center) which would have been characteristic of thermal tracking, even of the candle soot, had the building surfaces been cool due to poor insulation.

Excessive indoor humidity: If indoor humidity is too high we may be encouraging mold growth in a building or we may be inviting excessive levels of dust mite activity which in turn increases the level of allergens in the building.

Other Sources of Dark Stains on Indoor Walls & Ceilings

If you have frequent fireplace fires, cooking, or if you burn scented candles, if people smoke in your home, or if your oil-fired or gas-fired heating system is not working properly, the added soot particle load in the building air is not only a health concern (soot and potentially lead), it also will mark the building surfaces in this characteristic pattern.

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.

WHAT MOLD LOOKS LIKE
DIAGNOSE OTHER DARK STAINS on Indoor Surfaces

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