Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16 Wall Stains & Thermal Tracking
How to Recognize & Diagnose Stains on Building Walls

  • POST a QUESTION or READ FAQs about relating thermal tracking ghosting marks indoors to building insulation defects, air leaks, heat loss, and heating bills.

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Diagnose & fix stains on building interior walls.

This article describes the role of air leaks and soot deposition in indoor stains - clues that can sometimes explain high building heasting costs. Thermal tracking or ghosting stains may indicate insulation voids, air leaks, high indoor moisture, as well as serving as a collection point for airborne dust, dirt, soot, or other debris.

This article describes how to recognize & diagnose thermal tracking and other dark or sooty stains on building interior walls and discusses other 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. When investigating a building for a mold problem, you can save mold test costs by learning how to recognize stuff that is not mold but may be mistaken for more serious contamination.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How Air Leaks Cause Thermal Tracking or "Ghosting" Stains on Building Walls & Ceilings

Thermal bridging or thermal tracking soot stains on interior walls © Daniel Friedman

The black marks or stains or "sooting" observed in a characteristic pattern on building walls or ceilings are comprised of substances that cause black sooty marks on building interior ceilings or walls and sometimes also on carpets are from sources and causes that you can easily learn to recognize in buildings.

Indoor air contains dust particles, whether you see them directly or not. As this air passes over building surfaces, where a surface area is cooler than surrounding materials, these particles are usually deposited at a higher rate (due to higher moisture on the cooler surface).

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.

In this case the exterior wall surface (see that window at right in the photo) are cooler where the wall studs are located and warmer between the wall studs. Probably the wall cavity is insulated, and the insulated areas have a higher "R" value than the solid wood studs themselves.

So the wall is cooler where the studs are located, increasing condensation on those surface areas, and picking up a greater share of house dust in air moving across those surfaces.

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.

Wall Stains over Heat Sources such as Hot Water Baseboards

Photograph of thermal tracking on an indoor wall

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.

The stain and soot deposition pattern over heaters is complicated by variations in the heat and air movement coming out of the heat source, and may not always accurately map wall stud or insulation locations.

More wall stains from heat sources such as over an air supply register in a floor are described at OTHER STAINSon Indoor Walls & Ceilings and ceiling stains from heating or air conditioning registers are described

Why Black Stains May Appear on Building Interior Walls as Well as on Building Exterior Walls

Our photo at below left shows wide black rectangular stains on a wall where insulation was omitted. The photo at below right shows long narrow black stains on a wall where thermal tracking marked the location of wall studs, but where insulation was present. Why did both of these patterns appear in the same building?

High interior moisture (air handlers in wet crawl spaces, a wet basement) was most likely the determining factor in the extent of staining that occurred in this home. Moisture condensed (and caused adhesion of house dust particles) on cooler surfaces. In some areas the un-insulated wall was coolest and got stained (below left) while in other areas the wall studs were cooler than the insulated wall cavity, so the stud-areas got stained (below right).

Thermal tracking on a wall missing insulation © Daniel Friedman

Black, roughly rectangular stains on building interior or exterior walls: may be present wherever insulation has been omitted. While usually these stains are going to appear on the building exterior walls (which get colder) they may appear on interior walls as well. These stains on interior walls, which may be completely un insulated, may appear with greater severity where other construction details increase the flow of air and/or moisture in certain sections of wall cavity.

Thermal tracking at wall studs © Daniel Friedman

An example of the cause of this stain pattern is air movement up through the wall cavity from a cool, damp basement or crawl space. The heated building living space warms the open interior wall cavity, causing air in that cavity to rise by natural convection towards the building attic or roof cavity.

Air movement upwards in the wall cavity draws cooler air from the basement or crawl space. Where some wall sections have more air movement, less insulation, or greater moisture exposure, they may become more darkly stained than their neighbors or more darkly stained than other walls in the building interior.

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.

Black or Dark Spots on Walls: diagnose accurately before "fixing" or cleaning

Black spots on walls may be ghosting, thermal tracking, leak stains, or even metallic debris - wire wool "mold" !

Here are the photos before I identified the cause.

Dark spot stains on interior wall traced to steel wool pads (C) InspectApedia PW

They are sharp of the woodwork, but not as good as I'd like of the marks. I realise I should have used something with more definition like a ruler. If you'd like me to have another go then let me know. I can see that in less prominent situations, the wire wool could easily rust over time, leading to mysterious rust marks being discovered much later. - P.W. 7/8/2014

Dark spot stains on interior wall traced to steel wool pads (C) InspectApedia PW


I've had the same camera focus trouble. If you can find a thin flat metal ruler to place alongside a mark and then try again for a sharp photo that would be more useful, but I'll also go ahead and post these. Can we say something about just where in the home these marks appeared? for example are they only close to woodwork that was scrubbed with steel wool pads?

And how was paint applied later: by roller? I'm presuming this was a latex paint and that's why (reaction of the particles to moisture in the paint) we see the material telegraphing through the coating.

If you're curious & thus if you decide to try to capture some of the actual material on adhesive tape
(see DUST / MOLD TEST KIT INSTRUCTIONS ) you can mail that to me and I'll examine & photograph the material microscopically.

For other readers diagnosing black spots or stains on interior walls, because pets lying against walls can also leave black smudge marks, also

Reader follow-up:

I managed to get some clearer photos of the wire wool strands magnetically attached to the hidden nail heads - there was a setting on the camera that I didn't know about to focus close up. Here they are.

Wire wool (stainless steel cleaning pad debris) fragments collected on drywall nail head due to magnetic properties from hammering (C) Phiiip Westwell

Yes, thinking about where they are, all of the marks were either within 12 inches to the side, or below where some wire wool preparation had been done. I don't think any was above.

Wire wool (stainless steel cleaning pad debris) fragments collected on drywall nail head due to magnetic properties from hammering (C) Phiiip Westwell

The painting had already been done (brush and roller). I was varnishing after the completed painting, so the particles stuck (presumably through magnetism) to a completely smooth and dry surface. The bonding was also very weak and the fibres very light - holding the tip of a screwdriver two inches away was enough to make all the particles come through the air and attach to the screwdriver.

My understanding is that nails don't have any innate magetic force associated with them, but banging them into a surface can make them magnetic.


Indeed you're correct. Koch (2000) specifically states that hammering can cause molecular movement in metals and induce magnetism.

Induced magnetic effects of hammering on or striking nails, steel, or other metallic objects

Am I correct that you could simply wipe away these particles as they landed after the paint was dry?

Reader follow-up: metallic particles adhered to drywall nails simply wipe away

That's very interesting. It seems the opposite of what we were taught at school i.e. if a magnet gets too many knocks then it will lose its magnetism.

You are right about wiping away the particles. If the surface underneath is dry, then they can be wiped off with a dry cloth.

Magnetic Drywall Screws Explain Wall Spots of Metallic Fragments

Here's one final postscript.

Last weekend I met the man who had put up the walls and discussed this with him. He explained that he hadn't, in fact, used nails to put up the walls: it appears that everyone around here now uses screw guns to attach plasterboard to a frame, as it's considerably faster than nails (and screws can be unscrewed to correct errors, or course).

The screws come in magazines and are manufactured magnetic to that they remain attached to the screwdriver bit while being positioned on the wall. That explains why the wire strands formed around every single screw, not just the odd one or two,

Of course that makes the idea of magnetising nails less relevant, but I think it raises a more interesting question. These screws will not lose their magnetism just because they are driven into the wall, so all modern houses (round here, anyway) will now have a magnetic field around every place where the plasterboard is attached to the wall. Over time, I wonder what will happen as a result of that.

Lessons learned about diagnosing indoor black spots on walls or ceilings

Knowing the answers to these questions can help diagnos black spot stains indoors:


Continue reading at THERMAL TRACKING BRIDGING GHOSTING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see GHOSTING DARK STREAKS or LINES: CAUSES to diagnose lines or rectangular areas of stains on walls or ceilings



Or see STAIN & BIODETERIORATION AGENT CATALOG causes and types of damage that raise aesthetic and conservation issues on cultural artifacts, buildings, and art works.

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