Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16Carpet Stain Diagnosis
Diagnose Indoor Carpeting & Rug Stains: Air or Water Leaks, Mold, Pets, Spills, Weaving

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Carpet stain diagnosis, cure, prevention guide: this article describes how to identify and diagnose carpeting stains at the edge of carpets near walls 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.

We describe mold stains on carpeting and also other sources of carpet stains that may otherwise be hard to diagnose.

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.

Stains & Marks on Floor Carpeting - a Diagnostic Guide

Black stains on carpet at walls (C) Daniel Friedman JBArticle Series Contents

Often dark rug or carpet stains are mistaken for toxic indoor mold - but they may not be mold at all. Our page top photo shows stains on carpeting left when a storage cabinet was moved. Further inspection for water damage, leaks, and mold would be appropriate if you see marks like this. Some common sources of carpet stains include:

[Click to enlarge any image]

Building Air Leaks & Thermal Tracking Marks on Floor Carpeting Near Walls

Stained carpet due to thermal effects © Daniel Friedman

Carpeting Stains along the wall/floor juncture: Alan Carson, a home inspection educator and researcher, has pointed out that we often see dark marks at the edge of floor carpeting where it touches or is close to wall baseboard.

These marks are likely to be most severe where the carpet meets exterior walls.

These carpet stains at walls occur most often on the lower floors where we get infiltration (air moving in to the building space) from the basement or crawl space below. It is the counterbalance to the ex filtration (air moving out of the building space) issues found at the attic level.

As outdoor, basement, or crawl space air leaks into the building around wall-floor intersections, the edge of the carpet acts like a filter and collects dirt from the air leaking in.

These carpet stains are not mold: Some people mistake these "carpet edge stains" for toxic "black mold".

If the carpet stains are pretty much continuous along the floor/wall juncture of carpeting and not elsewhere, and particularly if they do not appear elsewhere and are not related to other building water leaks, it is not very likely that these stains are mold.

About these photographs of carpet stains: Thermal tracking or "soot tracking" is visible along the wall baseboard in the photograph at page top - notice that gray line about an inch wide along the carpet where it meets the wall?

The more ugly dark brown stains at right angles to the wall are not thermal tracking but indicate that this floor was wet, causing rust or bleeding from a cabinet which had been placed in this location. This photo is therefore interesting (if a bit confusing) because it shows two different types of carpet staining.

High indoor humidity: If indoor humidity is excessive (say regularly over 50 or 55% RH) 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.

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.

What Should We Do About Dark Black Shadows on Carpeting Around Walls or Furniture?

Black stains on carpet at walls (C) Daniel Friedman JBReader Question: We have a problem that none of our local "experts" (builder, carpet cleaner, pest guy) have any idea about or have ever seen. It presents as dark/black shadows around walls or an object that has laid on the carpet floor for awhile (months).

Our HVAC filters get black in It is very hard to get up. It is like something embedded in the carpet fiber. It occurs more upstairs than down.

The house is 4 years old, no pets, two adults, no smoking, in a new development in inland South Carolina, no apparent moisture problems, heat pump upstairs, gas down. [Photo at left].

We need advice on what to do. - J. B.

Reply: Decide if the Stains are Abnormal, Identify Probable Particle or Debris Sources, Correct the Source, Clean the Carpets

The fact that the dark stain is in such a straight line suggests that furniture or something else placed along the wall was protecting the "clean" area of carpeting from airborne soot or debris deposits, just as you suspect.

Start by taking a look at the common indoor stain types and sources we describe

at STAIN DIAGNOSIS on BUILDING INTERIORS - that might explain what you're seeing on your floors.

It's useful to make a distinction between dark stains that occur on carpets, floors, walls, ceilings due to deposition of ordinary house dust due to thermal tracking and stains that come from an abnormal or avoidable or even a potentially unsafe source such as gas fired heating equipment that is not working properly, leaks, or mold.


Also, from your description I wonder if you have a soot source - is your heating system oil-fired? Or worse, if it's gas fired and making soot that's very dangerous and needs immediate attention.

Watch out: soot produced from gas fired heating equipment of any kind is potentially very dangerous, even fatal, as it's a symptom of improper operation and possible carbon monoxide production.

Next Step: Initial Theory of Where the Soot Stains are Coming From: Ventless Fireplace

Here's our theory: The "dust" is soot from the gas pilot light in the downstairs ventless fireplace. It has been on since November. Off for a week. I have a filter element, installed in November, from the downstairs AprilAire that is black. The downstairs heat is gas - checked out OK last week.

Upstairs HVAC is a heat pump, also checked OK. Upstairs is little used, the heat/fan is normally off. House has always been dusty, not this black color. The soot floats upstairs and is conveyed down to the carpet by local downdrafts, against the cooler walls.

Anything in the way creates unusual patterns. Inside surface of all windows in house - black when cleaned. Outside not, garage not.

Does that theory make sense? How do we prove it? Should I send samples? If so, what's the procedure, how many, from where, cost?

Most important, how do we clean? A paid attempt, local ChemDry franchise, had never seen anything like it, wasn't sure that they could get it all out - didn't.

Reply: Correct apparent soot sources, check for other unsafe soot sources, clean, monitor

Your theory sounds plausible though without the benefit of expert onsite inspection of course one can only speculate.

Black looking carpet stains can be comprised entirely of house dust, or if as you may have done, you can find a source of soot, that is a likely particle source.

Part of understanding carpet "soot" stain diagnosis is the determination and interpretation of the stain location, pattern, and even time of appearance.

For example, if the stain appears only at the perimeter of building exterior walls we pose that that may be due to lower temperatures at the floor/wall juncture or even air leaks there.

I also agree with you that cooler exterior walls can cause downwards air movement in that location, depositing building dust or soot along the wall/floor juncture.

Other dark carpet stains such as in your photos can track to the location of furniture that has protected the not-stain area from particle settlement. Often we find darker carpet stains behind furnishings than in the occupied and walked-on space, perhaps also because those areas are less easily and less often reached for cleaning.

But your discovery of sooting at the gas pilot light on the ventless fireplace that has been sooting since November sounds to me like a good guess at a source of extra soot particles in the home, and your observation of a sooty deposit on windows supports that view too.

Set Priorities of Attention: Safety First

In terms of priority of attention and action, any gas appliance that is producing soot is potentially a dangerous carbon monoxide or combustible gas leak. So a first priority (after simply turning off the gas to the ventless fireplace) is to have that system examined and repaired to a safe condition.

I doubt that soot production is normal.

Watch out: see also CARBON MONOXIDE GAS

Is Carpet Stain Testing Appropriate?

While it is possible to collect suspect particles such as black carpet debris by using a vacuum-operated sampling cassette or even by careful use of clear adhesive tape, and while that same tape approach can be used for window surface debris, in my OPINION testing may not be warranted.



While particle identification would certainly be technically interesting (and a forensic lab fee may be as little as $50 or $100), even if a significant component of the dark carpet stain particles are identified as soot, we should not assume that the source you found is the only or most important one - not before performing a more thorough inspection of the building and its mechanical systems.

In other words, while we love particle testing and identification (I operate a forensic microscopy particle identification lab), you should not use a test or two as a substitute for a thoughtful inspection of the building. (It took thought to track down that fireplace as a soot source.)

What to Try Now

If you agree that lab testing is not really necessary, you might prefer to spend your money and energy on

I'm not sure just what process the local carpet cleaner used, but I agree that if it was ineffective, you need to try something else. Sometimes we find that a stain that is very resistant to one approach is removed easily by another.

A client spent a large effort using solvents to try to remove some brown goop that we thought was tile mastic. When she tried using plain water the goop came up easily - it was water soluble but was resistant to other solvents.

Cleaning of the carpeting: you may need to hire a carpet cleaning service that uses steam or even a rug shampoo and water extraction procedure. I'd avoid really wet carpet shampooing or any process that soaks or wets wall to wall carpeting if possible.

Discuss the suspected type of stain and what was tried before with your new carpet cleaner in deciding what cleaning process is necessary.

Inspecting all other gas-burning appliances in the building to be sure that they are operating safely and properly

I'm not sure what portable air cleaner model you have been using but our experience is that while a portable air cleaner will remove some indoor air particles, there is no evidence that those devices move and process enough air to remove high levels of airborne particles throughout a building.

However, if your home has central air heating and/or air conditioning, check and maintain your air filters.


If the sooting is from a problem source and not simply the common room perimeter carpet staining that occurs in buildings due to air movement, cooler wall/floor junctions and normal house dust deposition, and if you clean the carpets and fix the soot or debris sources you can find by inspection, there's a good chance that the problem will not recur.

If unusual carpet sooting stains recur, then further inspection and perhaps even testing would be appropriate.

Photographs & Diagnosis of Thermal Tracking and Other Stains on Carpeting

Carpet stains at edges and under furniture © Daniel FriedmanCarpets in a building are sometimes stained right along the building wall, particularly but not only at the exterior walls. Air leaks at the wall/floor juncture can occur due to building movement or simply sloppy original construction.

Air moving upwards through the building (due to upwards air convection currents) may draw incoming air at the wall/floor gap or cracks, thereby depositing dust and debris at that location.

The result is a dark stain at the edges of carpeting along the building walls.

Details about thermal tracking or "ghosting" are


Carpet stains at edges of or under furniture: 

Similar carpet stains due to air movement, possibly exacerbated by higher moisture where there is less air circulation, may occur under couches or other furniture whose skirts or other construction details interfere with air movement, causing a lower temperature (and possibly slightly higher moisture condensation) in those areas.

In our photo at above left you can see the mark from a couch foot near the right side of the photo, cleaner carpet exposed below a floor runner, and darker stained carpeting that developed below this piece of furniture.

Blotchy rounded carpet stain © Daniel Friedman

Blotchy carpet stains: When the furniture is moved the differences in carpet color or actual carpeting soiling and stains may be very obvious, especially where a couch skirt was in place.

Occasionally we also see more blotchy carpet stains with sharply defined edges where a spill has occurred or where a partial attempt was made to "wash" an area where thermal staining had previously occurred.

In this photo at right side a vertical dark line probably marks where a couch or chair skirt was in place.

[Click to enlarge any image]

Usually lab testing is an unnecessary approach when removing or diagnosing stains or contaminants on carpeting but sometimes when visual inspection and considering building history (leaks, spills, pets, air leaks, puffback or other soot producing events) isn't enough and where significant exenses are involved in cleaning or replacing carpeting we might test tape or dust samples of the carpet stain area as well as other samples of settled building dust.


Variations in Carpet or Rug Colours

What caused the light coloure "Stains" on this rug ? Diagnosis suggestions (C) InspectAPedia BBMight Be Traced to Repairs, Use of Different Yarns, Methods of Original Construction, Exposure to Light, Moisture, or Water

Reader Question: Dear Inspectapedia:I am amazed by your fabulously comprehensive website. Compliments! However, I still don't find what I am looking for -- and don't quite know what to look for. alas! - and urgently need your advice.

[Click to enlarge any image]

About three years ago I bought a beautiful blue-white Peking rug from a dealer in Manhattan for my bedroom. Suddenly about one and a half years ago small whitish/light-ish, almost bleached looking stains appeared.

They slowly expanded and expanded and are still expanding. I or noone else spilled anything on the rug.

There is no moisture or leak nearby. There is no mold or mildew looking (?) and smelling stuff on the rug.

The stains seem to grow not totally wildly but somewhat straight along the web (or waft??) of the rug. I am very, very worried by something as inexplicable spreading in my apartment and would like it inspected but I don't know to whom to turn. (I also seem to have some health problems - not respiratory though - lately. but they might be rather age-related.)

Could you please give me some advice? Of course, I would pay for the inspection if required.
I have attached some photos.

With many thanks in advance and best wishes, B.B. 15 Sept 2014

Reply: regular or rectangular or weave delineated color variations may be due to differences in rug yarns

No need to pay anyone for inspecting these photos nor the rug.

I can see one most likely explanation and offer a possible additional one for the light color "stain" patterns in your photos, provided we are referring to the light gray areas that have regular rectangular edges: this is a rug repair - re-weaving, or simply a use of a segment of yarn from a different batch that was not so dye-fast as other yarns used in the original weaving of the rug

. I've seen this exact pattern on antique, repaired rugs of various origins, especially some Moroccans.

What caused the light coloure "Stains" on this rug ? Diagnosis suggestions (C) InspectAPedia BB

Sometimes a spill or spill plus cleaning attempts will show up these yarn or repair differences further.

Flip over the rug and look at the back and you may see evidence of re-weaving repairs.

If you don't then I ascribe the difference to use of a different yarn at time of original weaving.
You can most likely confirm my opinion by showing your photos to an experienced rug expert in your area.

What caused the light coloure "Stains" on this rug ? Diagnosis suggestions (C) InspectAPedia BB

Unless you object, I'll post these photos at an appropriate spot at InspectApedia, keeping your information completely private of course - to invite further comment from others some of whom may be more expert.

Reader Follow-up:

I finally get to respond to your informative and incredibly fast reply re my rug spots. Sorry about the delay and thanks again.

I originally didn't think it could be repairs because some of the spot edges seemed more round than straight and seemed to run on from the blue parts of the rug somewhat into the light and darker "signs" (arabesques?) on it.

But then i looked again at the back of the rug and saw that they were really almost straight/rectangular. I couldn't spot whether the reason was re weaving with a slightly differently colored material because I don't know what to look for and what would indicate well-made repairs. But it is a very reasonable explanation.

However, I don't understand why the spots appear only now, after years - there definitely was no spill on the rug. And I also wonder what might have made the repair yarn, if it is that, first look exactly like the main blue color and then suddenly lighter/grey Would you have an explanation for that?

It seems that you are sure it is not microbes or any toxic material that leads to the color changes, right???

I'll try to go to the dealer, if he is still there, and ask him about the situation but I doubt that he would tell me anything.

by the way, I have another light blue rug in the apartment with off-white geometric designs. I think it is turkish. It now also seems to have lighter and darker streaks and sections. If I find the time I shall take pictures and send them if I may. I also didn't see the changes before.

Maybe it's just the light which I understand always bleaches blues more than other colors. That rug is a special case: i saw it at a dealer on a street fair and fell in love with the rare two-colored design.

When I turned it around, one could see that most of the off-white designs clearly originally were rust-colored.

So it seems the whole rug had been sun-bleached(?). I think it is gorgeous. The dealer himself wasn't there when I gave a down payment to his assistant and told me later he actually hadn't had in mind to sell it on the street but to give it to designers. - B.B. 9/19/14

Reply: why changes in rug colour may map individual sections of yarn or weaving

I'm not a rug expert, but I have some experience with these colour variations

. I've seen that different yarns will wear differently and will respond to light as well as spills differently. So a repair or even just a different yarn used in segments of a rug will often change appearance differently than their neighbors even in cases where the whole rug was exposed to the same conditions, say light, or wetting.

Sometimes it's a repair, sometimes just a different, perhaps small batch of yarn used in certain rug areas at the time of original weaving.

Mold Stains on Carpeting

Moldy carpet stains © Daniel Friedman

If you see carpet stains of any color, black, brown, green, yellow, in areas where there may have been water leaks from any source, you should suspect a mold growth problem, and further investigation of the carpet, the padding below the carpet, and the subfloor below that are all in order.

Mold stains on carpeting such as those in our photo at left are discussed in more detail


More photographs of thermal tracking and other stains on carpeting can be seen

at PHOTO GUIDE TO STAINS on Indoor Surfaces

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.

Other Causes of Carpet Stains & Marks

Carpet Surface Staining from Air Movement, Leaks, Moisture

Stained carpet due to thermal effects © Daniel FriedmanSkipping for now stains on carpeting that are due to spills of food, drink, or other stuff, and skipping pet stains from "accidents" on the rug by your dog, cat, or other pets, there are several common sources of carpeting stains and marks worth recognizing as they can tell us something about building conditions that may need attention.

Carpet stains from thermal tracking, air leaks, or areas of higher moisture that cause collection of house dust, candle or fireplace soot, heating system soot, may appear out of the normal walking-traffic areas.

If there are air supply registers in or near floors, air movement from the HVAC system may also deposit dust that appears as sooty stains around and close to the register.

Black or dark sooty stains on carpeting can appear from foot traffic but those marks will appear in the main walking path through a carpeted room.

Watch out: thermal-tracking-like stains on carpeting also appear along building exterior walls where there are air leaks, and may also appear as black streaks or smudges.


Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.


Continue reading at CARPET STAIN ID TESTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see CARPET STAIN DIAGNOSIS FAQs - questions & answers about stained carpets & rugs, posted originally at this page.

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