A Photo Guide to Stains on Indoor Surfaces in buildings
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A Photographic Guide to Diagnosing Indoor Stains on Carpet, Cabinet Doors, HVAC Registers
Phantom stains - indoor marks that are not an actual dirt, debris, or mold stain
Guide to stains and debris around heating and cooling air registers and baseboards and on air filters
How to diagnose discoloration, stains, or light spots on indoor carpeting; moldy carpeting?
Links to more photos of indoor ceiling or wall stains that are not toxic mold
Questions & answers about identifying the cause and finding the cure for indoor stains on ceilings, walls, floors, carpeting, furniture, and other indoor surfaces and materials

This article describes, provides photographs, & diagnoses the causes of interior wall, ceiling, flooring or carpeting stains and explains how to recognize their probable cause and source, including soot stains, house dust stains, pet or animal stains, and thermal tracking or thermal bridging stains associated with building air leaks, and building insulation defects.

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A Photographic Guide to Help Diagnose the Causes of Indoor Stains on Carpet, Cabinet Doors, HVAC Registers

Readers should also see STAIN DIAGNOSIS on BUILDING INTERIORS and for outdoor stains, see STAIN DIAGNOSIS on BUILDING EXTERIORSAlso see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES where we describe not only moss and lichens but black fungal stains on asphalt shingles and on other building surfaces.

Often dark indoor ceiling, wall, carpet or floor stains are mistaken for toxic indoor mold when they come from other causes instead.

When investigating a building for a mold problem, you can save mold test costs by learning how to recognize MOLD APPEARANCE - STUFF THAT IS NOT MOLD or is only Harmless Mold but may be mistaken for more serious contamination - save your money. 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 provide this library of photographs of things that are "not mold" and don't need to be tested. "black mold" often mistaken for "toxic fungal growth." Photos of HVAC and carpet stains (excluding the moldy carpet photos) were provided courtesy of Pat Belkin, Charlotte, NC.

Here are some examples of troubling indoor surface stains with some preliminary opinions about what these stains may be about. We emphasize preliminary opinion because here we comment on these photos before an expert diagnostic building inspection has been performed and before any lab samples have been collected and analyzed from these surfaces.

Whether or not such sampling and analysis are justified depends on the experience of the building occupants (health complaints or health vulnerability), the history of the building (exposure to leaks, damage, pets, contaminants), and other site investigative results. See When to hire a professional to investigate a building for help in deciding when to go further.

How to Recognize & Diagnose Phantom Stains on Indoor Surfaces

Often when people become concerned about indoor air quality, mold, or stains, they begin to study their building surfaces more carefully than ever before.

When this happens we sometimes find people reporting as "new" stains that were there from original construction, but where simply not noticed, or were not so particularly noticed before other concerns increased their level of attention.

This interesting photograph of a large yellowish stain on the surface of what appears to be a birch kitchen cabinet door surface (probably the door's interior surface) could be an example of this phenomenon.

What would support the "phantom stain" hypothesis for an item such as this cabinet door?

Stain pattern: If the stain appears in a regular pattern on companion surfaces such as other cabinet doors it may be an artifact of the door's manufacture

Stain location: If examination of other cabinet surfaces shows similar stains in varying locations

Manufacturing artifacts: If examination of sample cabinets from the same supplier or manufacturer, new, say in a showroom, show similar markings

Ability to remove surface staining material: If a tape sample cannot remove any debris from this are of darkened color

Stain particulate or chemical components: If a tape sample of surface debris removed particles which are determined to be finish coatings or wood fibers without fungal or chemical modification,

Presence of common causes of stains: If there are no moisture, food, air movement, or other suspect sources that have affected some of these cabinets but not others

Time of occurrence of stain: If the stain is under the finish-coating of the wood cabinet surface rather than something which was deposited on top of the coating (though indeed moisture can in some circumstances affect surfaces below their coating)

then this may be the case with this example photo of a yellowish stain on a birch ply cabinet door interior

How to Diagnose Stains & Discoloration on Heating or Cooling Supply or Return Registers or Plenums

Photograph of stains on a heating or cooling supply register

Photograph of paint overspray and house dust on a heating return plenum

Supply Register debris: The left photo above shows typical house dust deposition on a heating or cooling ceiling air supply register, where you can see brown debris adhered to the metal register surface. A second example is in our photo at left.

These particles adhere to the register surface due to either moisture from condensation or in this location, more likely due to static electricity as particles are moving across a normally dry surface. See Stains HVAC Supply Registers for more examples.

What to do about supply register debris: House dust, normally composed primarily of human skin cells and fabric fibers, is not usually an environmental or air quality issue, though at high levels on surfaces it can be diagnostic of building conditions such as high moisture or poor HVAC system maintenance.

We can reduce this debris deposition by duct and air handler cleaning and by better and constant maintenance of filters at the return air registers. If other information disclosed by the building investigation warrants, one have this debris screened for mold, allergens or other problematic particles by using a forensic laboratory whose technicians are expert in house dust analysis.

Return register & return air plenum debris: The right hand photo shows a combination of paint overspray (white particles on the black return plenum insulation liner), and house dust (brown debris on the metal frame intended to hold a return air filter).

What to do about return plenum debris: The brown dust and debris indicates that the air filter used at this location has been leaky. See An Analysis of the Effectiveness of Air Filters on HVAC Systems for further advice on air filters that do not leak in this location.

How to Diagnose Stains & Discoloration on Heating or Air Conditioning Filters

Photograph of indoor stains on a cabinet door

Air filter debris: This home air filter was taken from the central return air plenum and register cover shown in the photograph just above. The brownish stains on the filter are a typical color (brown to gray) of debris found on any indoor air filter. Such "stains' indicate that the filter is doing its job of reducing the level of airborne debris in the building.

What to do about air filter debris heating and air conditioning filters should be changed at least monthly whenever this equipment is in operation. In a dusty environment more frequent changes may be needed.

Also see An Analysis of the Effectiveness of Air Filters on HVAC Systems for further advice on air filters that do not leak in this location and also see When to hire a professional to investigate a building for help in deciding when to go further in inspecting, diagnosing, and testing particles and debris on a building air filter. In many circumstances further testing would not be justified.

How to Diagnose Mold Stains & Mold Growth on Floor Carpeting

These photographs show two common patterns of mold growth on the upper surface of a carpet that was left wet in a building. In the left photograph a brown mold is growing in a rather uniform pattern without definite edges on carpet in a closet. The black mold visible in the upper left and right portions of the photograph will probably be a different mold genera and species - this is an example of the dangers of careless sampling of mold in buildings since a lot of what people think is present in a building depends on exactly how mold samples are collected.

The right hand photo above shows two rather round black patterns of mold growth on floor carpeting in the same building. Even if no mold was visible on other carpet surfaces in this building it is likely that the carpeting, if it was wet, has become mold contaminated - a condition that might be confirmed by inspection of the carpet backing, padding, or other building surfaces.

Carpets or furniture that have been wet or had mold growth: Wall-to-wall carpets and upholstered furniture that have been soaked and/or have had mold growth on their surfaces, most likely cannot be adequately cleaned and should be replaced. See CARPET MOLD CONTAMINATION. Also see CARPET & other STAIN TESTS and CARPET TEST PROCEDURE.

Carpet padding and subfloor: And where carpeting has been wet, don't forget that the padding below the carpeting and even the floor and subfloor below may be damaged or moldy. See CARPET PADDING ASBESTOS, MOLD, ODORS.

Even carpeting which has not been wet may be a significant problem mold reservoir in a building if the carpeting has been exposed to a high level of airborne mold or other allergens. This condition occurs, for example, when a water-damaged moldy building has been remediated without proper dust and debris control.

The difference is that carpeting or upholstered furniture that has never been wet and that has not had mold growth on its surface, that is, it has been subjected to settled dust only, may often be adequately cleaned using HEPA vacuuming methods. See CARPET DUST IDENTIFICATION

Screening samples of carpeting and other building surfaces taken outside of the remediation work area both before and after a mold remediation project can protect both the remediation company and the building owner from unanticipated additional mold cleanup work after the initial mold remediation project has been completed.

A Another case of severe carpet staining due to furniture left on wet carpeting, accompanied by mold growth, is shown in this photograph.

How to Diagnose Stains & Discoloration on Rugs or Wall to Wall Carpeting

Possible thermal tracking stains are shown by the darkened debris on the floor carpet in the left photo above, where a grayish line appears to follow the point where the wall to wall carpeting abuts the building wall or wall baseboard trim. See Thermal Tracking for detailed discussion of this phenomenon and how to diagnose it.

Possible furniture footprint stains on carpeting are suggested by the right hand photo above. Sometimes a stain like this, particularly where it follows the same shape as an object which has been placed on the carpet, suggests that the stain was deposited from the object itself, or dirt on its surface, or bleed-out if the object was placed on a carpet left damp after carpet shampooing.

A Severe carpet staining due to furniture left on wet carpeting, accompanied by mold growth, is shown in this photograph.

How to Diagnose "Clean" Areas of Carpet Surrounded by Stains or Discoloration

"Clean" areas of carpet under furniture can also tell us what's been going on in a building. In the right hand photo above, the carpet appears darker inside the stain perimeter which suggests that the mark we see is either from a spill on the carpet or from the footprint of an object which was placed on the carpeting.

But if a mark on carpeting outlines an area of carpet which is lighter or cleaner than the surrounding carpeting, we usually find that an object which had been placed on the carpet was actually protecting that surface from settling dust, soot, or other debris in the building.

In this case we'd look further for an indoor source of high levels of airborne soot or other debris, such as a malfunctioning gas or oil fired heating system.

Because such heating systems could be unsafe (for example, risk of fatal carbon monoxide poisoning), the inspector should be one who is quite familiar with inspection methods and indicators of an unsafe or improperly-functioning heating or cooling system.

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.

Photos & Diagnosing Stains on Building Walls and Ceilings

Common stains on painted inteior walls and ceilings include

Black smudges and streaks on walls and ceilings - see THERMAL TRACKING & THERMAL BRIDGING where we describe indoor dust deposition patterns, soot stains, and other dark indoor stains often mistaken for mold (photo at left). In our thermal tracking stain article series we also distinguish candle and fireplace soot staining (Stains from Candles, Woodstoves, Fireplaces) from other indoor stain sources.
Dirt and soiling including from pets or other animals - See Pet Stains on Floors and Pet Stains on Walls
Human Occupant Stains on Walls. Human occupant stains may also be from smoking. In rooms occupied by heavy smokers we have found brown stains on walls and ceilings. If there are occasions of high indoor moisture and condensation on walls you may see concentrations of brown stains running down walls in a water pattern or droplets or small brown spots where moisture accumulated on ceilings.
Mold growth on building interior walls occurs on visible surfaces in buildings that have been wet or subject to high humidity, particularly on mold-friendly materials such as drywall and wood paneling. Watch out for hidden mold reservoirs in wallor ceiling cavities that have been wet - investigation may be justified. See MOLD APPEARANCE - WHAT MOLD LOOKS LIKE for photos of mold growing on indoor building surfaces of various materials.
Also see Other Stains on Indoor Walls & Ceilings

Photos & Diagnosing Stains on Building Floors other than Carpets

Common sources of stains on building floors include:

Rust marks left on vinyl tile or sheet vinyl where floors were wet below metal objects
Pet urine stains on wood flooring (photo at left).
We discussed and illustrated carpet stains above.
See Pet Stains on Floors and Pet Stains on Walls for details.

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Questions & answers about identifying the cause and finding the cure for indoor stains on ceilings, walls, floors, carpeting, furniture, and other indoor surfaces and materials

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THERMAL TRACKING
  What is Thermal Tracking
  Ceiling Thermal Tracking Marks
  Wall Thermal Tracking Stains
  Floor Carpet Thermal Tracking Stains
  Air Bypass Leaks Marks on Insulation
  Thermal Tracking to Diagnose IAQ
  Stains HVAC Supply Registers
  Pet Stains on Floors
  Pet Stains on Walls
  Human Occupant Stains on Walls
  Stains from Candles, Woodstoves, Fireplaces
  Other Stains on Indoor Walls & Ceilings
  What to Do About Thermal Tracking

STAIN & BIODETERIORATION AGENT CATALOG
STAINS on buildings - QUICK GUIDE
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
STAINS on INDOOR SURFACES: PHOTO GUIDE
STAINS & Thermal Tracking
STAIN DIAGNOSIS on ROOFS
STAIN DIAGNOSIS on STONE

Thanks to Pat Belkin, at Flooring Solutions for providing many of the photographs and suggesting some of the questions addressed in this web page article

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
- 2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
- 2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
  by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
  "2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
- 1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
  "The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
- Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
- 1993 ASHRAE Handbook Fundamentals (Hardcover), ISBN-10: 0910110964 or ISBN-13: 978-091011096
Dust from the World Trade Center collapse following the 9/11/01 attack: the lower floors of this building contained spray-on fire-proofing asbestos materials.
Energy Savers: Whole House Systems Approach to Energy Efficient Home Design [copy on file as /interiors/Whole_House_Energy_Efficiency_DOE.pdf ] - U.S. Department of Energy
"Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
"Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
"Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
"Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
"Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
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A Photographic Guide to Help Diagnose the Causes of Indoor Stains on Carpet, Cabinet Doors, HVAC Registers

How to Recognize & Diagnose Phantom Stains on Indoor Surfaces

How to Diagnose Stains & Discoloration on Heating or Cooling Supply or Return Registers or Plenums

How to Diagnose Stains & Discoloration on Heating or Air Conditioning Filters

How to Diagnose Mold Stains & Mold Growth on Floor Carpeting

How to Diagnose Stains & Discoloration on Rugs or Wall to Wall Carpeting

How to Diagnose "Clean" Areas of Carpet Surrounded by Stains or Discoloration

Photos & Diagnosing Stains on Building Walls and Ceilings

Photos & Diagnosing Stains on Building Floors other than Carpets

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair