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Causes of mold growth in HVAC ducts: This article expalins the cause, detection, and hazards of mold growth in fiberglass insulation in residential and light-commercial building and gives advice about dealing with moldy building insulation or ductwork.

This 720x photograph shows active fungal growth along the surface of a fiberglass insulation fiber collected the suspended ceiling of a building suffering wet conditions and moldy in-slab HVAC ducts.

Readers concerned with mold contamination in heating and air conditioning air handlers and ductwork should see BLOWER LEAKS, RUST & MOLD and PARTICLE & MOLD LEVELS in DUCTWORK where we describe how to test HVAC systems and ductwork for mold. Mold in HVAC ductwork is also discussed at  WHY DOES MOLD GROW in INSULATION?.

This website discusses health hazards associated with moldy fiberglass in buildings, with focus on fiberglass insulation, fiberglass fragments, fiberglass in heating and air conditioning duct work, and invisible but toxic mold growth in fiberglass which has been wet, exposed to high humidity, or exposed to other moldy conditions. Mold is often found in basement fiberglass insulation, crawl space fiberglass insulation, fiberglass wall insulation, heating or cooling duct fiberglass insulation, and attic or roof insulation in buildings which have either been wet or have been exposed to high levels of mold from other sources.

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Where and why does mold grow in fiberglass?

The left photo shows how fragile is the fiberglass insulation in some HVAC ducts. The rough surface attracts and collects organic and other particulate debris moving through the duct system (unless good filtration is installed at the return air inlets).

The surface of an HVAC duct lined with fiberglass cannot be mechanically cleaned - you can see what happens when someone tries to brush or vacuum it by looking at this photo. Once disturbed by improper "cleaning' efforts, the release of airborne asbestos in the building will certainly increase. If this insulation is wet by leaks or improper condensate handling, or if the building is exposed to high levels of airborne mold from another source, ducts that look like this are likely to become a problem mold reservoir and will need to be replaced.

The second photo at above right shows typical debris, usually skin cells and fabric fibers, which collects on the rough surface of exposed fiberglass inside ductwork. A return opening filter would have helped keep this duct clean and thus extend its life.

Water or condensate leaks into an HVAC duct system such as those shown by the above photographs of stains on the interior of this rooftop mounted commercial HVAC duct (left) and indoor residential air handler unit (right), are an invitation to mold or bacterial contamination in the system.

DF-OPINION: it is more (or less) likely that problematic mold will be found growing in or present in building insulation at a level sufficient to be a potential problem for building occupants in these conditions:

• Low-risk insulation: Mold requires moisture and organic material for nourishment, as well as other growing requirements that vary by mold genera and species. So perfectly clean, dry fiberglass or other building insulation is unlikely to harbor a problem mold reservoir.
  
  
• Wet or "dirty" building insulation: often is found to be moldy, either on the kraft paper or foil/paper insulation backing or in the insulation itself. Insulation which is old and has become populated or "soiled" with organic debris such as animal dander (a home housing animals, especially dogs) or insect fragments (an old house with old insulation) contains plenty of organic debris which can support fungal growth. When such insulation is wet by a single event such as a roof leak or basement flood or when it is exposed to recurrent high interior moisture conditions, the growth of problem mold is a real risk. If there are building occupant IAQ complaints, investigation of the insulation may be in order in this case.
  
  
• Clean fiberglass insulation exposed to moldy air: I have found very high levels of mold contamination in otherwise apparently clean fiberglass insulation in cases where the insulation has been exposed to high levels of moldy air.
  
  Aspergillus sp. and on less frequent occasions, Penicillium sp. or even basidiomycetes are found in insulation which may be rather new and which may appear perfectly clean to the naked eye. A typical scenario producing this condition is the presence of un-protected, exposed fiberglass insulation in the ceiling of a basement where there has been a significant mold contamination, perhaps even a mold remediation project. High airborne mold levels readily move small spores such as those in the Aspergillus/Penicillium group through building insulation by ordinary indoor air and convection currents.

Advice for Suspected or Known Mold-Contamination in HVAC Ductwork (Air ducts, heating ducts, air conditioning ducts)

If you think the heating or air conditioning (HVAC) system may be contaminated with mold, read the EPA's guide "Should You Have the Air Ducts in Your Home Cleaned?" before taking further action. Visit www.epa.gov/iaq/pubs/airduct.html, or call (800) 438- 4318 for a free copy.

The U.S. Environmental Protection Agency recommends:

“Do not run the HVAC system if you know or suspect that it is contaminated with mold - it could spread mold throughout the building”. [1]

buildings in areas where high humidity and high use of air conditioning such as Florida and Louisiana in the U.S. experience very high levels of condensate produced by the air conditioning system, so high that condensate sometimes blows into the ductwork itself rather than all draining successfully into the condensate drain system. According to the Florida Department of Health:

Unfortunately, it is thought that most, if not all, heating and air conditioning systems in Florida will support mold growth at some point. Stopping the use of an air conditioning system due to suspected mold growth would make most Florida buildings very uncomfortable during hot and humid weather.

Should you turn off an air conditioner if a mold problem in the system is found? Ideally, yes. The system should be shut down while cleaning or mold removal is performed. If the water and/or mold damage was caused by sewage or other contaminated water, then call a professional who has experience cleaning and fixing buildings damaged by contaminated water. [2]

Watch out: the U.S. EPA and other sources recommend: [among other details found in the articles listed in our references] that

Have your air ducts cleaned if they are visibly contaminated with substantial mold growth, pests or vermin, or are clogged with substantial deposits of dust or debris 

But fiberglass lined ductwork may be seriously damaged by mechanical cleaning, increasing the subsequent release of irritating airborne fiberglass particles into building air and actually reducing the resistance of such ductwork to future debris and moisture and even mold accumulation. In our opinion mold-contaminated ductwork that is metal lined can be cleaned successfully. Other ductwork should be replaced. And in any case you should address the cause of mold growth and correct that as well or the problem will simply repeat itself.

Readers concerned with mold contamination in heating and air conditioning air handlers and ductwork should see BLOWER LEAKS, RUST & MOLD and PARTICLE & MOLD LEVELS in DUCTWORK where we describe how to test HVAC systems and ductwork for mold. Mold in HVAC ductwork is also discussed at  WHY DOES MOLD GROW in INSULATION?.

Questions & Answers regarding this article

Questions & Answers about Mold in Air Ducts: cause, detection, cure, & prevention of moldy HVAC ductwork.

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INSULATION MOLD
  Mold in Fiberglass Insulation
  Mold on Books, Book Conservation
  Mold on Fiberboard Insulating Sheathing
  MOLD PREVENTION AFTER FLOODING
  MOLD RESISTANT CONSTRUCTION
  Mold in Foam Insulation
  Moldy insulation may look clean
  Why does mold grow in fiberglass?
  When to test insulation for mold
  How to Test for Mold in Insulation
  References, Fiberglass Hazards
  Vacuuming exposed insulation

• Should You Have the Air Ducts in Your Home Cleaned, U.S. Environmental Protection Agency, Indoor Environments Division, EPA-402-K-97-002, web search 8/5/11, original source: http://www.epa.gov/iaq/pdfs/airducts.pdf
• "Indoor Mold and Health, A Fungus AmonG Us", Florida State Department of Health, Florida Department of Health Division of Environmental Health Bureau of Community Environmental Health Radon and Indoor Toxics 4052 Bald Cypress Way, Bin A08 Tallahassee, FL 32311 1-850-245-4288 1-800-543-8279, web search 08/05/2011, original source: http://www.doh.state.fl.us/environment/community/indoor-air/Indoor_Mold_and_Health.pdf. Quoting:
  The Florida Department of Health has developed this brochure to address some of the most common questions and concerns about indoor mold, how it affects human health, and ways in which you can prevent or remove it.

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.
• Environmental Health & Investigation Bibliography - our technical library on indoor air quality inspection, testing, laboratory procedures, forensic microscopy, etc.
• Adkins and Adkins Dictionary of Roman Religion discusses Robigus, the Roman god of crop protection and the legendary progenitor of wheat rust fungus.
• Kansas State University, department of plant pathology, extension plant pathology web page on wheat rust fungus: see http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Leaf%20Rust.asp
• "A Brief Guide to Mold, Moisture, and Your Home", U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
• US EPA - Mold Remediation in Schools and Commercial Building [ copy on file as /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
• US EPA - Una Breva Guia a Moho - Hongo [on file as /sickhouse/EPA_Moho_Guia_sp.pdf - - en Espanol

Fiberglass in buildings: hazards, testing, cleanup, prevention: references & products

For more information about fiberglass as an indoor air quality concern see:

• Asbestos: How to find and recognize asbestos in buildings - visual inspection methods, list of common asbestos-containing materials (Asbestos is not fiberglass and vice versa).
• BASEMENT MOLD includes examples of moldy fiberglass insulation found in basements
• CRAWLSPACE MOLD includes additional examples of moldy fiberglass insulation found in crawl spaces
• Duct System Defects
• Fiberglass in Indoor Air, HVAC ducts, and Building Insulation, Indoor Air Quality Investigations, building insulation and HVAC duct work insulation hazards
• FIBERGLASS HAZARDS
• Fiberglass Particle Identification in the Fiberglass Test Laboratory
• Fiberglass References - Government Agencies & Authorities list of public documents on fiberglass
• Goodman Gray Flex Duct Deterioration and Failures
• INSULATION INSPECTION & IMPROVEMENT
• Insulation Identification Photographs - Fiberglass insulation photos, yellow, pink, green, white fiberglass identification in building attics, walls, ducts, other locations
• Insulation Identification Photographs - Cellulose insulation photos, Mineral wool insulation photos, rock wool insulation photos, cotton insulation photos, balsam wool insulation photos
• Insulation Identification Photographs - Vermiculite insulation photos
• Lab Identification of Fiberglass photographs and text assist in laboratory identification of fiberglass fibers and fragments in air, dust, or material samples in the laboratory using forensic microscopic techniques.
• Mold in Fiberglass building insulation, when, why, and how fiberglass becomes a reservoir of problem mold in buildings.
• AIR FILTERS, OPTIMUM INDOOR
• Owens Corning Flex Duct Deterioration and Failures
• World Trade Center Dust Particle Identification
• Fiberglass carcinogenicity: "Glass Wool Fibers Expert Panel Report, Part B - Recommendation for Listing Status for Glass Wool Fibers and Scientific Justification for the Recommendation", The Report on Carcinogens (RoC) expert panel for glass wool fibers exposures met at the Sheraton Chapel Hill Hotel, Chapel Hill, North Carolina on June 9-10, 2009, to peer review the draft background document on glass wool fibers exposures and make a recommendation for listing status in the 12th Edition of the RoC. The National Institute of Environmental Health Sciences is one of the National Institutes of Health within the U.S. Department of Health and Human Services. The National Toxicology Program is headquartered on the NIEHS campus in Research Triangle Park, NC. The National Institute of Environmental Health Sciences is one of the National Institutes of Health within the U.S. Department of Health and Human Services. The National Toxicology Program is headquartered on the NIEHS campus in Research Triangle Park, NC.
  Following a discussion of the body of knowledge, the expert panel reviewed the RoC listing criteria and made its recommendation. The expert panel recommended by a vote of 8 yes/0 no that glass wool fibers, with the exception of special fibers of concern (characterized physically below), should not be classified either as known to be a human carcinogen or reasonably anticipated to be a human carcinogen. The expert panel also recommended by a vote of 7 yes/0 no/1 abstention, based on sufficient evidence of carcinogenicity in well-conducted animal inhalation studies, that special-purpose glass fibers with the physical characteristics as follows longer, thinner, less soluble fibers (for 1 example, > 15 μm length with a kdis of < 100 ng/cm2/h) are reasonably anticipated to be a human carcinogen for the listing status in the RoC. The major considerations discussed that led the panel to its recommendation include the observations of tumors in multiple species of animals (rats and hamsters). Both inhalation and intraperitoneal routes of exposure produced tumors, although inhalation was considered more relevant for humans.
• Fiberglass insulation mold: occurrence of mold contamination in fiberglass insulation can be impossible to see with the naked eye, but can be significant
• World Health Organization International Agency for Research on Cancer - IARC Monographs on the Evaluation of Carcinogenic Risks to Humans - VOL 81 Man-Made Vitreous Fibers, 2002, IARCPress, Lyon France, pi-ii-cover-isbn.qxd 06/12/02 14:15 Page i - World Health Organization, 1/21/1998. - Fiberglass insulation is an example of what IARC refers to as man made vitreous fiber - inorganic fibers made primarily from glass, rock, minerals, slag, and processed inorganic oxides. This article provides enormous detail about fiberglass and other vitreous fibers, and includes fiberglass exposure data.
• http://monographs.iarc.fr/ENG/Monographs/vol81/mono81.pdf - the article (large PDF over 6MB)
  http://monographs.iarc.fr/ENG/Monographs/vol81/mono81-6A.pdf - article details
  http://monographs.iarc.fr/ENG/Monographs/vol81/mono81-6C.pdf - studies of cancer in experimental animals in re vitreous fibers such as fiberglass;
  http://monographs.iarc.fr/ENG/Monographs/vol81/mono81-6E.pdf - summary of data reported & evaluation
  http://monographs.iarc.fr/ENG/Monographs/vol81/mono81-6F.pdf for the article references
  To search the IARC monographs on various environmental concerns and carcinogens, use http://monographs.iarc.fr/ENG/Monographs/PDFs/index.php
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