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 Why is Mold Found in HVAC Ducts & in Fiberglass Duct Insulation
- Causes of mold growth in HVAC ductwork - mold in air ducts
- HVAC mold survey methodological error sources
- Mold genera / species most often found in HVAC air handlers & ducts
- What causes the occurrence of mold growth in fiberglass insulation in buildings: causes, hazards, cure, prevention
- Does toxic or allergenic mold actually grow in fiberglass or is it just moldy dust in fiberglass insulation?
- Procedure Guide for Testing or inspecting for moldy building insulation or moldy heating or air conditioner duct insulation
- EPA and other government advice about moldy ducts or suspected moldy HVAC ductwork
- Questions & Answers about Mold in Air Ducts: cause, detection, cure, & prevention of moldy HVAC ductwork
- References
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
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. We describe the types (genera/species) of mold most often found in HVAC ducts and the relationship between mold in ductwork and indoor air quality complaints by building occupants. We include authoritative citations for key research on mold contamination in HVAC ductwork.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Where and Why Does Mold Grow in HVAC Ductwork?
Mold growth & Mold Contamination are Common in HVAC Systems
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.
It is common for a careful inspection of air handlers and HVAC ductwork to find mold contamination on the duct interior as well as in the air handler on the blower assembly squirrel cage fan blades and other components.
In our experience as both field and lab investigators, several genera/species of mold are quite common in these environments (as well as in fiberglass and possibly some other building insulation products).
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 Our photo of a moldy air handler interior in a Florida home (left) is provided by home inspector & educator Mark Cramer.[3]
Where conditions support mold growth within the HVAC system air handler and ductwork, we also may find significant mold colonization of the surfaces of air supply registers or the ceilings around them. (Next photo, below).
HVAC Studies for Mold Contamation - Methodological Error Risks
Tests for mold in HVAC systems are vulnerable to methodology errors, particularly in the selection of sampling sites.
Variations in moisture and uneven distribution of organic debris and dust through the system are very important effects on what mold is found where.
For example, sampling immediately downstream from the blower assembly we are more likely to find more water-tolerant fungi (Cladosporium spp.) associated with condensate blow-off spun into that area by the blower fan.
Sampling further into the duct system may discover Aspergillus spp. that prefers somewhat drier surfaces. Ducts exposed to special conditions (flooding, greases or organics from cooking, etc) may support additional or different-dominant mold genera/species.
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 Fungal growth on fiberglass appears to be supported by the normal organic debris (skin cells that dominate house and other building dust), combined with moisture or even condensation and water found in the HVAC system.
But we also find mold contamination even in some clean-looking fiberglass building insulation, possibly supported by a combination of moisture and some organic resins or binders, or at times, simply having been absorbed by insulation that is installed in a very moldy environment whose mold is from another source reservoir. Details are at INSPECTION of INSULATION for MOLD.
Also see BLOWER LEAKS, RUST & MOLD and also PARTICLE & MOLD LEVELS in DUCTWORK.
And see SLAB DUCTWORK for the role of in-slab placement of air ducts in the formation of mold contamination in HVAC systems. |
Common Mold Genera/Species Found in HVAC Systems & Ductwork & Building IAQ Complaints
Experts studying both mold contamination in HVAC ductwork and related building indoor air complaints have confirmed our own lab experience that identifies Cladosporium spp. or C. herbarum, (most common), Aspergillus versicolor (common) A. flavus (common), and A. fumigatis. [5][6][7][8][9]
Those studies also point out that even when apparently modest levels of mold contamination traced to HVAC systems are removed (usually by removing the contaminated or "mold colonized" duct insulation or if that isn't possible, by replacing the ductwork) building IAQ complaints decline significantly.
Dirty HVAC Ducts That Cannot be Cleaned
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?.
Frequently Asked Questions (FAQs)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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.
- [3] Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com 11/06
- [4] pending
- [5] Ahearn, D.G., S A Crow, R B Simmons, D L Price, J A Noble, S K Mishra and D L Pierson, "Fungal colonization of fiberglass insulation in the air distribution system of a multi-story office building: VOC production and possible relationship to a sick building syndrome", Journal of Industrial Microbiology & Biotechnology, Volume 16, Number 5 (1996), 280-285, DOI: 10.1007/BF01570035. Abstract:
Complaints characteristic of those for sick building syndrome prompted mycological investigations of a modern multi-story office building on the Gulf coast in the Southeastern United States (Houston-Galveston area). The air handling units and fiberglass duct liner of the heating, ventilating and air conditioning system of the building, without a history of catastrophic or chronic water damage, demonstrated extensive colonization with Penicillium spp and Cladosporium herbarum. Although dense fungal growth was observed on surfaces within the heating-cooling system, most air samples yielded fewer than 200 CFU m–3. Several volatile compounds found in the building air were released also from colonized fiberglass. Removal of colonized insulation from the floor receiving the majority of complaints of mouldy air and continuous operation of the units supplying this floor resulted in a reduction in the number of complaints.
- [6] Ahearn, D.G., S.A. Crow, R.B. Simmons, D.L. Price, S.K. Mishra and D.L. Pierson, "Fungal Colonization of Air Filters and Insulation in a Multi-Story Office Building: Production of Volatile Organics", Current Microbiology Volume 35, Number 5 (1997), 305-308, DOI: 10.1007/s002849900259, Abstract:
Secondary air filters in the air-handling units on four floors of a multi-story office building with a history of fungal colonization of insulation within the air distribution system were examined for the presence of growing fungi and production of volatile organic compounds. Fungal mycelium and conidia of Cladosporium and Penicillium spp. were observed on insulation from all floors and both sides of the air filters from one floor. Lower concentrations of volatile organics were released from air filter medium colonized with fungi as compared with noncolonized filter medium. However, the volatiles from the colonized filter medium included fungal metabolites such as acetone and a carbonyl sulfide-like compound that were not released from noncolonized filter medium. The growth of fungi in air distribution systems may affect the content of volatile organics in indoor air.
- [7] Price,D. L., R. B. Simmons, I. M. Ezeonu, S. A. Crow and D. G. Ahearn, "Colonization of fiberglass insulation used in heating, ventilation and air conditioning systems", Journal of Industrial Microbiology & Biotechnology Volume 13, Number 3 (1994), 154-158, DOI: 10.1007/BF01584000, Abstract: The number of fungal species colonizing thermal and acoustic fiberglass insulations used in heating, ventilation, and air conditioning (HVAC) systems was fewer than that obtained from initial direct culture of these insulations. The colonization, determined by the microscopic observation of conidiophores with conidia, was primarily of acrylic-latex-facing material, but eventually the fungi permeated the fiberglass matrix. Isolates of Aspergillus versicolor were most often obtained from non-challenged insulation, whereasAcremonium obclavatum appeared to be the primary colonizing fungus in high-humidity (>90%) challenge chambers. At a lower humidity (about 70%) Aspergillus flavus was one of the more prominent fungi. Not all duct liner samples were equally susceptible to colonization and duct board appeared relatively resistant to colonization.
- [8] Simmons, R. B. and S. A. Crow, "Fungal colonization of air filters for use in heating, ventilating, and air conditioning (HVAC) systems", Journal of Industrial Microbiology & Biotechnology Volume 14, Number 1 (1995), 41-45, DOI: 10.1007/BF01570065, Abstract:
New and used cellulosic air filters for HVAC systems including those treated with antimicrobials were suspended in vessels with a range of relative humidities (55–99%) and containing non-sterile potting soil which stimulates fungal growth. Most filters yielded fungi prior to suspension in the chambers but only two of 14 nontreated filters demonstrated fungal colonization following use in HVAC systems. Filters treated with antimicrobials, particularly a phosphated amine complex, demonstrated markedly less fungal colonization than nontreated filters. In comparison with nontreated cellulosic filters, fungal colonization of antimicrobial-treated cellulosic filters was selective and delayed.
- [9] Ifeoma M. Ezeonu, Daniel L. Price, Sidney A. Crow and Donald G. Ahearn, "Effects of extracts of fiberglass insulations on the growth of Aspergillus fumigatus and A. versicolor", Mycopathologia Volume 132, Number 2 (1995), 65-69, DOI: 10.1007/BF01103777
, Abstract:
Water extracts of thermal and acoustic fiberglass insulations used in the duct work of heating, ventilation and air conditioning (HVAC) systems supported germination of conidia and growth of Aspergillus versicolor (Vuillemin) Tiraboschi 1908–9 and Aspergillus fumigatus Fresenius 1863. Urea, formaldehyde and unidentified organics were detected in the extracts. Formaldehyde in concentrations similar to those found in the extracts restricted the growth of both species in enriched media. A. versicolor, the more common species associated with fiberglass insulations, was more resistant to formaldehyde than A. fumigatus.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- 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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