Photo of water stains, rust, and possible insulation in air conditioner air handler fiberglass insulation (C) Daniel Fr4iedmanFiberglass HVAC Duct Mold Contamiantion
Why is Mold Found in HVAC Ducts & in Fiberglass Duct Insulation

  • WHY DOES MOLD GROW in INSULATION? - CONTENTS: Causes of mold growth in building insulation & 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
  • POST a QUESTION or READ FAQs about Mold in Air Ducts: cause, detection, cure, & prevention of moldy HVAC ductwork

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Causes of mold growth in HVAC ducts & in building insulation:

This article explains 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. Mold may grow at extensive or problematic levels in some building insulation materials used in walls, floors, ceilings as well as in HVAC air duct systems.

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.

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Where & Why Does Mold Grow in HVAC Ductwork & in Building Insulation?

Mold growth & Mold Contamination are Common in HVAC Systems

Duct mold (C) D Friedman

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.

[Click to enlarge any image]

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).

HVAC Duct mold (C) Mark Cramer D Friedman

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.

Moldy HVAC supply register (C) D Friedman

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



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]

Ductwork  mold (C) D Friedman Ductwork  mold (C) D Friedman

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

Photograph of damaged duct fiberglass lining Photograph of dirty fiberglass insulation fibers - higher risk of mold contamination

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.

Photograph of water damaged fiberglass HVAC duct lining Photo of moldy HVAC ducts

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:

Mold Growth on Fiberglass HVAC Duct Interiors

Below: a photograph of white or light gray mold growth on the interior surface of a fiberglass-lined air duct.

White deposits on fiberglass HVAC Duct interior (C) JC

While more investigation was required, we speculated that the Atlanta Georgia home where this duct mold was found had either suffered water or high moisture in the ductwork or there had been another source of high levels of Aspergillus, Penicillium, and perhaps other molds in the building's indoor air.

Details of this air duct mold contamination study are found at FIBERGLASS AIR DUCT MOLD TEST.

Advice for Suspected or Known Mold-Contamination in HVAC Ductwork

This topic is now found at Also see DUCT CLEANING ADVICE


Continue reading at WHEN to TEST INSULATION for MOLD or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BLOWER LEAKS, RUST & MOLD for Readers concerned with mold contamination in heating and air conditioning air handlers and ductwork



Or see PARTICLE & MOLD LEVELS in DUCTWORK where we describe how to test HVAC systems and ductwork for mold.




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WHY DOES MOLD GROW in INSULATION? at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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