Wet moldy building insulation (C) D Friedman BellefonteMold Contamination in Fiberglass in Insulation

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Mold contamination occurrences in fiberglass insulation:

This article, the head of a series of texts about mould contamination in building insulation, explains the cause, detection, and hazards of mold growth in fiberglass insulation in residential and light-commercial buildings.

We illustrate how to find or test for moldy insulation in buildings, the probable cause of mold contamination in building insulation, and how to recognize conditions that make that problem likely in a particular case. 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.

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.

Cause & Detection of Mold Contamination in Fiberglass Insulation in Buildings

Mold growth on an individual fiber in fiberglass insulation (C) Daniel Friedman

Mold and mold-odors in building wall and ceiling and even floor cavities can be traced to leaks into the cavity or in humid climates such as areas in the U.K. and North America if homes in those climates are not properly constructed to keep both leaks and humid air out of the wall, ceiling or floor cavity.

Our own study has shown that significant mold contamination can be found in some building insulation materials even when to the naked eye the insulation looks "clean".

Article Contents

The 720x photographs above and just below show 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.

Photograph of Moldy visible growing on a fiberglass insulation fiber  © Daniel Friedman

Fiberglass in building insulation is a topic I have been testing and studying for nearly twenty five years, after having first traced a building mold contamination reservoir to a hidden source in this material. I frequently find high levels of mold-contaminated fiberglass insulation in buildings which contain other large mold reservoirs. I have also detected high levels of problematic mold in fiberglass building insulation where other mold reservoirs were either not present or had been previously removed.

Our moldy building insulation photo at page top (contributed by a reader) shows an obvious case: very wet fiberglass insulation under a roof and extensive black mold growth on at least the insulation surface, probably inside the insulation as well.

But in a more subtle, and not easily visible form, problematic building contamination by mold is often found in otherwise clean-looking 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.

That "hidden" insulation mold is the focus of our discussion in this article.

Clean-looking Fiberglass Insulation may be Mold Contaminated if Exposed to Wet Conditions or A Secondary Airborne Mold Source

Except for some superficial "dust staining" that is often simply thermal tracking by house dust, the pink fiberglass insulation shown below (left) looked clean. Unlike our black insulation mold photo above, there was no visible mold on the pink fiberglass insulation shown at below left.

But vacuuming the center (most clean-looking area) of that mold and lab examination of the vacuum dust sample contents showed the long Penicillium/Aspergillus spore chains (below right) consistent with local problematic mold growth.

Photograph of crawl space insulation which testing found to be mold contaminated.

High levels of mold may be present in fiberglass insulation: We have measured very high levels of airborne problematic mold spores which were traced to a building reservoir of moldy fiberglass insulation.

Photograph of mold spores of Aspergillus sp. found in crawl space fiberglass insulation.

Recapping, the pair of photographs (above) shows fairly clean-looking fiberglass insulation over a crawl space which in fact had been subjected to flooding. While the insulation itself did not appear to have been flooded, and while there was no mold visible on or in this fiberglass insulation, a simple vacuum test demonstrated that the insulation was severely contaminated with Aspergillus sp. mold.

Fiberglass insulation can host active mold growth

The presence of both mold spore chains and conidiophores of Aspergillus sp. in the insulation test samples whose photos are shown above confirmed that not only was the crawl space ceiling fiberglass insulation moldy, but it was supporting active fungal growth.

Our screening samples confirmed that this mold was present in other building areas, most-likely emanating from this mold reservoir of mold-contaminated fiberglass insulation. In some of cases, non-visible mold contamination in fiberglass insulation has been enough to cause IAQ, health, or other mold-related complaints by building occupants, and in some cases

Photograph: Water stains on drywall suggest this fiberglass insulation may be mold-contaminated.
Mold contamination with Aspergillus sp. was confirmed by special sampling and lab methods.

In the partially-opened basement wall shown here, the water track stains on the cavity side of the exposed drywall (shown after a test cut was made) indicate that water passed in this wall from above.

Photograph of mold spores of Aspergillus sp. found in crawl space fiberglass insulation.

In this circumstance, even when the fiberglass insulation looks clean, I often find high levels of Penicillium sp. or Aspergillus sp. in this material. Comparison tests of fiberglass which is new at a building supply store or in homes where the insulation has not been wet nor infested with rodents or other pests, mold is rarely a problem.

The photo at above right shows a very dense presence of Pen/Asp spores and spore chains as well as a portion of a conidiophore (lower left) in this insulation test sample, indicating that mold appeared to be growing in the insulation, not simply accumulated there from another building mold reservoir.

Mold Growth Found on Fiberglass-Lined HVAC Duct Interiors

Mold on the Interior Surface of Fiberglass-lined HVAC Ducts

White deposits on fiberglass HVAC Duct interior (C) JC

[Click to enlarge any image]

Above: white "growth" found on the interior surface of fiberglass-lined HVAC ducts in a home in Atlanta, GA in 2016. Look closely to see the presence of clear adhesive tape being used to collect a sample from this surface to permit laboratory analyais.

Samples from this fiberglass duct were examined in our forensic laboratory. Below: an example of lab test results included these:

Fiberglass duct  surface mold test results (C) Daniel Friedman Fiberglass duct  surface mold test results (C) Daniel Friedman

Above first photo, sample 2, Aspergillus sp. conidiophore and hyphae at upper left, Penicillium conidiphore at lower right.

Above second photo sample 2, Cladorporium sp. in dense fungal growth on the surface.

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

Fiberglass Insulation May Have Been Contaminated by Airborne Mold Spores or Other Pathogens from Other Sources

Just below is an individual Curvularia-like mold spore to the left of what is probably a fragment of gypsum or drywall material.

More subtle in this photo are individual Penicillium/Aspergillus mold spores. Small and colorless these are harder to see unless the microscopist takes care and time. The combination of these particles in a vacuum sample of fiberglass insulation suggests that this insulation became contaminated by airborne debris, perhaps during demolition of moldy drywall.

Curvularia-like mold spore in fiberglass insulation (C) Daniel Friedman

Below is a large hyphal fragment in fiberglass insulation; from this observation we can be confident that wet conditions and growing mold have been present in the building where this sample was collected. The image does not mean that this fungus was necessarily growing in or on the fiberglass itself.

Hyphal fragment isolated from fiberglass building insulation (C) Daniel Friedman

Below are multiple Stachybotrys chartarum mold spores in a fiberglass insulation sample nd in the background I see some colorless Pen/Asp spores as well. This sample strongly suggests that there has been demolition of moldy materials, probably drywall, in the area where this fiberglass was exposed.

There is no evidence that these fungi are growing on or in this particular insulation sample. Any thick fibrous material can collect airborne particles and can be difficult or impossible to clean completely. This fiberglass insulation should be discarded.

Stachybotrys chartarum "black mold" contamination in fiberglass building insulation (C) Daniel Friedman

Below my photo illustrates a short three-spore chain of Penicillium or Aspergillus mold spores. These fungal contaminants are a particular concern because their small size (often in the 1u range) means they can be inhaled deeply into the lungs.

High exposure levels, or repeated exposure to high levels of this type of airborne mold can cause serious illness such as Aspergillosis.
See ASPERGILLOSIS for details.

Aspergillus or Penicillium spore chain in fiberglass insulation (C) Daniel Friedman

Because these spore chains are very fragile and break into individual spores quickly when airborne, when we find a spore chain of Penicillium or Aspergillus mold in an environmental sample I am confident that there has been or is currently nearby active growth of one or both of these mold genera/species.

I have on occasion found actual Penicillium or Aspergillus mold conidiophores growing in fiberglass insulation (as well as on or in a very large number of other building materials). These mold contamination problems occur in buildings that have been wet, flooded, or occasionally subject to very high humidity.

Where to Look for Mold in Building Insulation

For buildings which do not have other known mold reservoirs, special attention needs to be given to inspecting and testing for problematic mold in

Moldy building or duct insulation may look clean on visual inspection

Photograph of clean fiberglass insulation fibers - low risk of mold contamination

What can be tricky in investigations of mold contamination in building insulation is that severely mold-contaminated fiberglass insulation may look pretty clean to the naked eye.

Photograph of dirty fiberglass insulation fibers - higher risk of mold contamination

Special vacuum and agitation methods are needed to sample and test this material and special care is needed in choosing the sample or test location when looking for mold in fiberglass or other building insulation.

The left photo above shows clean fiberglass insulation fragments (taken from a sample of new fiberglass building insulation).

The right photo of a sample collected from fiberglass insulation in an older building exposed to moisture and leaks shows a high level of particulate debris, almost certainly including organic debris such as skin cells, animal hair (FIBER & HAIR IDENTIFICATION), and insect fragments which can form a base for mold growth.

Not All Stained Insulation Is Harmful and Other Building Materials Are Also at Risk

Watch out: not all stained or dirty-looking fiberglass is moldy or even contaminated. A very common source of black, gray or tan "stains" on building fiberglass is air bypass leaks that deposit house dust or building dust on or in the insulation as air passes over or through the material.

This is not mold contamination and that fiberglass does not need to be removed. Rather it makes sense to find and fix the air bypass leaks.

Air bypass leak stains on building insulation © Daniel Friedman


Keep in mind that the air bypass stains as well as mold, insect fragments or other contaminants described in this article can also be found on or in many building materials and in porous materials may be difficult or impossible to clean economically.

When to Test Building Insulation for Mold

Details of this topic are now at WHEN to TEST INSULATION for MOLD

We do not recommend routine mold testing of building fiberglass for mold in non-suspect cases. "Spot checks" by "mold testing" in buildings, if conducted without an expert diagnostic visual inspection and history gathering, are simply not reliable and thus not cost-justified.

We also do not recommend mold-testing of fiberglass or other building insulation that is visibly moldy (such as mold growth on kraft paper facings) nor of building insulation that has been wet. Such testing is not necessary and would not change the advice given by experts: Such insulation areas should be treated as follows:

  1. Remove the wet, moldy, dirty, or suspect insulation
  2. Clean the building cavity
  3. Correct the leak or other cause of contamination
  4. Install new insulation

See ATTIC MOISTURE or MOLD SOURCES for a discussion of common sources of moisture in attics that can cause moldy insulation, and similarly,

See BASEMENT LEAKS, INSPECT FOR for the same problem in basements.

Readers ducting cool air through a crawl space or who have damp or wet crawl spaces under their building should

So when might mold testing of building insulation be merited?

  1. If a physician treating someone suspected of building-related illness wants to know what mold genera/species to which the occupants may have been exposed
  2. If a building-related illness is know or suspected to be caused, aggravated, or contributed-to by possible contamination of building wall and ceiling cavities that have been wet or that have been exposed to airborne contaminants but the insulation and cavities appear clean, it may be useful to try vacuum-testing some representative samples of the insulation for mold. Vacuum-test the most-suspect area(s).
  3. See INSULATION MOLD CONTAMINATION TEST if testing is appropriate for your situation.


Continue reading at WHEN to TEST INSULATION for MOLD, advice about when where how and why to test building insulation for mold contamination., 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 FIBERGLASS AIR DUCT MOLD TEST - using tape to test & identify debris on HVAC duct interior surfaces

Or see INSPECTION of INSULATION for MOLD before testing - noting that contaminated building insulation may look clean to the naked eye.




Or see WHY DOES MOLD GROW in INSULATION? that includes photos and test results examining suspected mold on the surface of fiberglass-lined HVAC ductwork.

Suggested citation for this web page

FIBERGLASS INSULATION MOLD at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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