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.

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

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.

[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).

Our photo of a moldy air handler interior in a Florida home (left) is provided by home inspector & educator Mark Cramer.[3]

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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.

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 AIRBORNE 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 photo above 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 photo just above 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 photographs above and below 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.

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.

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 DUCT CLEANING ADVICE

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2017-10-14 - by (mod) -

Yeah well Anon, currently it's clear that Canadians are orders of magnitude smarter than their yank cousins, just look at how trumplandia, I mean the U.S., is being governed.

And I agree completely that external duct insulation is preferable: that makes it possible to actually clean the interior of metal ductwork work.

If you're suspecting air quality problems from your ducts nevertheless, inspect the air handler, blower fan, plenums, and accessible duct areas for a build-up of debris that might be worth cleaning; and of course check those air filters.

On 2017-10-14 by Anonymous

In Canada we use insulation on the outside of the duct work not on the inside lining the ducts
Can this help prevent this problem I am experiencing a scratchy throat

On 2017-04-12 - by (mod) -

Bill:

When you smell mold typically you're smelling volatile organic compounds or mold-volatile-organic compounds (MVOCs) that can permeate soft materials such as duct insulation. Considering how much material has been replaced, I'd think your onsite experts will want to

- examine/replace other ductwork in the HVAC system

- inspect for other leaks and thus possible hidden mold reservoirs.

Air tests for mold, particularly when results are negative, are almost entirely unreliable. Airborne dust, debris, mold levels can vary by several orders of magnitude depending on just how and when a "test" is conducted.

On 2017-04-11 by Bill Danford

I replaced my AC system and after almost a year I started to noticing musty smell. Almost like an old dirt smell. Checked the AC and found the unit and ductwork were saturated with water. After a lot of back and forth, they found the unit was not installed properly and water was flinging off the coil and going into the unit and ductwork.

They reinsulated the unit, replaced part of the supply and return ductwork and the smell still has not gone away. They determined the smell was coming from the unit and ultimately replaced the whole coil/blower and a short run of R/A ductwork and the supply plenum.

The smell is still very evident, musty/dirty smell. Could whatever is causing the smell penetrated more of the fiberboard ductwork and giving off this musty smell? House is in south Florida and has fiberboard ductwork with flex ducting down to registers. I had a mold company do an air test and the only thing they found was Asperillus/Penicillium-Like, Raw count 1, count/m3 - 13. Is that enough to cause the musty smell.

On 2016-10-04 by Mike

Is it the fault of the HVAC company if mold has started to grow in the fiberglass insulation that lines the evaporator coil? I also have an open sump pit in the basement that regularly has water in it. The mold found in our duct work is Aspergillus/Penicillium. The system is 9 years old, it has metal ductwork that is sealed at all the connection joints.

Question: any lawsuits or class action over HVAC insulating duct boards?

(Aug 26, 2014) Maggie said:
Are there any type of lawsuits for duct boards?

Insurance won't pay anything for the remediation of my home and I am at a loss on where to even start - no one seems to be able to tell me the source of my leak or what has caused the contamination or how long I've had the mold growth and I am now being told my whole house needs to be remediated because it's blown throughout the duct work for who knows how long and the levels are off the charts

Reply:

Maggie,

I'm doubtful that lawsuits are the best place to start attacking the problem you describe. I'd start by hiring either an experienced building inspector or HVAC system technician to diagnose the problem you face, including its cause, providing evidence and documentation, not just "opinion" lest you get nowhere. If necessary you may need an independent insurance adjuster to help out.

From just your note we don't know what contamination is the concern, nor how we know what's been blown through the building, at what levels, and causing what need for further cleaning.

(Aug 26, 2014) Anonymous said:

Thanks Dan. I have actually had all the mold testing along with air samples done and that's how I know it's all throughout my house. I have the type of mold and the counts etc. The insurance adjuster is coming to see if there's any possibility of them paying for any of it - but they say there's slim to no chance of them paying for any of it and if they do it is a minimal amount of the cleanup.

I am in the process of getting estimates from 3 mold remediation companies and an estimate from a HVAC company for replacement of the duct board and/or whole system. I spoke to a real estate agent about "resale" values and the effect of mold in the home and he is actually the one who suggested researching if there were any lawsuits relating to duct boards since insurance doesn't pay for anything etc. I did a google search but came up with nothing. The cost is very prohibitive to wanting to get your house clean so it's all pretty discouraging.

Reply:

If your HO policy excludes mold damage you're not covered.

WATCH OUT: DO NOT allow a superficial inexpert "cleanup" if in fact there is a large area of mold contamination - inexpert work may further spread contamination and require still more professional and costly cleanup. Before proceeding with cleanup you need a reliable mold remediation plan defining the scope of work, containment, etc.

(Aug 27, 2014) Maggie said:

Yes the insurance company came out today to do their inspection and nothing is covered. I have 4 companies coming tomorrow to provide me with quotes. Two of them work with insurance companies all the time and are supposed to be two of the top mold removal companies (Rainbow Intl and ServPro) the other is Advanta and the 4th is AAS Restoration.

If you've heard of any of these companies and have any reviews or thoughts on them I would appreciate any feedback. I've read Angies list reviews and other online reviews and really haven't seen too many bad reviews on any of them.

I also had a duct cleaning service come in that was NADCA certified and they were telling me about some type of service that they spray and paint and the manufacturer of the product guarantees no new regrowth or old leaks of mold for 10 years. I'm guessing this would be the cheapest but not sure if it is the most effective. If you have an opinion on that too I would appreciate your input.

ServPro also told me they use a different testing company because the mold inspection company I used tends to be very "over the top" with their recommendations. For instance because my readings on the 3rd floor were high they suggested having the whole house HEPA vaccumed. ServPro suggested the whole house may not require "total cleaning"

He stated some companies will then try to tell you you have to have absolutely everything cleaned or thrown away - like sheets, clothes and if you have a stuffed animal it would be thrown away. Tables could be cleaned but some furniture couldn't - so sometimes the "over the top" recommendations are not necessary and companies may try to get me to do more than necessary.

Not sure if you have thoughts on that either but if I do the minimum cleaning - in other words replace duct work clean extremely high level areas with HEPA vaccuming etc. how much damage will I leave if I don't HEPA clean whole house. I of course realize these are all broad opinions on items that you are not seeing or reports you are reading. Thank.

Reply: "throw everything away" is usually inappropriate advice

I agree with your ServPro rep that "throw everything away" is usually inappropriate advice. Some items such as hard surface items are usually easily cleaned; some soft goods can be laundered or drycleaned. Other items such as water damaged carpets, padding, drywall, upholstered couches are tossed out.

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

Or see these

Recommended Articles

• BLOWER LEAKS, RUST & MOLD for Readers concerned with mold contamination in heating and air conditioning air handlers and ductwork
• FIBERGLASS AIR DUCT MOLD TEST - using tape to test & identify debris on HVAC duct interior surfaces
• FIBERGLASS INSULATION MOLD - home
• INSPECTION of INSULATION for MOLD before testing - noting that contaminated building insulation may look clean to the naked eye.
• INSULATION MOLD CONTAMINATION TEST
• INSULATION STAINS - AIR BYPASS LEAKS
• TEST CHOICES for MOLD in FIBERGLASS
• TEST PROCEDURE for MOLD in FIBERGLASS
• VACUUM TEST INSULATION CONTAMINANTS
• WHEN to TEST INSULATION for MOLD
• 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

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

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to MOLD CONTAMINATION & REMEDIATION

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