Johns Manville Micro-Aire round flexible ductwork, ca. 1977 (C) HowlandMicro-Aire® FS Fiberglass HVAC Ducts
Johns Manville MicroAire Ductwork

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Fiberglass air ducts produced by Johns Manville, sold under the Micro-Aire and Micro-Aire FS brand are described here.

We discuss both older or "original" Micro-Aire® duct products like the one shown at the top of this page and current Micro-Aire HVAC duct materials that include flex-duct and rigid-board fiberglass duct construction.

This article series describes the different types of fiberglass heating & cooling ducts including fiberglass duct wrap, flexible fiberglass insulated ductwork, fiberglass duct board, and fiberglass duct inspection, installation, cleaning, and restoration.

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.

Johns Manville Micro-Aire® HVAC Ducts

Johns Manville Micro-Aire(R) Mat-Faced Fiberglass Duct Board Type 475 & 800 at and cited in detail in this article, Johns Manville Corp. (Berkshire Hathaway)  10/30/18Current & Recent Micro-Aire® Air Duct Description & Installation Guides

The illustration shown here is of Johns Manville's current Micro-Aire® Mat-Faced fiberglass duct board, type 475 & Type 800, used both as a rectangular HVAC duct product and in related forms as an insulator inside or forming air handlers and supply or return air plenums.

Johns Manville describes this product as follows:

Micro-Aire® duct board is produced from strong glass fibers, bonded with a thermosetting resin.

The airstream side of Micro-Aire duct board features a black fiber glass mat, which minimizes visibility of the duct system at supply air and return air outlets while providing excellent durability in high-velocity conditions.

The exterior surface features a fire-resistant foil-scrim-kraft facing extending the full width of the male edge to serve as an integral closure flap for section joints.

Micro-Aire duct board is molded with double-density, male/ female edges for secure connections.

The airstream surface of Micro-Aire duct board is treated with an antimicrobial agent specifically registered with the EPA for HVAC applications to resist potential growth of fungus or bacteria on the airstream surface.
- HVAC Insulation, Micro-Aire® Data Sheet 2018/10/30 cited below.  

Cleaning Fiberglass Duct-board Air Ducts

If cleaning is necessary, the airstream surface may be cleaned using standard industry-recognized dry methods. See the North American Insulation Manufacturers Association (NAIMA) “Cleaning Fibrous Glass Insulated Air Duct Systems”  -(Johns Manville 2018)

Watch out: OPINION: using aggressive mechanical cleaning of fiberglass HVAC ducts risks causing severe damage to the ductwork and may also cause fiberglass hazards in indoor air.

Older Johns Manville Micro-Aire® Air Ductwork Condition Assessment

Johns Manville Micro-Aire FS Fiberglass Flexible ductwork data tag (C) HowlandThis Johns Manville Micro-Aire® FS fiberglass air duct was installed in a U.S. home in 1977. (Photos courtesy of an anonymous reader)


My older home (circa 1977) has Johns-Manville “Micro-Aire FS” Fiberglas wrapped ducting in the attic from my furnace.

It has an Underwriters Lab label No. 2627092 Does it contain asbestos?

Is the old plastic type interior lining stable.

They used this instead of galvanized duct.

Should it be replaced if I add on an AC unit?

The product is 40 years old. Is it common that deterioration may have allowed leaks to develop? Do you think it would withstand a pressure test?

Please let me know. Thanks. - Anonymous by private email 2019/02/06

Reply: OPINION: the ductwork in your photos does not contain asbestos but it should be replaced

Johns Manville Micro-Aire round flexible ductwork, ca. 1977 (C) HowlandI would have at least see some photos of the duct, labeling, runs, connections, and visual evidence of deterioration. [The reader then provided the photos shown here.]

Plastic materials used in some other duct systems such as GOODMAN GRAY FLEXDUCT ( a different design ) deteriorated badly and could be seen by visual inspection to be broken or shredded.

Fiberglass duct-board that is foil faced and whose fiberglass is fabricated far more rigid than fiberglass-insulated flexduct is also in my opinion much more durable and, properly-fabricated, less likely to be leaky.

I am sorry to throw your money at this but if that were my house I would replace ALL of that ductwork if it looks quite dirty, frizzy inside, or if it has ever been wet or otherwise contaminated.

Some of your photos show that the duct interior has been damage.

Your first photo [above] shows lost fiberglass and exposed wire coil inside the duct. I think the fiberglass surface appears roughened.

I suspect someone tried mechanical cleaning. That loosens and damages the fiberglass, resulting in still more-rapid accumulation of house dust (not itself hazardous) in the future and in addition increases the chance of release of airborne fiberglass into building air.

While large fiberglass fragments may not be more than an irritant, small fragments (omitted by too many air and dust test consultants and labs) are potentially harmful and maybe even carcinogenic.


When the fiberglass is exposed on the inside of the ducts they cannot be cleaned mechanically.

Your second and third photos show that the exterior wrap on the duct is badly deteriorated and is bound to be leaky. That looks like a plastic wrap. Is or was that original?

Johns Manville Micro-Aire round flexible ductwork, ca. 1977 (C) Howland

The NADA National Air Duct Cleaners Association also notes that when ducts need to be cleaned, physically removing the contaminant is preferred, and the use of chemical treatments, sprays, coatings, is not a substitute but rather might be supplementary.

Do Johns Manville Micro-Aire ducts present an asbestos hazard?

To address your question, I found no evidence that asbestos was a component in the fiberglass duct about which you asked.

Carless research on asbestos in air ducts patenented by Johns Manville, Owens Corning and others can lead to confusing findings in part because of this patent classification that appears in some of those patents:

B32B19/02 Layered products comprising a layer of natural mineral fibres or particles, e.g. asbestos, mica the layer of fibres or particles being impregnated or embedded in a plastic substance.

And indeed asbestos was a key ingredient in transite (asbestos cement) pipes used for air ducts.


However the word "asbestos" doesn't appear in the product description in patents I cite below.

Here is a representative Micro-Aire patent assigned to the Johns Manville Corporation in 1978.

Notice that the word "asbestos" doesn't appear in the patent

Pearson's rigid duct patent 1963 at InspectApedia.comThe history of insulating fabrics that became used on HVAC ducts in North America is much older, as we see in

Here are all of the relevant patents I could find for Johns-Manville “Micro-Aire FS” Fiberglas wrapped ducting pertinent Johns Manville Patents

And related patents

The Toas patent disclosure (last in the list above) also includes these non-patent but relevant citations for understanding the properties of Johns Manville fiberglass duct products and their history.

Rigid & Flexible Fiberglass / Micro-Aire® Duct Construction, Cleaning & Inspection References


Continue reading at FIBERGLASS DUCT, RIGID CONSTRUCTION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see these

Fiberglass HVAC Duct Articles


Or see DISINFECTANTS & SANITIZERS, SOURCES if you need to sanitize or seal air duct interiors

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