Cleaning, Replacing Fibrous Glass Duct Insulation

Fibrous glass duct liner has been simultaneously promoted and denigrated over the years and countless articles have been written on the subject.  Regardless of what ones personal view is on the material, fibrous glass duct liner is a product that has been widely used for many years and will continue to be used into the foreseeable future.

The intent of this article is not to take a position for or against the product, but to address the reality that fibrous glass is in place and continues to be installed and that it must be managed like any other component of an HVAC system.

The issue of cleaning fibrous glass insulation is a relative one and is subject to the condition of the materials surface and the integrity of the fiber matrix.  Duct liner that has a relatively intact surface membrane is typically cleanable from the standpoint of settled dust and debris.  The cleaning process can not be as aggressive as that which would be used in cleaning metal duct work and the results will only be for the surface and will not remove any significant amounts of debris that has found its way into the material.

The North American Insulation Manufacturers Association (NAIMA) recommends three methods, which include contact vacuuming, air washing and power brushing.  It has been our experience that contact vacuuming provides the best results with the  least damage to the material.  Contact vacuuming with soft brushes and light pressure penetrates more deeply into the uneven surface and tends to capture more dust and debris.  The method also provides for better results in the duct joints, seams and corners.  Air washing is less effective in our experience at removing the debris and does not perform well in the corners or seams.  This method however is less likely to damage the insulation itself compared to power brushing.  Power brushing provides better agitation of the surface but can not effectively reach the corners, seams and joints.  This method if not performed with great care can result in damage to the insulation.  For more information on the manufactures recommended cleaning practices, contact NAIMA for a copy of their guidance document.

While we have talked about cleaning duct liner that is relatively intact, damaged or degraded insulation poses a new set of variables.  When the surface coating of the duct liner is degraded cleaning may result in more damage to the material and actually worsen the condition.  In such cases encapsulates or coating materials may need to be considered as part of the restoration process.

Encapsulating products differ from coatings in that the encapsulates are formulated to penetrate the fiber matrix and bind the material together while the coating products are designed to create a new air streamside surface for the insulation.  No coating should be applied without first cleaning the surfaces as best as possible.  The decision to apply products such as coatings or encapsulates must be made on a case-by-case basis.  Will the added weight of the product delaminate the insulation?  What will be the impact on the thermoaccoustic performance?  And which product is best suited for the particular situation?

Once the decision is made to apply a coating or encapsulant you must select the proper product.  Not just any coating or encapsulant can be used in a ventilation system.   Fire codes often require that materials within the ventilation system meet specific test requirements for smoke generation and flame spread:  this is often referred to as Underwriters Laboratories Standard 181.  Products that cannot meet this standard should not be used in ventilation systems.

The issue of water damage or mold contaminated duct liner has been a hot topic.  The question rages on, Can it be cleaned and can it even grow mold?  The reality is simply that duct liner that has active mold growth or has had active mold growth should not be saved but removed.  The key to this statement is mold growth.  The presence of mold spores in duct liner is no reason to remove the insulation.  Many of us have seen consultants take swab samples of duct surfaces or press a Rodac plate in the duct dust then culture the sample and "low and behold" something grew;  I frankly would be more concerned if nothing grew because we expect to find dust and mold spores in virtually all HVAC systems short of clean rooms and certain health care applications.

When abating mold contaminated duct liner the guidelines established by EPA, New York City Department of Health and the American Conference of Governmental Industrial Hygienists should be the minimum practices employed for mold remediation.  These guidelines are similar and set the current standards of practice and care for the mold remediation industry.

When talking about water damaged insulation, the rule of thumb that is often followed is the same with any porous material including duct liner, sheet rock or carpeting to name a few.  If these materials get wet and remain wet for more than 48 hours [they] should be removed.  EPA has followed this line of thinking as far back as 1992 when they published their Building Air Quality Guide.

Duct liners can fail.  Whether failure is due to installation errors, abuse or other influences there are circumstances where the option to restore the material is not feasible.   Some common situations where the product will fail include insufficient adhesion to the metal, badly abraded surfaces and delaminating coatings.  In such cases where the decision is to remove the liner the removal method should be designed to safeguard the building environment.  The liner material will release fibers as it is scraped off of the sheet metal and the dust that is within the fibrous glass matrix will become airborne.  It is our practice to use the methods of abating mold when removing the duct liner.  The mold remediation guidelines lend themselves well to this application because they safeguard the workers and the building environment.

When duct liner is removed it may or may not be necessary to replace it.  If the liner is for thermal control the duct will need to be either internally or externally reinsulated.  If the duct liner is solely for acoustic reasons then its replacement can be at the discretion of the building owner.  When reinstalling fibrous glass duct liner it is important to follow the recommended  guidelines established by the manufacturers and the Sheet Metal and Air Conditioning Contractors, National Association (SMACNA).

Today there are at least several non-fibrous glass products being marketed, as replacements to duct liner and some appear worthy of consideration when reinsulation is necessary.  One of the more popular materials is Elastomeric insulation, which is a rubber based material and marketed under at least one brand name.  These products are closed cell and have very low moisture absorption in comparison to fibrous glass and some come with a pre-adhesive making installation very simple.  Other types of material include Polyethylene/Polyolefins.  These materials are a closed cell and have low water permeability.  The apparent advantage to these products over fibrous glass is the moisture content and durability.  While these products are inviting, they do have some limitations over the fibrous glass duct liners and that is their thermal and acoustic performance.  Both the elastomerics and the polyethylene/polyolefin's offer less thermo acoustic performance at the same thickness than the fibrous glass.

Currently there is a debate between the manufacturers of these replacement products as to their ability to meet the UL 181 requirements for smoke generation and flame spread.  The data from both products states that they meet the requirements however the manufacturers of the elastomeric material claim that the polyethylene/polyolefin's are high molecular weight waxes and subject to dripping thus "fool" the test procedures.  Unfortunately I do not have the answer to this debate so it is up to the installer and building owner to make the decision.  Regardless of the product you choose it must meet all local codes.

As times change so may the use of some products.  Currently the American Institute of Architects specified that fibrous glass duct liner should not be used in certain healthcare HVAC systems.  ASHRAE has raised discussion on the proximity of duct insulation to moisture sources.  Fibrous glass duct liner, whether you agree or disagree with it's application in HVAC systems, is a reality and must be incorporated in an ongoing preventative maintenance program like all aspects of an HVAC system.

Charles W. Cochrane is president of Cochrane Ventilation Inc. In Wilmington, Mass., and is  a past president of the National Air Duct Cleaners Association.