Sorting out the options in fiber glass duct insulation

Key concepts Fiber glass insulation is used in HVAC systems for temperature, noise, and condensation control, and energy conservation. Four primary types of products are duct liner, duct wrap, duct board, and flexible fiber glass ducts.

By David Tomchak March 1, 2000

Key concepts

Fiber glass insulation is used in HVAC systems for temperature, noise, and condensation control, and energy conservation.

Four primary types of products are duct liner, duct wrap, duct board, and flexible fiber glass ducts.

Studies show fiber glass insulation does not contribute to microbial growth and has a positive impact on building IAQ.

For more than 40 yr, fiber glass insulation has been used as a component in air duct systems to maintain comfort and indoor environmental quality. HVAC fiber glass insulation is specified for four primary reasons:

Temperature control Delivering heated or cooled air at a temperature level best suited to building needs

Acoustical control Absorbing noise generated by air handling equipment and by air moving through ducts

Condensation control — Preventing condensation that could damage insulation and other HVAC system components

Energy conservation — Reducing HVAC system operating costs and energy use by lowering heat loss or gain through air duct walls.

More than 99% of all HVAC insulation products use fiber glass technology. The material remains an optimal choice in terms of safety, cost-effectiveness, and performance.

Types of insulation

There are four basic types of HVAC insulation products: duct liner, which is used in the interior of sheet metal ducts; duct wrap, which is used on the exterior of sheet metal ducts; fiber glass duct board, which is used to fabricate duct; and flexible fiber glass ducts.

1. Duct liner. Fiber glass duct liner products come as flexible blankets or rigid boards of fiber glass insulation. Either a coating or a fibrous mat may form the airstream surface. Liners are offered in a variety of thickness and density combinations from 1/2­2 in. Their tough airstream surface resists puncturing, tearing, and surface wear during fabrication, installation, operation, and cleaning.

Most duct liners contain an EPA-registered biocide to make them suitable for IAQ-sensitive jobs. Although most liner products are intended for application in square and rectangular ductwork, products specifically designed for round duct are available.

2. Duct wrap. Fiber glass duct wrap is applied to the outside of sheet metal duct. It is a blanket-type thermal insulation composed of glass fibers bonded together with a thermosetting resin. Duct wrap can be used for both round and rectangular duct. It normally has a vapor-retardant facing of foil scrim kraft (FSK) or vinyl. Unfaced duct wrap insulation is also available when vapor-retardant facing is not required. Rigid board insulation with reinforced FSK or all-service jacket facings (ASJ) are used as exterior insulation on large metal ductwork.

3. Duct wrap. Fiber glass ducts are fabricated from 1, 11/2, or 2-in. thick boards. The boards consist of insulation materials made from resin-bonded inorganic glass fibers. The outside surface of the board has a manufacturer-applied reinforced aluminum/FSK laminate facing that serves as an air barrier and water vapor retardant. Some products are available with mat-faced or coated airstream surfaces. Many contain a safe, effective EPA-registered biocide to protect against the growth of mold and mildew. Surfaces on these enhanced products can be easily cleaned using industry-approved methods.

4. Flexible fiber glass ducts . Flexible fiber glass insulated ducts consist of a spiral-wire-reinforced inner air core wrapped with fiber glass insulation and jacketed with a vapor-retardant reinforced foil or plastic film. These ducts provide an efficient, economical way to connect trunk ducts and room diffusers or registers. Flexible fiber glass ducts should be used in restricted lengths.

Effect of insulation on IAQ

A number of myths about the safety of fiber glass insulation have arisen over the years. Most question the impact of the material on a building’s indoor air quality (IAQ) levels. It is unquestioned that the presence of water in HVAC systems is what contributes to mold growth. And mold is no more likely to grow on fiber glass than on any other surface in the duct system.

A study conducted by Duke University backs up this claim. Researchers examined the role of such environmental factors as temperature, humidity, airflow, operating hours, and presence of liquid in the enhancement of microbial growth in a ventilation system. They consistently found that microbial growth is greatest in wet zones, such as areas close to cooling coils, mixing boxes, and places where cold air mixes with hot, humid air and creates condensation.

Keeping the surface temperature above the dew point can greatly reduce the wet conditions that contribute to microbial growth. Fiber glass insulation can help control condensation, inhibit mold growth, and positively contribute to a building’s IAQ.

Some specifiers have suggested that microbes use the binder and glass in duct liner as a food source and that microbial infestation can cause duct liner to degrade. A Harvard University School of Public Health study indicates this is not the case. The binders are not a nutrient for mold and do not amplify growth.

Others believe that glass fibers erode from the interior surface of fiber glass duct liner and board and become an irritant to workers. Study results from 25-yr ago are consistent with current research. The World Health Organization has declared that fibers do not have an adverse health effect on building occupants. The University of Nevada-Las Vegas also looked into the amount of exposure to occupants from manmade vitreous fibers when fiber glass insulation was used in air handling systems. That research confirmed that fiber erosion is not a significant issue.

Ensuring that an HVAC system continues to provide efficient, quiet air delivery, occupant comfort, and cost-effectiveness is best ensured with regular system maintenance. High-efficiency filtration is very important, as are regular inspections, adjustments, system balancing, and the draining of cooling coil trays.

— Edited by Jeanine Katzel, Senior Editor, 630-320-7142,