Fiberglass Duct Liner for Efficient and Quiet HVAC Duct Systems
The introduction of HVAC systems established the need for duct insulation. A forced air duct system is designed to be a conduit for the supply and return of fresh and conditioned air to and from occupied spaces of a building. Insulation is used to control duct-borne noise, reduce heat loss or heat gain through duct walls, and minimize water vapor condensation. An internally-lined duct system safely conveys fresh and conditioned air, quietly, throughout occupied spaces at the correct design temperature.
What is fiberglass duct liner?
Fiberglass acoustical duct liner is fibrous glass insulation used as a sound absorbing and thermal energy insulating liner for interior surfaces of ducts, plenums, and other air handling equipment, that handle air up to 250°F (121°C). Reference: ASTM C1071
Where is duct liner used?
Many types of systems, buildings, and spaces within higher-occupancy commercial buildings where acoustical comfort, thermal comfort, and safety are key including:
- Conference Rooms
- Hospitals and Healthcare
- Performance Halls/Theaters
- Churches/Temples/Houses of Worship/Religious Venues
- Recording Studios
- Office Spaces
- Education Facilities
- High Velocity Air Distribution Systems
The duct systems that serve the space conditioning needs of these uses are prime-candidate buildings and systems for the use of fiberglass acoustical and thermal duct liner.
The performance and physical properties required of fiberglass duct lining materials are detailed in ASTM C1071 Standard Specification for Fibrous Glass Duct Lining Insulation (Thermal and Sound Absorbing Material). There is no minimum or maximum standard velocity rating for fiberglass duct liners required. ASTM C1071 only details the proper test method used to determine if a particular product can be rated for the maximum airstream velocity stated for a given product. All duct lining materials do not offer the same velocity ratings. There are no positive or negative duct air pressure ratings attendant with fiberglass duct lining products. The pressure-class details of the duct wall products and construction methods employed by the designer or duct fabricator determine the operational limitations of the system related to air pressure.
There are significant differences in acoustical and thermal duct liner materials from other flexible mechanical insulations. Fiberglass duct liner is the most cost effective and technically effective method to provide acoustical attenuation in duct systems, when compared to other options like increasing duct sizing or adding sound attenuators. The requirements given in ASTM C1071 ensure that a fiberglass acoustical and thermal duct liner is designed and tested for safe continuous operation with no degradation when exposed to a moving air stream, internally, in HVAC air distribution systems for both heating and cooling systems. Fiberglass duct liner products are subject to more rigorous required testing than other flexible fiberglass insulation, for properties critical to air distribution system performance.
Fiberglass duct liner delivers proven performance for durability and safety as demonstrated by these important properties:
Material Testing for:
Quality and Safety
UL 181 – Service Velocity/Erosion Resistance
ASTM C665, C1617 – Corrosiveness to HVAC air distribution components
ASTM C1304 -- Odor
ASTM C1338/G21/G22/UL 2824 – Microbiological Resistance
ASTM C167 – Dimensional/Density Tolerances
ASTM C423/E795 – Sound Absorption, Noise Reduction
ASTM C177/C518/C1114 – Thermal Conductivity/Heat Flow Limitation
ASTM C411– Upper Service Temperature, Stability
Moisture and Condensation
ASTM C1104 – Water Vapor Sorption
Why Fiberglass for Duct Lining?
Fiberglass acoustical and thermal duct liner provides high-performance as an acoustical absorber, is designed to be durable in service, is inert in reaction to metals typically used in HVAC duct construction and affords a high level of fire safety. Fiberglass duct lining provides these features while being highly cost effective and highly versatile.
Fiberglass duct liners are often misunderstood by the construction community – owners, designers and installers alike. Products that make good sound energy control materials also generally make good thermal energy control materials, too. This is especially true of fiberglass products, duct liner included.
One function of duct liner is as an acoustical insulation, designed to reduce the transfer of acoustical energy from one location to another. This function involves removing sound transmission from operating HVAC air distribution systems, enhancing the Acoustics segment of Indoor Environment Quality (IEQ).
Another function of duct liner is as a thermal insulation, designed to retard heat gain and loss with the primary goals of energy savings, condensation control, and enhancing the Thermal Comfort segment of IEQ. Duct liner is usually a less cost-effective method of providing thermal energy control. If the acoustical control in an HVAC air distribution system is not required, another form of insulation to provide the thermal control function might be considered.
A highly common practice in air distribution ducts is to line and externally insulate air distribution components to provide the most cost-effective combination of the two materials. For example, if a duct system required acoustical control and thermal control to an R-8 level of thermal insulation, a designer might select 1” thick duct liner that provides an installed R-value of 4.2, then add a layer of fiberglass duct wrap to the external surface of the duct that provides an installed R-value of 3.8 or greater. The combined thermal value will meet or exceed the need for the thermal insulation value, while delivering high-quality and cost-effective acoustical comfort. This combination provides highly effective acoustical and thermal energy control.
Based on the current fiberglass duct liner products in the market, fiberglass duct liner can be used in HVAC duct and equipment, serving HVAC air distribution systems that have airstream velocities up to 6,000 feet per minute. As stated earlier, not all duct liners are rated to the same velocity level. Confirm the performance of the products being considered. Fiberglass duct liner is meant for service in HVAC air distribution systems at a maximum service temperature of 250°F only. It is not intended for kitchen exhaust, fume hood exhaust, solids conveyance systems, or other similar ducts.
Fiberglass duct liner products exist to line rectangular/square, spiral-round, spiral-flat-oval geometries in either single-wall or double-wall duct construction. Other air distribution system components that might receive installation of duct liner include air handlers, unitary equipment cabinets, fan coils, VAV boxes, sound attenuators, plenums and transfer ducts, and air terminal devices like boots, boxes, and diffusers.
Forms of Duct Liner Material
Fiberglass duct liner materials are supplied in blanket form as rolls, and rigid form supplied as boards. Blanket/roll products offer a wide range of thicknesses and widths to meet performance and application requirements. Rigid/board form is somewhat limited to meet the needs of larger plenum spaces.
Duct Sound Transmission and Acoustic Performance
The types of unwanted sound that duct systems can experience or generate that make a space acoustically uncomfortable include:
- Crosstalk Noises: Airborne origin which uses air waves to transmit from a room to another, e.g. voices or music traveling through diffusers in adjacent rooms located on the same duct line
- Insertion Noises: Airborne origin and/or structure-borne origin transmitted by vibrations from one room to another, e.g. noise generated by the fan of the HVAC system
- Other Noises/System-borne Noise: Whistling noises, rumbling noises, popping noises
Sound or noise attenuation is the loss of energy from sound waves or damping of sound, an interruption that diminishes the volume and quality of the sound wave, the depletion of sound energy.
Sound or noise attenuation for HVAC air distribution can take the forms of Sound Diffusion, Sound Absorption and Vibration Reduction (Noise Management).
The effectiveness of a duct liner is typically measured in one of three ways:
- The Sound Absorption Coefficient (SAC) – the amount of sound energy absorbed upon striking a particular surface. The SAC represents a percentage of sound energy absorbed at a specific frequency.
- The Noise Reduction Coefficient (NRC) – the average of SACs, at the octave band center frequencies of 250, 500, 1000 and 2000 Hz.
- The Sound Insertion Loss– the reduction of sound pressure level due to placement of duct liner in the sound path between the sound source and the receiver. This value is measured in decibels.
Density vs. Thickness Performance
A common perception is that increasing density is the best way to increase the acoustical absorption properties of fiberglass duct liner. However, increasing the thickness of the duct liner generally provides higher sound transmission reductions than increases in density. It is important to understand the frequency ranges being considered since increased density may be more effective at higher frequencies.
Fabrication/Installation of Fiberglass Duct Liner
Duct liner blankets can be applied manually or automatically. Rigid duct liner is applied manually.
Maintenance and Cleaning
Fiberglass duct liner is designed to withstand proper cleaning, being made with airstream surfaces that are resistant to damage, per NAIMA AH112 Cleaning Fibrous Glass or Lined Sheet Metal Ducts.
There are three widely-endorsed practices for cleaning duct liner:
- Contact Vacuuming - A portable vacuum is used and contact of the brush head to the duct liner surface dislodges and removes dirt and debris.
- Air Washing - Compressed air is supplied to a hose with a ‘skipper’ nozzle. The compressed air propels the nozzle inside the duct, dislodging dirt and debris. The dislodged dirt is drawn out of the system by vacuum equipment.
- Power Brushing - Rotating brushes loosen dirt and debris which is drawn into a vacuum, as in the air washing method.
Fiberglass duct liner is recognized as highly effective in performance by many industry design professionals and has been installed successfully in many buildings around the world. Fiberglass duct liner is a highly-engineered and tested material and is a safe choice for both occupants and acoustic abatement performance. When used as intended, a long and trouble-free service life is the result.
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
- 2021 ASHRAE Handbook—Fundamentals
- 2019 ASHRAE Handbook—HVAC Applications
- 2020 ASHRAE Handbook—HVAC Systems and Equipment
- Committee C16 Subcommittees (astm.org)
- Standard Specification for Fibrous Glass Duct Lining Insulation (Thermal and Sound Absorbing Material) (astm.org)
National Fire Protection Association (NFPA)
- NFPA 90A: Standard for the Installation of Air-Conditioning and Ventilating Systems
- NFPA 90B: Standard for the Installation of Warm Air Heating and Air-Conditioning Systems
North American Insulation Manufacturers Association (NAIMA)
- AH112 Cleaning Fibrous Glass or Lined Sheet Metal Ducts - Duct Insulation | Air Handling | Insulation Information (insulationinstitute.org)
- AH113 The Facts About Mold Growth - Duct Insulation | Air Handling | Insulation Information (insulationinstitute.org)
- AH114 The Facts About Airborne Fibers - Duct Insulation | Air Handling | Insulation Information (insulationinstitute.org)
- AH124 Fibrous Glass Duct Liner Standard -- Design, Fabrication and Installation Guide - Duct Insulation | Air Handling | Insulation Information (insulationinstitute.org)
- AH138 Fibrous Glass Duct Liner - Duct Insulation | Air Handling | Insulation Information (insulationinstitute.org
Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA)