Duct liner is an insulation material applied directly to the interior surfaces of sheet metal HVAC ductwork. Composed of fiberglass, mineral wool, or closed-cell foam, it is bonded to the inside walls of the duct using adhesives and mechanical fasteners. Its primary functions are to reduce thermal losses or gains as conditioned air travels through the duct system and to attenuate noise generated by airflow and HVAC equipment.
Technical Specifications
Duct liner is available in common thicknesses of 1 inch and 2 inches, with thermal resistance (R-value) typically ranging from R-4 to R-8 per inch depending on the material and density. The surface facing the airstream is coated or faced to resist fiber erosion caused by moving air. Air velocity limits are critical to maintaining liner integrity: supply ducts generally should not exceed 5,000 feet per minute (fpm), while return ducts are limited to approximately 2,500 fpm. Exceeding these thresholds can degrade the liner surface and release particulates into the airstream.
Many duct liner products feature antimicrobial coatings designed to inhibit the growth of mold, mildew, and bacteria on the liner surface. Because the liner is in direct contact with conditioned air, maintaining indoor air quality is a key design consideration. Proper installation, including fully sealed edges and secure adhesion, is essential to prevent moisture accumulation and material deterioration over time.
Applications
Duct liner is widely used in commercial and institutional HVAC systems where both thermal performance and noise control are priorities. Common applications include:
- Supply and return ductwork connected to air handling units (AHUs) and fan coil units (FCUs)
- Plenums and duct sections near mechanical rooms where equipment noise is most pronounced
- Ductwork routed through unconditioned spaces such as attics, crawlspaces, and parking garages where thermal insulation is critical
- Interior duct runs in occupied spaces where sound attenuation contributes to occupant comfort
Duct liner differs from duct wrap, which is applied to the exterior of ductwork. While both provide thermal insulation, duct liner offers superior acoustical performance because it directly absorbs sound energy within the airstream.
Relevant Standards and Codes
Duct liner products and their installation are governed by several industry standards. UL 181 establishes fire safety and performance requirements for factory-made duct materials, including liners. NFPA 90A, the Standard for the Installation of Air-Conditioning and Ventilating Systems, sets requirements for materials used within duct systems in terms of flame spread and smoke development ratings. ASHRAE standards also provide guidance on thermal insulation requirements for ductwork based on climate zone and system configuration.
Practical Significance
Properly installed duct liner contributes to measurable energy savings by reducing the temperature differential between conditioned air and its surroundings as it moves through the distribution system. In buildings where ductwork passes through unconditioned zones, the thermal benefit is especially significant. The acoustical dampening properties also reduce the need for separate silencers or sound attenuators, potentially lowering overall system costs. However, duct liner requires periodic inspection to ensure the facing remains intact and that no moisture-related issues have developed, particularly in high-humidity environments.