Flexible duct, known as flex duct, is very convenient ducting for attaching supply air outlets to rigid trunk ductwork, and, in many cases, flex duct comprises the entire duct system. Flex duct made for HVAC applications is typically constructed of a plastic inner liner attached to a metal wire helix (or coil) to make round, flex duct. The duct comes with a layer of fiberglass blanket insulation already attached around the duct. The insulation is covered and protected by a polyethylene or foil vapor barrier. Flex duct is typically available in insulation values of R-4, R-6, and R-8. For residential HVAC systems, insulated flex duct typically comes in diameters of 4 inches through 10 inches; above 10 inches, it comes in even sizes of 12, 14, 16, etc., up to 22 inches measured at the radius of the metal helix.
Figure 1 - Flex duct consists of a plastic inner liner attached to a metal coil, covered by a layer of fiberglass blanket insulation, which is covered by foil or plastic vapor barrier.
Ideally, ducts should be located inside the conditioned space in dropped ceiling duct chases. However, ducts are often installed in crawlspaces or attics. In attics, the ideal location would be laying ducts on the ceiling rafters or on the truss supports. Note: in humid climates, the ductwork should not lay in or be buried in loose fill or fibrous insulation due to the risk of mold from condensation if the ducts should develop an air leak or if the duct insulation is compromised in some way that allows cold spots to form. Building America recommends, and DOE Challenge Home requires, that in all climates ducts that are to be “buried” are insulated with R-8 duct insulation than encapsulated with an additional minimum 1.5 inches of closed-cell spray foam insulation, then covered with at least 2 inches of blown-in insulation.
Figure 2 - Flex ducts may rest on ceiling joists or truss supports.
If that cannot be accommodated, then the ducts are typically hung from the ceiling with strapping. Properly supporting flex duct is vital to supply good air flow and to maintain the integrity of the flexible duct material. When the flex duct is improperly supported by spacing the supports too far apart or by using supports straps that are too narrow or too tight, the metal helix and plastic inner liner of the flex duct can be damaged. Hanger straps need to be wide enough to prevent the straps from compressing the insulation and cutting into the inner liner. Straps must be adequately spaced to prevent sagging of the ducts.
Manual D Residential Duct Systems (ACCA 2009) sets standards for flex duct design and installation. See Appendix 17 for a detailed discussion of duct installation. The Flexible Duct Performance Standards (Fifth Edition) by the Air Diffusion Council (ADC) provides guidance to designers, architects, engineers, contractors, and installers.
For more on flex duct installation, see No Kinks or Sharp Bends in Flex Duct Installation, Sufficient Cavity Space for Flex Ducts, No Excessive Coiled or Loped Flex Ducts and Sealed and Insulated Flex Ducts.
How to Properly Support Flex Duct that is Hung in an Attic or Crawlspace
1. Hang flex duct using the support straps and methods specified by the manufacturer. The hanger or saddle material in contact with flex duct should be at least 1.5 inches wide and should be sufficiently loose to prevent restriction of the internal diameter of the duct when the weight of the supported section rests on the hanger or saddle material.
Figure 3 - Hanger straps should be at least 1.5 inches wide.
2. Space supports at 5-foot intervals or closer to ensure that the flexible duct does not sag more than 1/2 inch per lineal foot or more than 2.5 inches in a 5-foot span.
Figure 4 - Supports should be installed at least every once 5 feet. The maximum allowable sag in flexible duct is one-half inch per foot.
3. Place strap supports at each duct connection to rigid metal duct. For example, in Figure 5, a hanger is placed at the trunk line where a flex connection is made.
4. Ensure that strapping, framing, or other supports are in place to support all air devices (such as diffuser boxes, register boxes, or grille boxes) and all duct peripherals (such as junction boxes, wyes, plenums, etc.).
5. Support the flex duct between a metal connection and a bend by allowing the duct to extend straight for a distance equal to or greater than the radius of the flex duct being installed before making a bend. The support will hold the duct straight for an adequate distance from the connection to avoid possible damage to the flex duct by the metal edge of the collar.
Figure 5 - Ensure that hanger straps or other supports are in place to support the flex duct between a metal connection and a bend, to support trunk lines at a branch duct connection, and to support supply registers and other duct connections.
6. Stabilize vertically installed flex duct with support straps at a maximum distance of 6 feet apart.