2009 IECC, International Energy Conservation Code. International Code Council, Washington, D.C.
Support at Intervals for Flex Ducts
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Scope
Duct Distribution Quality Installation
Flexible ducts supported at intervals as recommended by manufacturer but at a distance ≤ 5 feet:
- Install supports at a minimum of every 5 feet to prevent sagging.
- Install supports at least 1 inch wide.
- Install supports without compressing the duct and the duct insulation.
Notes:
Duct Air Flow Basics
- Each turn, kink, or compression of ductwork reduces air flow.
- If the recommended amount of air flow is not delivered to the room, it could lead to homeowner comfort complaints.
Duct Air Flow Tips
- Design ducts in compliance with Manual D.
- Use balancing dampers in flex ducts to control flow. For metal ducts, butterfly dampers may be used to control air flow.
- To prevent kinks at the duct and boot connection, ENERGY STAR recommends using metal duct elbow instead of flex duct.
- Webbed trusses between floors allow for ducts to freely pass through the floor system without compromising the structure.
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Description
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.
Ensuring Success
Plan for duct layout in the initial plans. Lay flexible ducts on ceiling rafters or in duct chases if possible. If ducts are hung, inspect that duct is supported every five feet with strapping that is at least 1.5 inches wide.
Climate
No climate specific information applies.
Right and Wrong Images
Presentations
None Available
Videos
None Available
CAD Images
None Available
Compliance
ENERGY STAR Version 3, (Rev. 6)
HVAC System Quality Checklist, Duct Quality Installation. Flexible ducts supported at intervals as recommended by manufacturer but at a distance less than or equal to 5 ft.
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.
Flex duct installation is not specifically addressed in the 2009 IECC.
Section M1601.4.3 Support. Metal ducts need to be supported by ½-inch wide 18-gage metal straps on 12-gage galvanized wire 10 feet or less, or other approved method. Non-metal ducts need to be supported per the manufacturer’s installation instructions.*
Flex duct installation is not specifically addressed in the 2012 IECC.
Section M1601.4.3 Support. Metal ducts need to be supported by ½-inch wide 18-gage metal straps on 12-gage galvanized wire 10 feet or less, or other approved method. Non-metal ducts need to be supported per the manufacturer’s installation instructions.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.
Case Studies
None Available
References
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2009 IECC—International Energy Conservation Code
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2009 IRC—International Residential Code for One and Two Family Dwellings
2009 IRC, International Residential Code for One and Two Family Dwellings. 2009. Fifth Printing. International Code Council, Washington, D.C.
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2012 IECC—International Energy Conservation Code
2012 IECC, International Energy Conservation Code. International Code Council, Washington, D.C.
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2012 IRC—International Residential Code for One and Two Family Dwellings
2012 IRC, International Residential Code for One and Two Family Dwellings. 2012. Second Printing. International Code Council, Washington, D.C.
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Advanced Strategy Guideline: Air Distribution Basics and Duct Design
Burdick, A. 2011. Advanced Strategy Guideline: Air Distribution Basics and Duct Design. DOE/GO-102011-3461. Prepared by IBACOS for the U.S. Department of Energy Building America.
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DOE Challenge Home National Program Requirements
DOE. 2012. DOE Challenge Home National Program Requirements (Rev. 02). Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C.
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ENERGY STAR Qualified Homes, Version 3 (Rev. 06) Inspection Checklist for National Program Requirements
EPA. 2012. ENERGY STAR Qualified Homes, Version 3 (Rev. 06) Inspection Checklist for National Program Requirements. U.S. Environmental Protection Agency, Washington, D.C.
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Flexible Duct Performance and Installation Standards
ADC. 2010. Flexible Duct Performance and Installation Standards, Fifth Edition. Air Diffusion Council.
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HVAC System Quality Installation Rater Checklist
EPA. 2011. HVAC System Quality Installation Rater Checklist. U.S. Environmental Protection Agency, Washington, D.C.
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Manual D—Residential Duct Systems
Rutkowski, H. 2009. Manual D—Residential Duct Systems, 3rd Edition. Prepared by the Air Conditioning Contractors of America (ACCA).
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Recommended Practices for Properly Installing Flexible Duct
ACTO Rubber Products Inc. and Air Diffusion Council. 2011. Recommended Practices for Properly Installing Flexible Duct. Presented at HVACR & Mechanical Instructor Workshop by ACTO Rubber Products Inc., and the Air Diffusion Council, March 13, 2011.
