Support at Intervals for Flex Ducts

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Climate

For best performance, install ducts within the home’s thermal envelope. If installed in an uninsulated attic in dry climates, ducts may be laid on the ceiling deck. In the humid, mixed-humid, and marine climates, the ducts must be encapsulated in closed-cell spray foam if installed on the ceiling deck. Or, ducts can be hung from the attic ceiling with strapping or supported with saddle supports. In humid climates, ducts in vented attics should be hung or supported in the hottest part of the attic and contact of the ducts with other objects in the attic should be avoided to reduce the possibility of condensation forming on the ducts.

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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.

 

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.

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.   (Image Courtesy of Calcs Plus)

Ideally, ducts should be located inside conditioned space, i.e., within the thermal envelope of the home. This could be in a dropped ceiling duct chase; in an insulated and air sealed attic, basement, or crawlspace; or between the floors of a two-story home. However, ducts are often installed in uninsulated, unsealed crawlspaces or attics that are outside the thermal envelope or conditioned space of the home. In these locations, the ducts must be thoroughly air sealed and insulated and properly installed and supported to avoid air flow losses, energy losses, and moisture problems.

In attics in dry climates, ducts can be laid on the ceiling joists or ceiling deck and covered with blown insulation (Figure 2). In humid or mixed-humid climates, ducts cannot be laid on any flat surface (such as the ceiling deck or ceiling joists whose cavities are filled with insulation) because of concerns about the formation of condensation on the outside of the ducts under the right dew point conditions. The only exception to this is if the ducts are first sealed, insulated, and completely encapsulated in at least 1.5 inches of closed-cell spray foam (Figure 3 Shapiro et al. 2013). Building America recommends, and DOE Challenge Home requires, that in all climates, if ducts are to be “buried,” they must first be insulated with R-8 duct insulation then encapsulated with at least 1.5 inches of closed-cell spray foam insulation, then covered with at least 2 inches of blown insulation.

 

 flex ducts may rest on ceiling joists or truss supports

 

Figure 2 - Flex ducts may rest on ceiling joists or truss supports in dry climates if supported at least every 4 feet.  (Image Courtesy of Calcs Plus)

 

Sealed, insulated ducts may be encapsulated

 

Figure 3 - Sealed, insulated ducts may be encapsulated in 1.5 inches of closed-cell spray foam and buried in blown attic insulation. Reference

 

If the buried and encapsulated duct technique cannot be accommodated, then the ducts are typically hung from the attic ceiling with strapping or supported in saddles anchored to the attic framing. The flex duct must be adequately supported to ensure the duct is not crimped and to maintain both air flow through the duct and the integrity of the duct coil and insulation material. When the flex duct is improperly supported, for example by spacing the support straps or saddles too far, the ducts can sag, restricting air flow. When the support straps are too narrow or too tight, the metal helix or plastic inner liner of the flex duct can be crushed or damaged, restricting air flow and compromising the insulation. If the insulation is overly compressed, cold spots can occur on the exterior of the duct where condensation can form in hot, humid attics. Crushed ducts can eventually fail causing air leakage and large energy losses. Hanger straps or saddles need to be wide enough to prevent the straps from compressing the insulation and cutting into the inner liner. Straps or saddles must be adequately spaced to prevent sagging of the ducts.

In hot-humid climates, if ducts are to be installed in an uninsulated, vented attic, hang the ducts in the hottest part of the attic to maintain the surface temperature of the ducts above the dew point and minimize contact of the ducts with other objects in the attic (Bailey and Bailey 2013).

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 2010) 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 DuctsNo 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.  Support flex duct using straps or saddle supports or as specified by the manufacturer. Hanger straps in contact with flex duct should be at least 1.5 inches wide, preferably wider, (ADC 2010) 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. Factory-installed suspension systems integral to the flexible duct are an acceptable alternative hanging method when the manufacturer’s recommended procedures are followed. Ducts can also be supported by a saddle-shaped support held up by a pole that is attached to attic framing. The supporting width of the saddle must exceed 5.5 inches along the length of the duct (ICC-ES 2007).
 

Hanger straps should be at least 1.5 in. wide 

Hanger straps should be at least 1.5 inches wide.   (Image Courtesy of Calcs Plus)

 

2.  Space supports at 4-foot intervals or closer to ensure that the flexible duct does not sag more than 1/2 inch per lineal foot between the supports.

Supports installed every 4 feet

Supports should be installed at least every once 4 feet. The maximum allowable sag in flexible duct is one-half inch per foot.  (Image Courtesy of Calcs Plus)

3.   Place supports at each connection between flex duct and rigid metal duct. For example, in Figure 6 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. 

 

: Install supports every 5 feet so that maximum allowable sag in flexible duct is no more than one-half inch per foot

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.   (Image Courtesy of Calcs Plus)

6.  Support duct before and after sharp bends (see figure below).

Support before and after sharp bends

Install duct supports before and after any sharp bends in ducts.  (Image Courtesy of Calcs Plus)

7. Support any vertically installed sections of flex duct with support straps installed at no more than 6 feet apart. 

Ensuring Success

Plan for duct layout in the initial plans so that framing obstacles can be minimized. Install ducts within the thermal boundary of the home if possible. If installed in an uninsulated attic in dry climates, ducts can be laid on the ceiling deck and buried in blown insulation. In humid climates, encase the ducts in closed-cell spray foam. If ducts are hung, inspect that ducts are supported at least every 4 feet with strapping that is at least 1.5 inches wide (wider is better) and with no more than ½ inch of sag per lineal foot between supports.

Scope

Flexible ducts supported at intervals as recommended by mfr. but at a distance ≤ 5 ft

Duct Distribution Quality Installation

If ducts are to be installed in a vented, uninsulated attic or crawl space, hang the ducts with supporting straps or saddles of sufficient width and frequency to adequately support the ducts so that the ducts do not sag and are not pinched or compressed in a manner that would cause damage to the ducts, the duct insulation, or the duct air barrier covering.

Follow the Air Conditioning Contractors of America (ACCA) Manual D recommendations for flex duct installation. ACCA recommends that ducts should be installed with no significant sag or snaking and that installation methods should follow the guidance provided in the ADC (Air Diffusion Council) Flexible Duct Performance and Installation Standard, 5th edition, 2010, Sections 4 and 5. The ADC Standards (5th edition, Section 4.6) make the following requirements regarding support of flexible duct.

  • The ducting should be supported at the manufacturer’s recommended intervals, but the supports should be no greater than 4 feet apart and there should be no more than ½ inch sag per foot between the supports. A connection to rigid duct or equipment is considered a support joint.
  • Long horizontal duct runs with sharp bends should have additional supports before and after the bend.
  • Supports should be at least 1.5 inches wide.
  • Straps should not compress the inner core or constrict air flow. The supports should not excessively compress the vapor barrier and insulation material because compressing the insulation could lead to condensation at that point.
  • Factory-installed support systems that are part of the flex duct can be used if manufacturer’s procedures are followed.
  • Flexible ducts may rest on ceiling joists or truss supports, if the manufacturer’s support spacing guidance is followed.
  • A support should be installed between a metal connection and a bend and the duct should be allowed to extend straight for at least one duct diameter before making the bend.
  • Vertically installed ducts shall be supported every 6 feet at a minimum.

ENERGY STAR Notes

ENERGY STAR Ver. 3, Rev. 07 permits ducts to be installed in uninsulated spaces. The ENERGY STAR Ver. 3, Rev. 07 HVAC System Quality Installation Rater Checklist states in Item 2.4 “Flexible ducts supported at intervals as recommended by mfr. but at a distance ≤ 5 ft.”

DOE Challenge Home Notes

DOE Challenge Home requires that ducts be installed in conditioned space, i.e., within the home’s thermal boundary, in homes that use ducts as a heating and cooling air distribution system.

Training

Right and Wrong Images

Presentations

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Videos

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CAD Images

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Compliance

ENERGY STAR Version 3, (Rev. 07)

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.

DOE Challenge Home

Exhibit 1, #3. Mandatory Requirements. Ducts must be installed in conditioned space or encapsulated in spray foam and buried in attic insulation.

2009 IRC

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.*

2012 IRC

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.

More Info.

Case Studies

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: September 2013

    Case study providing information about buried and encapsulated ducts.

References and Resources*

  1. Author(s): International Code Council
    Organization(s): International Code Council
    Publication Date: March 2014

    Standards criteria supporting ICC HVAC duct support standards.

  2. Author(s): Burdick
    Organization(s): IBACOS, NREL
    Publication Date: December 2011

    Document providing guideance and considerations for duct design in an energy efficient house.

  3. Author(s): Bailey
    Organization(s): Bailey Engineering Corporation
    Publication Date: March 2014

    Case study about high humidity, mold and condensation on surfaces in S. Florida, and how building characteristics cause and amplify these problems.

  4. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  5. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  6. Author(s): Air Diffusion Council
    Organization(s): Air Diffusion Council
    Publication Date: January 2010

    Standard providing a comprehensive approach to evaluating, selecting, specifying and installing flexible duct in HVAC systems.

  7. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: December 2013

    Standard outlining industry procedure for sizing residential duct systems.

  8. Author(s): Shapiro, Zoeller, Mantha
    Organization(s): CARB
    Publication Date: August 2013

    Document covering the technical aspects of buried and insulated ducts (BEDs), as well as the advantages, disadvantages, and risks of BEDs compared to alternative strategies.

  9. Author(s): ACTO Rubber Products Inc., Air Diffusion Council
    Organization(s): Air Diffusion Council
    Publication Date: March 2011

    Presentation about proper installation of flexible ducts.

Last Updated: 03/28/2014

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