Sufficient Cavity Space for Flex Ducts

Scope

Flexible ducts in unconditioned space not installed in cavities smaller than outer duct diameter; in conditioned space not installed in cavities smaller than inner duct diameter
Flexible ducts in unconditioned space not installed in cavities smaller than outer duct diameter; in conditioned space not installed in cavities smaller than inner duct diameter

Ensure that flex ducts are not located in building cavities where they will be compressed or crushed.
When flexible ducts are installed in unconditioned areas of the home, they should not be squeezed into any cavities that are smaller than the outer diameter of the duct, including the duct insulation; in conditioned space, they should not be installed in cavities smaller than the inner duct diameter (ENERGY STAR).

  • Consider duct layout in the initial framing design stage to plan for short, straight duct runs with no compressions. 
  • Install flex duct in the straightest line possible; pull ducting taut and provide adequate supports per code. 
  • Ensure that the radius of each bend is no less than the diameter of the flexible duct. 
  • Coordinate with the plumber and the electrician to avoid crushing ducts when other services are installed. 
  • Design ducts in compliance with Manual D.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

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.  Reference

For proper performance, flex ducts in unconditioned space should not be installed in cavities smaller than the outer diameter of the duct, including the insulation. Compression of the insulation around the duct should be avoided, especially in humid climates where compressed insulation could encourage condensation to form. Even when ducts are installed in conditioned space, they should not be installed in cavities smaller than the inner duct diameter. 

During the design phase, duct routing should be identified. Open-web trusses between floors allow for ducts to freely pass through the floor system without compromising the structure. Manual J and D calculations should be performed to identify the size of ducting needed and thus how much space is required for the ductwork. Remember to take into account the thickness of the insulation (Figure 2). For example, if R-6 flex duct is used, then the exterior diameter of the duct will increase by four inches. Plan air handler locations, making sure there is clear access for the flex duct (or other specified ducting) to run from the mechanical space (or room) to the duct zone (Figure 3). 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, Support at Intervals for Flex Ducts, No Excessive Coiled or Loped Flex Ducts and Sealed and Insulated Flex Ducts.

How to Install Flex Ducts within Appropriate Cavity Space

  1.  Design ducts in compliance with Manual D. Once the appropriate duct diameter is determined, establish the duct layout in coordination with the framing plan. Ensure that adequate space is allowed for all trunk and branch duct runs.

R-6 flexible duct has 2 inches of insulation around the inner liner so a 12-inch duct requires a 16x16-inch chase

Figure 2 - R-6 flex duct has 2 inches of insulation around the inner liner. If the required duct run is a 12-inch round, the exterior radius will be about 16 inches. At a minimum, a 16x16-inch duct chase or opening would be required along the full length of the duct.  Reference

Ensure that the air handler location does not conflict with duct routing

Figure 3 - Ensure that the air handler location does not conflict with duct routing.  Reference

If a dropped soffit is used to house a duct, the soffit space must equal the duct diameter plus the insulation thickness

Figure 4 - The height and width of a dropped soffit used to house ductwork should be the duct diameter plus the thickness of the insulation to prevent compression of the duct.  Reference

  2.  If room is tight and there is fear of compressing the flex duct when making a 90-degree turn, use a metal elbow. 

Consider using a metal duct elbow instead of flex duct at boot connections to prevent compressions

Figure 5 - Use a metal duct elbow instead of flex duct at boot connections to prevent compressions.  Reference

  3.  Coordinate with the framer, plumber, and electrician for effective duct installation with adequate space.

Ensuring Success

Consider duct layout in the initial framing design stage to plan for short, straight duct runs where the duct is adequately supported and has adequate space to avoid constriction of flow. Inspect before drywall is installed to ensure that ductwork is not constricted. Use a duct blaster test to detect leakage and to confirm proper air flow at each duct supply outlet.

Climate

 No climate specific information applies.

Training

Right and Wrong Images

Presentations

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CAD Images

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes

ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Rater Field Checklist

6. Duct Quality Installation 

6.1 Ductwork installed without kinks, sharp bends, compressions, or excessive coiled flexible ductwork33 

Footnote:

(33) Kinks are to be avoided and are caused when ducts are bent across sharp corners such as framing members. Sharp bends are to be avoided and occur when the radius of the turn in the duct is less than one duct diameter. Compression is to be avoided and occurs when flexible ducts in unconditioned space are installed in cavities smaller than the outer duct diameter and ducts in conditioned space are installed in cavities smaller than inner duct diameter. Ducts shall not include coils or loops except to the extent needed for acoustical control.

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

DOE Zero Energy Ready Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

2009 IECC

Flex duct installation is not specifically addressed in the 2009 IECC.

2012 IECC

Flex duct installation is not specifically addressed in the 2012 IECC.

2015 IECC

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Additional Scope Language for Retrofit Applications

Additions 

If new flex duct is added to an existing home, either as part of an addition or an equipment upgrade, ensure that the flex duct is installed in runs that are as short, straight, and direct as possible and if any duct runs through building cavities verify that the cavities are large enough to have sufficient space to contain the duct without compressing it, as noted in the Description tab.

Existing Buildings 

In an existing building with flex ducts and an under-performing HVAC system, inspect for compression of the flex duct. Symptoms may include minimal air flow to a terminal, excessive noise, or fan burnout due to increased static pressure.

If the ducts are located in the attic, review the guide Pre-Retrofit Site Assessment of Ceilings, Attics, and Roofs.

If the ducts are located in the basement or crawlspace, review the guide Pre-Retrofit Site Assessment of Crawlspaces and Basements.

For more information about ducts see the U.S. Department of Energy’s Standard Work Specifications.

Additional Description Language for Retrofit Applications

For the installation of flex duct in retrofit and existing building situations, follow all best practices and precautions described for new construction. A typical cause of crushed or kinked flex duct is a constricted route through building cavities that do not accommodate the shape and diameter of the flex duct. As with new construction, this can also happen at transitions such as at connections to the air handler and/or at the boots connecting a duct to a register or grille. Consider using a metal duct elbow instead of flex duct at boot and equipment connections to prevent compressions. Refer to installation images in the Description tab of this guide.

Additions 

See Existing HVAC System Upgrade or Expansion for a discussion on when and how to add to an existing distribution system. Coordinate the duct layout for a new space with the design of the structure and other systems to ensure there is space for proper routing. As with new construction, use ACCA Manual D and provide a schematic of the layout to the homeowner for future reference.

Existing Buildings 

See Pre-Retrofit Assessment of Existing HVAC Systems for a discussion of how to determine whether a flawed duct design may be contributing to poor HVAC system performance. If a flex duct is connected to a large, inflexible device or piece of equipment through a narrow cavity or opening, the flex duct will naturally collapse to conform to the obstruction or to fit the opening. Since it is often vital structure that causes the pinch-point, there are three alternatives:

  1. Re-route the duct. Consider offsetting the added static friction that results from the longer run by increasing the duct diameter. At a minimum, ensure all curves are gradual, fully extend the flex, and provide ample supports to prevent sagging.
  2. Move the duct by finding a new location for the supply register (sometimes possible) or for the obstacle (seldom possible).

A transition piece can be installed to fit the size and shape of the space. This can be accomplished with a non-flexible fitting in alternate dimensions that provides similar pressure drop. For instance, a round duct may transition to a flat rectangular shape to pass through a soffit constriction. This will require a well-sealed connection on each side of the new transition piece.

Additional Ensuring Success Language for Retrofit Applications

When re-routing ductwork, access the ducts directly from the basement, crawlspace, or attic if possible. If the ducts are behind finish materials like drywall, it is tempting to make the smallest possible openings in existing ceilings and walls in an effort to reduce disruption. However, flex duct is likely to collapse if forced into place from a distance. It will be impossible to fully examine the entire duct layout if openings are too small. Instead, cut openings in drywall large enough to fit the head and shoulders of the installer so that the flex duct can be manipulated into the best possible orientation. If flex ducts are disconnected, when reattaching them, fasten the inner liner to the collar with mechanical fasteners and seal with tape or mastic, then cover with insulation and the plastic sheeting vapor barrier and seal the plastic to the trunk duct or boot collar with metal tape or mastic, as described in the guide Sealed and Insulated Flex Ducts.

Although an existing duct system is generally not required to be brought up to code when making repairs (double check this with the local code official), you can take this opportunity to check the ducts for duct leakage and seal all accessible seams and joints with mastic and/or metal tape. You can also add duct insulation where it is lacking or deficient, especially if you see evidence that condensation has been occurring.

Additional Climate Language for Retrofit Applications

Starting with the 2009 International Energy Conservation Code (IECC), supply ducts located in unconditioned space must be insulated to at least R-8, and return ducts must be insulated to R-6. Although an existing system is generally not required to be brought up to code (double check this with the local AHJ), this is the time to add insulation to deficient systems, especially if you see evidence that condensation has been occurring.

In humid climates, uninsulated or under-insulated ducts in unconditioned space can create a source of moisture damage. When warm, humid air comes in contact with the cold surface of a duct delivering chilled air, the water vapor can condense to liquid water that can cause rust in the ducts and mold in insulation, framing, and other building materials. 

See Sealed and Insulated Flex Ducts  for guidance.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

None Available

References and Resources*

  1. 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.
  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): DOE
    Organization(s): DOE
    Publication Date: April, 2017

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: December, 2015

    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).

  5. 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.
  6. 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.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Last Updated: 08/21/2017

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