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Ducts Buried in Attic Insulation

Scope

When HVAC ducts are installed in a vented attic in a dry climate, bury the ducts in attic insulation to protect them from temperature extremes in the unconditioned attic space.

  • Install ductwork so that it is in direct contact with (i.e., laying on) the ceiling and/or truss lower cords.
  • Use metal, flex, or fiber board ducts that are insulated to code. The duct insulation should include a vapor barrier cover.
  • Mechanically fasten and mastic-seal all duct connections.
  • Test total duct leakage. Add additional sealant if necessary.
  • Install loose-fill insulation to cover the ducts and the attic floor to meet or exceed the code-required R value for attic insulation.
  • If using this technique in a humid or marine climate, the ducts must be encapsulated with spray foam before installing the blown insulation. See Encapsulated Ducts and Ducts Buried in Attic Insulation & Encapsulated for more information.
  • This technique fulfills the DOE Zero Energy Ready Home program requirement that ducts be installed in conditioned space.

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

For homes with ducted heating and cooling systems, the best place to locate the duct system from an HVAC performance standpoint is within the conditioned space of the home, either in dropped ceilings, or between floors, or in a sealed and insulated basement, crawlspace, or attic. If the ducts must be located within a vented attic, one option for protecting the ducts and helping to minimize heat transfer between ducts and the unconditioned attic is to lay the ducts on the attic floor and cover them with the same blown insulation that is used to cover the attic floor. The ductwork should be installed so that it is in direct contact with the ceiling and/or truss lower cords. Metal, flex, or fiber board ducts can be used. The ducts should be covered with R-8 or higher duct insulation and the duct insulation should include a vapor barrier cover. All duct connections should be mechanically fastened and mastic-sealed. The ducts should be tested for total duct leakage and any leaks should be sealed. Once these preparations are made, the ducts can be buried in loose-fill insulation (typically blown fiberglass or cellulose) at the same time that the attic floor is being insulated. The insulation levels should meet or exceed the code-required R value for attic insulation. Figure 1 shows a duct that is partially, fully, or deeply buried in blown attic insulation.

If using this technique in the humid or marine climates, the ducts must be encapsulated with spray foam before installing the blown insulation. See Encapsulated Ducts and Ducts Buried in Attic Insulation & Encapsulated for more information.

 

Buried ducts are laid on the attic floor then covered with attic floor insulation
Figure 1. Buried ducts are laid on the floor of a vented attic then covered with blown attic floor insulation (Steven Winter Associates 2013).

The effective R-value of a buried duct installation depends on the size of the ducts, the R-value of the duct insulation, and the depth of the loose-fill insulation. Table 1 shows effective R-values for 8-inch round ducts at three attic insulation levels and three duct insulation levels (R-4.2, R-6, and R-8).

Effective R-Values for Buried Ducts at Three Levels of Attic and Duct Insulation
Table 1. Effective R-Values of Buried Ducts at Three Attic and Duct Insulation Levels (Steven Winter Associates 2013).

Typically, the duct design will consist of one or more main supply trunk(s) and perpendicular duct branches serving each of the ceiling registers. If the trunk is perpendicular to the truss bottom chords, then the duct branches can be parallel and rest directly on ceiling. If the main truck is placed parallel to the ceiling supports, then the branches will need to run perpendicular to and rest on top of the truss chords. Either configuration will work, giving the designer the flexibility to select whichever method works best for a particular circumstance. In every case, a compact, low-profile layout should be a primary goal.

How to Install Buried Ducts After Ceiling is in Place

1.   Install ceiling gypsum board prior to installing buried ducts (FIgure 2).

Ceiling gypsum board, HVAC ducts, and other services have been installed but not attic insulation.
Figure 2. Ceiling gypsum board, HVAC ducts, and other services have been installed but not attic insulation. (Steven Winter Associates 2013)

 

2.   Install ductwork across lower truss cords or rafters or resting on the ceiling drywall. Install insulated ducts that have a minimum of R-8 duct insulation and in integral vapor barrier. Install the ductwork in accordance with a low-profile compact duct design (Figure 3).

This duct is insulated with a minimum of R-8 insulation
Figure 3. This duct is insulated with a minimum of R-8 insulation. (Steven Winter Associates 2013).

3.  Properly fasten and seal ducts at all connections. To attach flex duct, pull back outer liner, fasten inner liner over collar with tool-tightened tension tie, mastic seal the connection. Pull insulation and outer liner over the joint and seal to attached duct or boot with mastic or foil tape. The outer liner should not be attached with a tie as shown here but should be connected with mastic or foil tape to avoid compressing the insulation.

The inner liner of the flex duct is fastened to the collar with a tension tie, the connection is sealed with mastic, then the outer layer is pulled over and sealed with mastic or foil tape (Steven Winter Associates 2013).
Figure 4. The inner liner of the flex duct is fastened to the collar with a tension tie, the connection is sealed with mastic, then the outer layer is pulled over and sealed with mastic or foil tape (Steven Winter Associates 2013).

4.  Test total duct leakage with a duct blaster to ensure that the ducts have been adequately sealed (total duct leakage < 3 cfm25 per 100 ft2 of conditioned space) (Figure 5).

Total duct leakage test
Figure 5. A duct blaster is used to test total duct leakage (Steven Winter Associates 2013).

5.  Air-seal the ceiling plane penetrations, including sealing the duct register boots to the gypsum board ceiling using canned spray foam (Figure 6).

Ceiling penetrations air sealed
Figure 6. Duct boots and other ceiling penetrations are air-sealed using spray foam. (Steven Winter Associates 2013).

6.    Install loose-fill insulation to the specified depth, and verify that the ducts are covered to the level desired (Figure 7).

Loose fill installed
Figure 7. Loose-fill attic insulation is installed to the specified depth. (Steven Winter Associates 2013)

How to Install Buried Ducts before the Ceiling Is in Place

1. Install ductwork across lower truss cords or rafters. Install insulated ducts that have a minimum of R-8 duct insulation and an integral vapor barrier. Install the ductwork in accordance with a low-profile compact duct design. Where ducts are running parallel to ceiling framing, provide temporary blocking to hang ducts approximately 2 inches above the ceiling plane to allow for the thickness of the spray foam. (Figure 8).

Ducts installed before ceiling
Figure 8. These ducts were installed before the ceiling drywall was installed. Ducts are supported by temporary strapping across the lower truss cords. (Steven Winter Associates 2013).

2.   Connect, seal, and test ducts as described above in Steps 3 and 4. 

3.  Install ceiling gypsum board after installing ducts.

4. Air seal ceiling plane and and install attic floor insulation as described in Steps 5 and 6 above.

Ensuring Success

Buried ducts can be used in dry climates. For humid and marine climates, ducts should be encapsulated in at least 1.5 inches of closed-cell spray foam before burying in blown insulation. As with all advanced systems, the key to optimal implementation lies with the initial planning. To accommodate a buried ducts strategy, the designer must also consider how best to incorporate a low-profile design, where the system layout is specifically designed to place ducts as low as practical to allow ductwork to hug the drywall ceiling where possible. Other best-practice measures, which assist in achieving the desired low-profile layout, should be incorporated into the building, including compact HVAC distribution and right-sized HVAC sizing. Specifically, smaller ducts (lower duct height) and inboard registers (shorter duct runs) mean that there is less ductwork to bury.

Return trunks and branches could be treated in the same manner; however, to keep the HVAC distribution system at a minimum, while simultaneously providing good comfort and proper airflow, the use of central returns is recommended. Return air paths from bedrooms and other spaces can be accommodated by low-profile jump ducts.

Climate

Buried & encapsulated ducts may be installed in all climate zones, including moist (A) and marine (C) climate zones. The ccSPF insulation mitigates condensation concerns in these climates. 

IECC Climate Zone Map
IECC Climate Zone Map

Training

Presentations

  1. Design Options for Locating Ducts within Conditioned Space
    Author(s): Zoeller
    Organization(s): Steven Winter Associates

    Presentation with technical training covering several possible approaches to locating ducts within the home’s air and thermal barriers, and then dig into design considerations and details for the spray foam encapsulation approach.

Videos

None Available

CAD Images

None Available

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, Version 3 (Rev 08)
Buried ducts have the same requirements as all other ductwork installed in unconditioned attics.

Rater Field Checklist, 6. Duct Quality Installation

6.3 All supply and return ducts in unconditioned space, including connections to trunk ducks, are insulated to ≥ R-6.

6.4 Rater-measured total duct leakage meets one of the following two options: 

6.4.1 Rough-in:  The greater of ≤ 4 CFM25 per 100 sq. ft. of conditioned floor area (CFA) or ≤ 40 CFM, with the air handler and all ducts, building cavities used as ducts, and duct boots installed. In addition, all duct boots are sealed to the finished surface; rater-verified at final.

6.42 Final:  The greater of ≤ 8 CFM25 per 100 sq. ft. of CFA or ≤ 80 CFM, with the air handler and all ducts, building cavities used as ducts, duct boots, and register grilles atop the finished surface (e.g., drywall, flooring) installed.

6.5 Rater-measured duct leakage to the outdoors is ≤ 4 CFM25 per 100 sq. ft. of CFA or ≤ 40 CFM25.

DOE Zero Energy Ready Home
Buried ducts alone are not compliant with the DOE Zero Energy Ready Home. Buried & encapsulated ducts, however, may be compliant.

2009 IECC  /  2009 IRC

IECC R403.2/IRC N1103.2 Ducts.

IECC R403.2.1/IRC N1103.2.1 Insulation (Prescriptive). Supply ducts in attics shall be insulated to a minimum of R-8. All other ducts shall be insulated to a minimum of R-6.

               Exception:  Ducts or portions thereof located completely inside the building thermal envelope.

IECC R403.2.2/IRC N1103.2.2 Sealing (Mandatory).  All ducts, air handlers, filter boxes, and building cavities used as ducts shall be sealed.  Joints and seams shall comply with Section M1601.4 of the International Residential Code. [Exceptions may apply.]

               Duct tightness shall be verified by either of the following:

  1. Post-construction test:  leakage to outdoors shall be less than or equal to 8 cfm (226.5 L/min) per 100 ft2 (9.29 m2) of conditioned floor area or a total leakage less than or equal to 12 cfm (339.8 L/min) per 100 ft2 of conditioned floor area when tested at a pressure differential of 0.1 inches w.g. (25 Pa) across the entire system, including the manufacturer’s air handler enclosure. All register boots shall be taped or otherwise sealed during the test.
  2. Rough-in test:  Total leakage shall be less than or equal to 6 cfm (169.9 L/min) per 100 ft2 of conditioned floor area when tested at a pressure differential of 0.1 inches w.g. (25 Pa) across the roughed in system, including the manufacturer’s air handler enclosure. All register boots shall be taped or otherwise sealed during the test. If the air handler is not installed at the time of the test, total leakage shall be less than or equal to 4 cfm (113.3 L/min) per 100 ft2 of conditioned floor area.

    Exceptions: Duct tightness test is not required if the air handler and all ducts are located within conditioned space.

IECC R403.2.3/IRC N1103.2.3 Building cavities (Mandatory).  Building framing cavities shall not be used as supply ducts.

2012 IECC  / 2012 IRC

IECC R403.2/IRC N1103.2 Ducts.  Ducts and air handlers shall be in accordance with IECC R403.2.1 through R403.2.3 (IRC N1103.2.1-N1103.2.3).

IECC R403.2.1/IRC N1103.2.1 Insulation (Prescriptive).  Same as 2009 IECC/IRC.

IECC R403.2.2/IRC N1103.2.1 Sealing (Mandatory).  Ducts, air handlers, and filter boxes shall be sealed.  Joints and seams shall comply with either the International Mechanical Code or International Residential Code, as applicable [Exceptions may apply.]

Exceptions:

1. Air-impermeable spray foam products shall be permitted to be applied without additional joint seals.

2. Where a duct connection is made that is partially inaccessible, three screws or rivets shall be equally spaced on the exposed portion of the joint so as to prevent a hinge effect.

3. Continuously welded and locking-type longitudinal joints and seams in ducts operating at static pressures less than 2 inches of water column (500 PA) pressure classification shall not require additional closure systems.

Duct tightness shall be verified by either of the following:

  1. Post-construction test: Total leakage shall be less than or equal to 4 cfm (113.3 L/min) per 100 ft2 (9.29 m2) of conditioned floor area when tested at a pressure differential of 0.1 inches w.g. (25 Pa) across the entire system, including the manufacturer’s air handler enclosure. All register boots shall be taped or otherwise sealed during the test.
  2. Rough-in test.  Total leakage shall be less than or equal to 4 cfm per (113.3 L/min) per 100 ft2 (9.29 m2) of conditioned floor area when tested at a pressure differential of 0.1 inches w.g. (25 Pa) across the system, including the manufacturer’s air handler enclosure. All registers shall be taped or otherwise sealed during the test. If the air handler is not installed at the time of the test, total leakage shall be less than or equal to 3 cfm (85 L/min) per 100 ft2 of conditioned floor area.

    Exception: The total leakage test is not required for ducts and air handlers located entirely within the building thermal envelope.

IECC R403.2.2.1/IRC N1103.2.2.1 Sealed air handler.  Air handler shall have a manufacturer’s designation for an air leakage of no more than 2 percent of the design air flow rate when tested in accordance with ASHRAE 193.

IECC R403.2.3/IRC N1103.2.3 Building cavities (Mandatory).  Building framing cavities shall not be used as ducts or plenums.

2015 and 2018 IECC  /  2015 and 2018 IRC

IECC R403.3/IRC N1103.3 Ducts.  Ducts and air handlers shall be in accordance with Sections R403.3.1 through R403.3.5 (IRC N1103.3.1-N1103.3.5).

IECC R403.3.1/IRC N1103.3.1 Insulation (Prescriptive).  Supply and return ducts in attics shall be insulated to a minimum of R-8 where 3 inches (76 millimeters) in diameter and greater and R-6 where less than 3 inches (76 millimeters) in diameter.  Supply and return ducts in other portions of the building shall be insulated to a minimum of R-6 where 3 inches (76 millimeters) in diameter or greater and R-4.2 where less than 3 inches (76 millimeters) in diameter. 

Exception:  Ducts or portions thereof located completely inside the building thermal envelope

IECC R403.3.2/IRC N1103.3.2 Sealing (Mandatory).  Ducts, air handlers, and filter boxes shall be sealed.  Joints and seams shall comply with either the International Mechanical Code or International Residential Code, as applicable.

Exceptions [In 2015 IECC/IRC only; these exceptions were not included in the 2018 IECC/IRC.)

  1. Air-impermeable spray foam products shall be permitted to be applied without additional joint seals.
  2. For ducts having a static pressure classification of less than 2 inches of water column (500 Pa), additional closure systems shall not be required for continuously welded joints and seams, and locking-type joints and seams of other than the snap-lock and button-lock types.

IECC R403.3.2.1/IRC N1103.3.2.1 Sealed Air Handler.  Air handlers shall have a manufacturer’s designation for an air leakage of no more than 2% of the design air flow rate when tested in accordance with ASHRAE 193.

IECC R403.3.3/IRC N1103.3.3 Duct testing (mandatory).  Ducts shall be pressure tested to determine air leakage by one of the following methods:

  1. Rough-in test.  Total leakage shall be measured with a pressure differential of
    0.1 inch water gage (25 Pa) across the system, including the manufacturer’s air handler enclosure if installed at the time of the test.  All registers shall be taped or otherwise sealed during the test.
  2. Post-construction test.  Total leakage shall be measured with a pressure differential of 0.1 inch water gage (25 Pa) across the entire system, including the manufacturer’s air handler enclosure.  Registers shall be taped or otherwise sealed during the test.

Exception:

A duct air leakage test shall not be required where the ducts and air handlers are located entirely within the building thermal envelope

 [New Exception added to 2018 IECC/IRC] A duct air leakage test shall not be required for ducts serving heat or energy recovery ventilators that are not integrated with ducts serving heating or cooling systems.

A written report of the results of the test shall be signed by the party conducting the test and provided to the code official.  [Because the ducts will be installed outside the building thermal envelope, this exception would not apply.]

IECC R403.3.4/IRC N1103.3.4 Duct leakage (Prescriptive).

The total leakage of the ducts, where measured in accordance with Section R403.3.3, shall be as follows:

  1. Rough-in test.  The total leakage shall be less than or equal to 4 cfm per (113.3 L/min) per 100 ft2 (9.29 m2) of conditioned floor area where the air handler is installed at the time of the test. Where the air handler is not installed at the time of the test, the total leakage shall be less than or equal to 3 cfm (85 L/min) per 100 ft2 of conditioned floor area.
  2. Post-construction test: Total leakage shall be less than or equal to 4 cfm (113.3 L/min) per 100 ft2 (9.29 m2) of conditioned floor area.

IECC R403.3.5/IRC N1103.3.5 Building cavities (Mandatory). Building framing cavities shall not be used as ducts or plenums.

2009, 2012, 2015, and 2018 IRC

Ducts are designed, constructed, and installed in accordance with the provisions of IRC M1601 and  M1602, ACCA Manual D, and manufacturers’ guidance.    

Retrofit: 2009, 2012, 2015, and 2018 IECC

Section R101.4.3 (Section R501.1.1 in 2015 and 2018 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Retrofit: 2009, 2012, 2015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

2008 Title 24 California Building Energy Efficiency Standards
Buried ducts may be used to comply with the 2008 Title 24 California Building Energy Efficiency Standards or take credit for additional duct insulation. The Residential ACM Manual states that ducts shall have a minimum insulation value of R-4.2 prior to burial. The ACM Manual also provides tables for effective R-value of buried ducts based on attic insulation level and nominal duct diameter.

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.

SCOPE

In existing homes with vented attics and HVAC ducts that are installed on the attic floor, bury ducts in blown attic insulation when adding or increasing attic insulation.

Before adding blown insulation,

  • Inspect and test the ducts for missing or torn duct insulation; sagging, crushed. or excess flex duct; disconnected or leaking ducts.
  • Repair and air seal ducts as needed before installing spray foam insulation.
  • See the following for more information about insulating and air sealing metal, flex, or fiber board ducts.
  • Flex duct runs should be pulled taut to reduce internal airflow friction. See the following guides to remedy installation problems prior to sealing or adding insulation to an existing flex duct system:

If ducts are hung in attic rather than laying on the attic floor, see the guide Encapsulated Ducts.

For more information, view the U.S. Department of Energy’s Standard Work Specifications regarding ducts.

Also see the Solution Center guide Pre-Retrofit Site Assessment of Attics, Ceilings and Roofs.

DESCRIPTION

Make any needed repairs as described above then follow installation guidance for new homes in the Description tab.

COMPLIANCE 

See Compliance tab. 

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

  1. Author(s): PNNL
    Organization(s): PNNL, CARB, Steven Winter Associates
    Publication Date: September, 2013

    Case study on a DOE Building America 2013 top innovation describing research by the Consortium for Advanced Residential Buildings (CARB), a Building America research team led by Steven Winter Associates, on HVAC ducts located in vented attics that are encapsulated in foam and buried in attic insulation.

  2. Author(s): CARB
    Organization(s): CARB
    Publication Date: November, 2013

    Case study exploring how using buried and/or encapsulated ducts can reduce duct thermal losses in existing homes.

References and Resources*

  1. Author(s): California Energy Commission
    Organization(s): California Energy Commission
    Publication Date: January, 2008

    Document intended to help owners, designers, builders, inspectors, examiners, and energy consultants comply with and enforce California’s 2008 energy efficiency standards for low-rise residential buildings.

  2. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  3. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  4. Author(s): Department of Energy
    Organization(s): DOE
    Publication Date: April, 2017

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

  5. Author(s): U.S. Environmental Protection Agency
    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): Shapiro, Zoeller, Mantha
    Organization(s): CARB, Steven Winter Associates, SWA
    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.

Contributors to this Guide

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

Steven Winter Associates, lead for the Consortium for Advanced Residential Buildings (CARB), a DOE Building America Research Team

Last Updated: 04/05/2018