Encapsulated Ducts

Scope Images
Encapsulated ducts are sprayed with ccSPF in an unconditioned attic
Encapsulated ducts are sprayed with ccSPF in an unconditioned attic
Scope

When HVAC ducts are installed in a vented attic in any climate, encapsulate the ducts in closed-cell spray polyurethane foam (ccSPF) to protect them from temperature extremes in the unconditioned attic space.

  • Where possible, install ductwork so that it is in direct contact with (i.e., laying on) the ceiling and/or truss lower cords. Where all or some portion of the ducts cannot be laid on the attic floor (due to truss design or some other obstacle), then hang the ducts using straps or saddles to properly support the ducts.
  • Mechanically fasten and mastic-seal all duct connections.
  • Test total duct leakage. Add additional sealant if necessary.
  • Completely encase the ducts in closed-cell spray polyurethane foam (ccSPF) to the desired foam depth.
  • This technique is appropriate for all climates including the humid or marine climates. 
  • 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 bury the ducts in the attic floor insulation. Where all or some portion of the ducts cannot be laid on the attic floor (due to truss design or some other obstacle), then the ducts can be hung or supported above the attic floor and encapsulated in closed-cell spray polyurethane foam insulation.

 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. The ducts should be hung using straps or saddles to properly support the ducts. See the guide Support at Intervals for Flex Ducts for more on properly supporting ducts. Once these preparations are made, the ducts can be encapsulated in at least 1.5 inches of closed-cell polyurethane spray foam. Encapsulating the ducts in spray foam increases the R-value of the ductwork and reduces air leakage. The effective R-value of an encapsulated duct installation depends on the size of the ducts, theR level of the duct insulation, and the thickness of the ccSPF insulation. For example, the effective R-value of an 8-inch round duct encapsulated with 1.5 inches of ccSPF is R-12.7.

How to Install Encapsulated Ducts

1.   Install ductwork with a minimum of R-8 duct insulation in accordance with low-profile duct design (Figure 1).

This duct is insulated with a minimum of R-8 insulation

Figure 1. This duct is insulated with a minimum of R-8 insulation. (Steven Winter Associates 2013).

2.  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 (Figure 2). 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.

Connection mastic-sealed  Connection mastic-sealed  Connection mastic-sealed
Figure 2. 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).

3.  Test total duct leakage to ensure that the ducts have been adequately sealed (total duct leakage < 3 cfm25 per 100 ft2 of conditioned space, Figure 3). Testing should be performed before encapsulation because it may be difficult to correct sealing issues after the application of spray foam.

Total duct leakage testing

Figure 3. A duct blaster is used to test total duct leakage (Steven Winter Associates 2013).

4.  Apply at least 1.5 inches of ccSPF to all duct surfaces, including trunks, branches, and register boots. Ducts should be entirely encapsulated (see the Scope image).

Ensuring Success

To minimize air leaks and maximize effective R-values, ccSPF must be applied to all surfaces to the ductwork. All duct connections should be mastic sealed and ducts should be tested for leakage with a duct blaster before the ccSPF is applied.

The 2009 IRC allows exposed installations of ccSPF in attics, but the spray foam must be specifically approved for installation without an ignition barrier.

Climate

Encapsulated ducts may be installed in all climate zones, including moist (A) and marine (C) climate zones.

The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide. 

IECC climate zone map
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC).
Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)

 

Presentations
Author(s)
Zoeller
Organization(s)
Steven Winter Associates
Description
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
CAD

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 3/3.1 (Rev. 09)

National Rater Field Checklist

HVAC System.
6. Duct Quality Installation - Applies to Heating, Cooling, Ventilation, Exhaust, & Pressure Balancing Ducts, Unless Noted in Footnote.
6.3 All supply and return ducts in unconditioned space, including connections to trunk ducts, are insulated to ≥ R-6.35
6.4 Rater-measured total duct leakage meets one of the following two options. Alternative in Footnote 37: 36, 37, 38​
6.4.1 Rough-in: The greater of ≤ 4 CFM25 per 100 sq. ft. of CFA or ≤ 40 CFM25, with air handler & all ducts, building cavities used as ducts, & duct boots installed. In addition, all duct boots sealed to finished surface, Rater-verified at final.39
6.4.2 Final: The greater of ≤ 8 CFM25 per 100 sq. ft. of CFA or ≤ 80 CFM25, with the air handler & all ducts, bldg. cavities used as ducts, duct boots, & register grilles atop the finished surface (e.g., drywall, floor) installed.40
6.5 Rater-measured duct leakage to outdoors the greater of ≤ 4 CFM25 per 100 sq. ft. of CFA or ≤ 40 CFM25.36, 38, 41​

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 3) Duct distribution systems located within the home’s thermal and air barrier boundary or an optimized location to achieve comparable performance.

Footnote 14) Exceptions and alternative compliance paths to locating 100% of forced-air ducts in home’s thermal and air barrier boundary are:

  1. Up to 10’ of total duct length is permitted to be outside of the home’s thermal and air barrier boundary.
  2. Ducts are located in an unvented attic, regardless of whether this space is conditioned with a supply register.
  3. Ducts are located in a vented attic with all of the following characteristics: [​Note that in either of these designs the HVAC equipment must still be located within the home’s thermal and air barrier boundary.]
    1. In Moist climates (Zones 1A, 2A, 3A, 4A, 5A, 6A and 7A per 2015 IECC Figure R301.1) and Marine climates (all “C” Zones per 2015 IECC Figure R301.1), minimum R-8 duct insulation with an additional minimum 1.5” of closed-cell spray foam insulation encapsulating the ducts; duct leakage to outdoors ≤ 3 CFM25 per 100 ft2 of conditioned floor area (in addition to meeting total duct leakage requirements from Section 4.1 of the ENERGY STAR HVAC Rater checklist); and ductwork buried under at least 2” of blown-in insulation.
    2. In Dry climates (all “B” Zones per 2015 IECC Figure R301.1), minimum R-8 duct insulation; duct leakage to outdoors ≤ 3 CFM25 per 100 ft2 of conditioned floor area (in addition to meeting total duct leakage requirements from Section 4.1 of the ENERGY STAR HVAC Rater checklist); and ductwork buried under at least 3.5” of blown-in insulation.
  4. Systems which meet the criteria for “Ducts Located in Conditioned Space” as defined by the 2018 IECC Section R403.3.7
  5. Jump ducts which do not directly deliver conditioned air from the HVAC unit may be located in attics if all joints, including boot-to-drywall, are fully air sealed with mastic or foam, and the jump duct is fully buried under the attic insulation.
  6. Ducts are located within an unvented crawl space.
  7. Ducts are located in a basement which is within the home’s thermal boundary.
  8. Ductless HVAC system is used.

2009 International Energy Conservation Code (IECC)  /  2009 International Residential Code (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.

    

Retrofit: 

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 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, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 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.

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

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.

Existing Homes

SCOPE

In existing homes with vented attics and HVAC ducts that are hung in the attic, encase ducts in closed-cell spray polyurethane foam.

Before installing spray foam, inspect the ducts for missing or torn duct insulation; disconnected or leaking ducts; sagging or crushed ducts; or excess flex duct. 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:

Test ducts for air tightness using a duct blaster test.

Repair and air seal ducts as needed before installing spray foam insulation.

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
Author(s)
PNNL
Organization(s)
PNNL,
CARB,
Steven Winter Associates
Publication Date
Description
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...
References and Resources*
Author(s)
Shapiro,
Zoeller,
Mantha
Organization(s)
CARB,
Steven Winter Associates,
SWA
Publication Date
Description
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.
Author(s)
CARB
Organization(s)
CARB
Publication Date
Description
Reference covering the installation of ductwork in unconditioned attics to reduce the overall heating and cooling costs of residential buildings.
*For non-dated media, such as websites, the date listed is the date accessed.
Building Science Measures
Building Science-to-Sales Translator

HVAC Ducts in Unvented Attics =

Image(s)
Technical Description

Traditional vented attics can reach extreme temperatures compared to living spaces and are inhospitable to heating and cooling equipment and ducts. Comfort systems work much better when they are inside conditioned space. That’s why unvented attics that are fully insulated provide a more efficient location for comfort systems.


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