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Ducts Buried in Attic Insulation

    Scope
    Scope Images
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    Diagram of buried ducts
    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 links to related codes and standards and voluntary federal energy-efficiency program requirements.

    Description
    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.

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    Buried ducts are laid on the floor of a vented attic then covered with spray foam and blown attic floor insulation
    Buried ducts are laid on the floor of a vented attic then covered with spray foam and blown attic floor insulation
    Buried ducts are laid on the floor of a vented attic then covered with spray foam and blown attic floor insulation
    Table 1. Effective R-Values of Buried Ducts at Three Attic and Duct Insulation Levels. (Source: Steven Winter Associates.)

    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. (Source: Steven Winter Associates.)

    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).
    Image
    Ceiling gypsum board has been installed
    Ceiling gypsum board has been installed
    Ceiling gypsum board has been installed
    Figure 2. Ceiling gypsum board, HVAC ducts, and other services have been installed but not attic insulation. (Source: Steven Winter Associates.)
    1. 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. (Source: Steven Winter Associates.)
    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. 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.
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    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).
    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).
    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. (Source: Steven Winter Associates.)
    1. 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. (Source: Steven Winter Associates.)
    2. Air-seal the ceiling plane penetrations, including sealing the duct register boots to the gypsum board ceiling using canned spray foam (Figure 6).
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    Duct boots and other ceiling penetrations are air-sealed using spray foam
    Duct boots and other ceiling penetrations are air-sealed using spray foam
    Duct boots and other ceiling penetrations are air-sealed using spray foam
    Figure 6. Duct boots and other ceiling penetrations are air-sealed using spray foam. (Source: Steven Winter Associates.)
    1. 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. (Source: Steven Winter Associates.)

      strong>How to Install Buried Ducts before the Ceiling Is in Place

      1.  
        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. (Source: Steven Winter Associates.)
        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.
    Success
    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
    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. 

    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. 
     

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    IECC climate zone map
    IECC climate zone map
    IECC climate zone map
    Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)
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    Climate Zone Map from IECC 2021
    Climate Zone Map from IECC 2021
    Climate Zone Map from IECC 2021
    Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)
    Training
    Right and Wrong Images
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    Right: Ducts are completely buried beneath insulation to the depth specified in the plans
    Right: Ducts are completely buried beneath insulation to the depth specified in the plans
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    Wrong: Ducts are not laid across the lower truss cords or ceiling, but are hung from the rafters by straps. As a result, ductwork is not buried
    Wrong: Ducts are not laid across the lower truss cords or ceiling, but are hung from the rafters by straps. As a result, ductwork is not buried
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    Wrong: Duct is hung between truss members by strapping and not buried
    Wrong: Duct is hung between truss members by strapping and not buried
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    Right: Ductwork is installed in direct contact with lower truss cords. In this picture the main trunk is laying on the truss cords and branch ducts are temporarily held with strapping across truss cords. Ducts are well sealed with mastic
    Right: Ductwork is installed in direct contact with lower truss cords. In this picture the main trunk is laying on the truss cords and branch ducts are temporarily held with strapping across truss cords. Ducts are well sealed with mastic
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    Wrong: Ducts are held from the rafters with strapping and a large beam prevents ducts from properly lying on the ceiling plane
    Wrong: Ducts are held from the rafters with strapping and a large beam prevents ducts from properly lying on the ceiling plane
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    Right: Side-entry boots are used to keep ductwork close to the ceiling
    Right: Side-entry boots are used to keep ductwork close to the ceiling
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    Wrong: Ducts are held above the ceiling plane with strapping to provide a good angle into top-entry boots. Once the ceiling insulation is added, this duct will protrude from the ceiling insulation and will not be buried
    Wrong: Ducts are held above the ceiling plane with strapping to provide a good angle into top-entry boots. Once the ceiling insulation is added, this duct will protrude from the ceiling insulation and will not be buried
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    Right: ductwork is fully encapsulated with ccSPF prior to ceiling installation and burial
    Right: ductwork is fully encapsulated with ccSPF prior to ceiling installation and burial
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    Wrong: Ductwork is not fully encapsulated with ccSPF insulation. The duct jacket is still fully visible at sections of the duct
    Wrong: Ductwork is not fully encapsulated with ccSPF insulation. The duct jacket is still fully visible at sections of the duct
    Image
    Wrong- A tie strap should not be used over the duct outer liner because it can compress the insulation. Tuck in the fibrous insulation and seal the outer liner to the connecting duct with mastic or foil tape (Steven Winter Associates 2013).
    Wrong- A tie strap should not be used over the duct outer liner because it can compress the insulation. Tuck in the fibrous insulation and seal the outer liner to the connecting duct with mastic or foil tape (Steven Winter Associates 2013).
    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.
    Compliance
    Retrofit
    Existing Homes

    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. 

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    More Info.

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    Case Studies
    Author(s)
    Pacific Northwest National Laboratory,
    Steven Winter Associates
    Organization(s)
    PNNL,
    CARB,
    SWA
    Publication Date
    Description
    Case study describing a DOE Building America Top Innovation on HVAC ducts located in vented attics that are encapsulated in foam and buried in attic insulation.
    References and Resources*
    Author(s)
    National Renewable Energy Laboratory,
    U.S. Department of Energy
    Organization(s)
    NREL,
    DOE
    Publication Date
    Description
    Fact sheet describing best strategies to bring ductwork on single-story homes with a slab-on-grade foundation in hot, dry climates into the conditioned envelope of the home intending to highlight the pros and cons of multiple ductwork integration strategies, including traditional duct systems in...
    *For non-dated media, such as websites, the date listed is the date accessed.
    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

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    Building Science Measures
    Building Science-to-Sales Translator

    Vented Attics with Buried HVAC Ducts =

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    Technical Description

    Comfort systems installed in vented attics work much better when they are buried in insulation. Vented attics can reach extreme temperatures. Deeply burying the ducts in the attic's insulation provides significant protection from these harsh conditions. The ducts need to be tightly sealed to ensure optimum performance.


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