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Cantilevered Floor

    Scope
    Scope Images
    Image
    Air seal the top, bottom, and sides of a cantilevered floor cavity and ensure that insulation is in full contact with all sides without voids.
    Scope

    Air-seal the top, bottom, and sides of a cantilevered floor cavity and ensure that insulation is in full contact with all sides without voids.

    • Install a rigid air barrier or other supporting blocking to separate the cantilever from the conditioned space above. Seal all seams, gaps, and holes in the air barrier with caulk or foam. 
    • Block and seal any open floor joists abutting the cantilever floor cavities.
    • Install insulation without misalignments, compressions, gaps, or voids to fill the cantilever floor cavity, making full contact with the top, bottom, and sides of the cantilevered floor cavity 
    • Cover the bottom of the insulated cantilever floor cavities with a rigid, weather-resistant solid blocking material such as plywood or house siding.

    See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

    Description
    Description

    A cantilevered floor is a floor that sticks out past the foundation or supporting wall below. It may be a first- or second-story bump-out, a bay window, or a room over an open porch. Floor joist bays that extend from the building out under the cantilevered floor are sometimes left unsealed and uninsulated by the builder, allowing outside air to flow into the interior space and conditioned air to escape (Figure 1). 

    If these floor joist bays remain open, cold air can flow between the floors of the house.
    Figure 1. If these floor joist bays remain open, cold air can flow in the joist space between the floors.

    Sometimes cantilevered floors are insulated but not air-sealed. A rigid air barrier (rigid foam insulation, OSB, plywood, or drywall) must be installed across any open floor joist bays to prevent air from blowing through the insulation and rendering it ineffective (Figure 2). 

    These floor joist bays have been properly air sealed with caulked rigid foam insulation.
    Figure 2. These floor joist bays have been properly air-sealed with caulked rigid foam insulation.

    Plywood subflooring above the cantilever should be caulked at the edges and seams. The cantilever floor cavity must be filled with insulation that completely touches the underside of the floor. Insulating foam sheathing and/or house sheathing or siding should be attached to the underside of the cantilever floor joists as a protective covering. Air-sealing and insulation materials may be installed by framers, insulators, and/or siding installers. This task should be included in the contract for the appropriate trade depending on the workflow at specific job sites.

    Air barrier effectiveness is measured at the whole-house level. High-performance branding programs and the IECC code require that builders meet specified infiltration rates at the whole house level. See the Compliance Tab for these specified infiltration rates.

    Whenever possible, try to avoid running HVAC ducts and plumbing pipes through cantilevered floors. However, when HVAC ducts and/or plumbing pipes must be located in cantilevered floors, ensure that all penetrations are properly air-sealed and that the ducts and/or pipes are well insulated.

     

    How to Air-Seal and Insulate a Cantilevered Floor

    1. Create an air barrier between the interior conditioned space and the cantilever by cutting a rectangle of rigid foam or other rigid air barrier material to fit into each floor joist bay cavity (Figure 3). Make a backstop for the rigid air barrier by tacking furring strips to the joists at the plane with the foundation or house wall.

      Rigid foam and caulk are used to seal each joist bay cavity.
      Figure 3. Rigid foam and caulk are used to seal each joist bay cavity beneath a cantilevered floor.
    2. Insert rigid air barrier pieces into each joist bay, nail in place and caulk to air-seal all four edges of of each piece (Figure 4).
    3. Caulk the subfloor to the floor joists at the perimeter of the cantilevered floor and at any seams in the subfloor (Figure 4 and Figure 5). Seal any wiring or piping holes through perimeter joists or subfloor with caulk or spray foam.

      All seams are caulked to completely air seal the subfloor of the cantilever.
      Figure 4. All seams are caulked to completely air-seal the subfloor and joist bay of the cantilever.
    Cantilever floor with insulation and air blocking.
    Figure 5. Cantilever floor construction detail showing where caulk should be installed in connection with framing and rigid air barriers to provide continuous air blocking.
    1. Install unfaced batt insulation in each floor joist bay (Figure 6). Use insulation that will completely fill the cavity; it must be in contact with the top and bottom air barrier (i.e., the subfloor above and rigid sheathing below) with no compressions or voids. Alternatively, insulate each cavity with open- or closed-cell spray foam that is aligned with the underside of the subfloor above. Install to a depth equivalent to the required exterior wall R-value. In cold climates, use closed-cell foam to ensure moisture within the floor is mitigated.

      Fill the joist bays with fiberglass batt or spray foam.
      Figure 6. Fill the joist bays of the cantilevered floor with fiberglass batt or spray foam.
    2. Cover the underside of the cantilever with rigid foam insulation (Figure 7). Use caulk and fasteners to attach rigid foam to joists and to air-seal at the edges. Tape foam at seams.

      Cover rigid foam with siding or with 3/8-inch exterior plywood.
      Figure 7. Cover the cantilever floor cavity insulation with rigid foam.
    3. Cover the rigid foam with siding or with 3/8-inch exterior plywood that is pressure-treated, painted, or primed on all exposed sides (Figure 8).

       

      Cover rigid foam with siding or with 3/8-inch exterior plywood.
      Figure 8. Cover the rigid foam on the underside of the cantilever with plywood that is pressure-treated, painted, or primed on all exposed sides.
    4. If you have plumbing pipes in the cantilevered floor (not recommended) and live in a cold climate, ensure adequate insulation on the exterior side of the pipes to protect them from freezing. One option is to box in the pipes with a rigid foam box that is caulked to the subfloor to allow warmth from the house to reach the pipes.
    Success
    Ensuring Success

    Air-sealing of cantilever floor joist bays should be inspected by the site supervisor before the insulation and covering are installed. Blower door testing conducted as part of whole-house energy performance testing may indicate whether air leakage at cantilever floors has been successfully sealed. An infrared camera may also be used to detect air leakage.

    Climate
    Climate

    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. 

    Image
    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)
    Image
    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
    Image
    Wrong – The rigid air barrier material between the cantilever and the conditioned space is not air sealed at the seams.
    Wrong – The rigid air barrier material between the cantilever and the conditioned space is not air sealed at the seams.
    Image
    Right – All holes, gaps, and seams of the rigid air barrier are air sealed.
    Right – All holes, gaps, and seams of the rigid air barrier are air sealed.
    Image
    Wrong – Cantilever is not blocked or insulated.
    Wrong – Cantilever is not blocked or insulated.
    Image
    Right – Cantilever has been properly insulated, air sealed, and cavity has been blocked.
    Right – Cantilever has been properly insulated, air sealed, and cavity has been blocked.
    Image
    Wrong - Cantilevered floor joist bay cavities are not air sealed with a solid air barrier aligned with the exterior wall.
    Wrong - Cantilevered floor joist bay cavities are not air sealed with a solid air barrier aligned with the exterior wall.
    Image
    Wrong - Cantilevered joist bay cavities are not air sealed with a solid air barrier, allowing outside air to flow between floors.
    Wrong - Cantilevered joist bay cavities are not air sealed with a solid air barrier, allowing outside air to flow between floors.
    Image
    Wrong - Plumbing penetrations are not sealed in sub-floor and it is not recommended to run plumbing in a cantilevered floor.
    Wrong - Plumbing penetrations are not sealed in sub-floor and it is not recommended to run plumbing in a cantilevered floor.
    Image
    Wrong - OSB subfloor left exposed to the elements.
    Wrong - OSB subfloor left exposed to the elements.
    Videos
    CAD
    CAD Files
    Insulated cantilever floor - cavity insulation with 1 1/2 inch rigid insulation closure
    Insulated cantilever floor - cavity insulation with 1 1/2 inch rigid insulation closure
    Download: DWG PDF
    Insulated cantilever floor - cavity insulation with 1 inch rigid insulation closure
    Insulated cantilever floor - cavity insulation with 1 inch rigid insulation closure
    Download: DWG PDF
    Compliance
    More

    More Info.

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    Case Studies
    References and Resources*
    Author(s)
    U.S. Environmental Protection Agency,
    ENERGY STAR
    Organization(s)
    EPA
    Publication Date
    Description
    Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.
    Author(s)
    Advanced Energy
    Organization(s)
    Advanced Energy
    Publication Date
    Description
    Information sheet containing images and descriptions of cantilevers.
    Author(s)
    Taggart Jim,
    Sikora Jeannie,
    Wiehagen Joseph,
    Wood Amber
    Organization(s)
    NAHB Research Center,
    National Renewable Energy Laboratory,
    NREL,
    U.S. Department of Energy,
    DOE
    Publication Date
    Description
    Research study providing a comparison of selected retrofit activity typically done, versus that same retrofit activity approached from an integrated high performance remodeling and quality management perspective.
    Author(s)
    Baechler Michael C,
    Gilbride Theresa L,
    Hefty Marye G,
    Cole Pam C,
    Love Pat M
    Organization(s)
    Pacific Northwest National Laboratory,
    PNNL,
    Oak Ridge National Laboratory,
    ORNL,
    U.S. Department of Energy,
    DOE
    Publication Date
    Description
    Guide describing measures that builders in the cold and very cold climates can take to build homes that have whole-house energy savings of 40% over the Building America benchmark with no added overall costs for consumers.
    *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.

    Sales
    Building Science Measures
    Building Science-to-Sales Translator

    Fully Aligned Air Barriers = Whole-House Draft Barrier

    Image(s)
    Technical Description

    A whole-house draft barrier is a continuous layer of air-tight materials that block air leaks. This barrier can be integrated with other materials to also function as a water barrier, thermal barrier, and vapor barrier. For example, rigid foam insulation can be used to block thermal flow as well as air flow when seams are sealed with tape, caulk, adhesives, or liquid-applied sealants. Some rigid foams have an integrated water control layer as well. Additionally, drywall can serve as an interior air barrier when the seams are taped and spackled, and caulk, spray foam, or gaskets are used to seal around wiring, plumbing, and other penetrations. It also serves as the vapor barrier when finished with paint. Insulation should be in full contact with the air barrier layer.

    Whole-House Draft Barrier
    Sales Message

    Whole-house draft barriers block air flow that can undermine the thermal protection with a complete high-performance insulation system. What this means to you is less wasted energy along with enhanced comfort, quiet, and durability. Wouldn’t you agree it would be a shame to only get a partial return on your investment in advanced insulation?

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