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 a 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 related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Single-Family New Homes, and Indoor airPLUS.
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 house out under the cantilevered floor are sometimes left unsealed and uninsulated by the builder, allowing outside air to flow through the home and conditioned air to escape (Figure 1). Sometimes cantilevered floors are insulated but not air sealed. Air barriers must be put in place across any open floor joist bays to form an air barrier between the cantilever and the rest of the house to prevent air from blowing through the insulation, which renders the insulation ineffective. Blocking material (rigid foam, OSB, plywood, or drywall) should be installed (Figure 2). 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.
When designing the home, HVAC ducts and plumbing pipes should not be located in cantilevered floors.
How to Air Seal and Insulate a Cantilevered Floor
- Create an air barrier between the house and the cantilever by cutting a rectangle of rigid foam to fit into each floor joist bay cavity (Figure 3). Make a backstop for the foam by tacking furring strips to the joists at the plane with the foundation or house wall.
Figure 3. Rigid foam and caulk are used to seal each joist bay cavity beneath a cantilevered floor. - Insert rigid foam pieces into each joist bay, nail in place and caulk to air seal all four edges (Figure 4).
- Caulk the subfloor to the floor joists at the perimeter of the cantilevered floor and at any seams in the subfloor (Figure 4). Seal any wiring or piping holes through perimeter joists or subfloor with caulk or spray foam.
Figure 4. All seams are caulked to completely air seal the subfloor of the cantilever. - Install unfaced batt insulation in each floor joist bay (Figure 5). Use a thickness 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.
Figure 5. Fill the joist bays of the cantilevered floor with fiberglass batt or spray foam. - Cover the underside of the cantilever with rigid foam insulation (Figure 6). Use caulk and fasteners to attach rigid foam to joists and to air seal at the edges. Tape foam at seams.
Figure 6. Cover the cantilever floor cavity insulation with rigid foam. - 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 7).
Figure 7. Cover the rigid foam on the underside of the cantilever with painted plywood. - 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.
Figure 8. Cantilever floor construction detail showing where caulk should be installed in connection with framing and rigid air barriers to provide continuous air blocking.
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
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.
Training
CAD
Compliance
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 Single-Family New Homes, Version 3/3.1 (Rev. 11)
National Rater Field Checklist
Thermal Enclosure System.
2. Fully-Aligned Air Barriers.7 At each insulated location below, a complete air barrier is provided that is fully aligned as follows:
Floors: At exterior vertical surface of floor insulation in all climate zones and, if over unconditioned space, also at interior horizontal surface including supports to ensure alignment. Alternatives in Footnotes 12 & 13.11, 12, 13
2.6 Floors above garages, floors above unconditioned basements or crawlspaces, and cantilevered floors.
Footnote 7) For purposes of this Checklist, an air barrier is defined as any durable solid material that blocks air flow between conditioned space and unconditioned space, including necessary sealing to block excessive air flow at edges and seams and adequate support to resist positive and negative pressures without displacement or damage. EPA recommends, but does not require, rigid air barriers. Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 in. or 1.5 in., respectively, to qualify as an air barrier unless the manufacturer indicates otherwise. If flexible air barriers such as house wrap are used, they shall be fully sealed at all seams and edges and supported using fasteners with caps or heads ≥ 1 in. diameter unless otherwise indicated by the manufacturer. Flexible air barriers shall not be made of kraft paper, paper-based products, or other materials that are easily torn. If polyethylene is used, its thickness shall be ≥ 6 mil.
Footnote 11) EPA highly recommends, but does not require, an air barrier at the interior vertical surface of floor insulation in Climate Zones 4-8.
Footnote 12) Examples of supports necessary for permanent contact include staves for batt insulation or netting for blown-in insulation. Alternatively, supports are not required if batts fill the full depth of the floor cavity, even when compression occurs due to excess insulation, as long as the R-value of the batts has been appropriately assessed based on manufacturer guidance and the only defect preventing the insulation from achieving the required installation grade is the compression caused by the excess insulation.
Footnote 13) Alternatively, an air barrier is permitted to be installed at the exterior horizontal surface of the floor insulation if the insulation is installed in contact with this air barrier, the exterior vertical surfaces of the floor cavity are also insulated, and air barriers are included at the exterior vertical surfaces of this insulation.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable 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 2) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels and achieve Grade 1 installation, per RESNET standards. See the guide 2015 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.
Exhibit 2 DOE Zero Energy Ready Home Target Home.
The U.S. Department of Energy’s Zero Energy Ready Home program allows builders to choose a prescriptive or performance path. The DOE Zero Energy Ready Home prescriptive path requires builders to meet or exceed the minimum HVAC efficiencies listed in Exhibit 2 of the National Program Requirements (Rev 07), as shown below. The DOE Zero Energy Ready Home performance path allows builders to select a custom combination of measures for each home that is equivalent in performance to the minimum HERS index of a modeled target home that meets the requirements of Exhibit 2 as well as the mandatory requirements of Zero Energy Ready Home Exhibit 1.
Exhibit 2, Insulation and Infiltration) Whole house leakage must be tested and meet the following infiltration limits:
- Zones 1-2: ≤ 3 ACH50;
- Zones 3-4: ≤ 2.5 ACH50;
- Zones 5-7: ≤ 2 ACH50;
- Zone 8: ≤ 1.5 ACH50;
- Attached dwellings: ≤ 3 ACH50.
Footnote 13) Steel-frame ceilings, walls, and floors shall meet the insulation requirements of the 2015 IECC – Table 402.2.6
Footnote 23) Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.
2009-2021 IECC and IRC Insulation Requirements Table
The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, 2018, and 2021 IECC and IRC, can be found in this table.
Retrofit: 2009, 2012, 2015, 2018, and 2021 IECC
Section R101.4.3 (in 2009 and 2012). 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.)
Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.
2009, 2012, 2015, 2018, and 2021 International Residential Code (IRC)
Table N1102.4.2 (N1102.4.1.1 in 2012, 2015, 2018, and 2021 IRC) Air Barrier and Insulation Installation, Floors: Insulation in floors (including above garage and cantilevered floors) is installed to maintain permanent contact with underside of subfloor decking. A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material. Junction of foundation and wall sill plates, wall top plate and top of wall, sill plate and rim-band, and rim band and subfloor are sealed. Corners, headers, and rim joists making up the thermal envelope are insulated.
Table R1102.1.1 (Table R1102.1.2 in 2015, 2018, and 2021 IRC) Insulation and Fenestration Requirements – meet or exceed the insulation levels listed in this table.
Retrofit: 2009, 2012, 2015, 2018, and 2021 IRC
Section R102.7.1 Additions, alterations, or repairs. 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 the requirements of this code, unless otherwise stated. (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.
Air Barrier Association of America (ABAA) 07261
Self-Adhered Sheet Air Barrier. 2006. Air Barrier Association of America, Walpole, MA. This specification for self-adhered sheet air barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.
Fluid-Applied Air and Vapor Barrier. 2012. Air Barrier Association of America, Walpole, MA. This specification for air barriers that are fluid-applied and also act as vapor barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.
Closed Cell, Medium-Density Spray Polyurethane Foam Air Barrier. 2011. Air Barrier Association of America, Walpole, MA. This specification for closed cell, medium-density spray polyurethane foam air barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.
Fluid-Applied Vapor Permeable Air Barrier. 2012. Air Barrier Association of America, Walpole, MA. This specification for fluid-applied vapor permeable air barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.
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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.
The following authors and organizations contributed to the content in this Guide.
Pacific Northwest National Laboratory
Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team
Sales
Fully Aligned Air Barriers = Whole-House Draft Barrier
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.
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?