Cantilevered Floor

Scope

Fully Aligned Air Barrier

1. Install a rigid air barrier or other supporting blocking to separate the cantilever from the conditioned space.*
2. Seal all seams, gaps, and holes of the air barrier with caulk or foam. 
3. Install insulation without misalignments, compressions, gaps, or voids and align it with the sub-floor, the rigid air barrier (A), and the exterior face of the cavity.
4. Once insulated, enclose the cavity with a rigid air barrier material.

* ENERGY STAR highly recommends using a rigid air barrier, but it is not a requirement.

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. ENERGY STAR recommends, but does not require, rigid air barriers. Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 inches or 1.5 inches, 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 inch 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.

Examples of supports necessary for permanent contact include staves for batt insulation or netting for blown-in insulation. Batts that completely fill a cavity enclosed on all six sides may be used to meet this requirement without the need for supports, even though some compression will occur due to the excess insulation, as long as the compressed value meets or exceeds the required insulation level. Specifically, the following batts may be used in six-sided floor cavities: R-19 batts in 2x6 cavities, R-30 batts in 2x8 cavities, R-38 batts in 2x10 cavities, and R-49 batts in 2x12 cavities. For example, in a home that requires R-19 floor insulation, an R-30 batt may be used in a six-sided 2x8 floor cavity.

Fully-aligned air barriers may be installed at the exterior surface of the floor cavity in all climate zones if the insulation is installed in contact with this exterior air barrier and the perimeter rim and band joists of the floor cavity are also sealed and insulated to comply with the fully-aligned air barrier requirements for walls.

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. 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. 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 can be installed on the underside of the cantilever. 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.

Figure 1 - These floor joist bays have been properly air sealed with caulked rigid foam insulation

Figure 2 - If these floor joist bays remain open, cold air can flow between the floors of the house

How to Air Seal and Insulate a Cantilevered Floor

1. 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. Make a backstop for the foam by tacking furring strips to the joists at plane with the foundation or house wall.
2. Insert rigid foam pieces into each joist bay, nail in place and caulk to air seal all four edges.

Figure 3 - Rigid foam and caulk are used to seal each joist bay cavity

3. Caulk the subfloor to the floor joists at the perimeter of the cantilevered floor and at any seams in the subfloor. 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

4. Install unfaced batt insulation in each floor joist bay. 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 and rigid sheathing below) with no compressions or voids. Alternatively, spray foam each cavity with open- or closed-cell spray foam to the required exterior wall R-value. In cold climates, use closed-cell foam.

Figure 5 - Fill the joist bays with fiberglass batt orspray foam

5. Cover bottom of cantilever area with rigid foam insulation. 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 cavity insulation with rigid foam

6. Cover rigid foam with siding or with 3/8-inch exterior plywood that is pressure-treated, painted, or primed on all exposed sides.

Figure 7 - Cover the rigid foam with painted plywood

7. If you have plumbing pipes in the cantilevered floor (not recommended) and live in a cold or very cold climate, ensure adequate insulation on the exterior side of pipes to prevent freezing pipes. 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 with insulation and air blocking

Ensuring Success

Air sealing of cantilever joist bays should be inspected by the site supervisor before the insulation and soffit 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 during a blower door test, if a sufficient temperature difference exists between the conditioned space of the house and the outdoors to see the leakage.

Climate

DOE Challenge Home

Exhibit 2: DOE Challenge Home Target Home. Infiltration (ACH50): Zones 1-2: 3; Zones 3-4: 2.5; Zones 5-7: 2; Zone 8: 1.5. Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.

International Energy Conservation Code (IECC) Climate Regions

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Compliance

ENERGY STAR Version 3, (Rev. 6)

Thermal Enclosure Checklist, Fully-Aligned Air Barriers. A complete air barrier shall be provided that is fully aligned with the insulation at interior surface of floors in all climate zones, including supports to ensure permanent contact and blocking at exposed edge. Examples of supports necessary for permanent contact include staves for batt insulation or netting for blown-in insulation. Alternatively, batts that completely fill floor cavities enclosed on all six sides may be used to meet Items 2.2 and 3.2, 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. Fully-aligned air barriers may be installed at the exterior surface of the floor cavity in all climate zones if the insulation is installed in contact with this exterior air barrier and the perimeter rim and band joists of the floor cavity are also sealed and insulated to comply with the fully-aligned air barrier requirements for walls.

DOE Challenge Home

Exhibit 2: DOE Challenge Home Target Home. Certified under ENERGY STAR Qualified Homes Version 3. Insulation levels shall meet or exceed the 2012 IECC (Table R402.1.1) and achieve Grade 1 installation, per RESNET standards. Infiltration (ACH50):  Zones 1-2: 3; Zones 3-4: 2.5; Zones 5-7: 2; Zone 8: 1.5.  Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol. Steel-frame ceilings, walls, and floors shall meet the insulation requirements of the 2012 IECC – Table 402.2.6.

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.

ABAA 07262

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.

ABAA 07263

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.

ABAA 07265

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.

2009 IECC

Table 402.4.2 Air Barrier and Insulation Inspection Component Criteria, Floors: Air barrier is installed at any exposed edge of insulation.* Table 402.4.2, Air barrier and thermal barrier:  Air permeable insulation is not used as a sealing material.*

2009 IRC

Table N1102.4.2 Air Barrier and Insulation Inspection Component Criteria, Floors: Air barrier is installed at any exposed edge of insulation.* Table N1102.4.2, Air barrier and thermal barrier:  Air permeable insulation is not used as a sealing material.*

2012 IECC

Table R402.4.1.1 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.* 

2012 IRC

Table N1102.4.1.1 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.* 
 

     
*Due to copyright restrictions, exact code text is not provided.  For specific code text, refer to the applicable code.  

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