Air seal the floor above an unconditioned basement or crawlspace and make sure floor insulation is in full contact with this subfloor air barrier.
- Seal all seams, gaps, and holes in the subfloor air barrier with caulk or foam.
- Air seal rim joists.
- Install insulation in floor joist bays without misalignments, compressions, gaps, or voids.
- Install supports to keep insulation in permanent contact with the air barrier above, for example, metal staves for batt insulation or netting for blown insulation.
- If spray foam insulation is used for the floor cavity insulation, the spray foam can serve as the air barrier if it is at least 5.5 inches thick if open-cell or at least 1.5 inches thick if closed-cell spray foam insulation.
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.
Floors can account for one-fourth to one-third of the building enclosure’s surface area. When defects in the air barrier and insulation system exist, heat loss through floors over unconditioned basements or vented crawlspaces can cause uncomfortably cold floors and increases in space conditioning costs as well. Heat loss in floors can be caused by
- Conduction losses, also known as thermal bridging, when heat transfers through framing members dues to a lack of insulation
- Air leakage, due to the lack of an effective air barrier or unsealed holes in the air barrier
- Misalignment of the air barrier and thermal barrier, which allows gaps to exist, for example between batt insulation and the drywall, which allow air to flow in wall cavities due to convective currents or holes in the air barrier, which rob the insulation of its effectiveness (EPA 2011).
To prevent these heat losses, the insulation layer in the floor should be continuous and consistent and should be completely aligned with (in continual contact with) the air barrier separating the house from the unconditioned space below. When basements or crawlspaces are uninsulated the air barrier is typically the subfloor, which consists of plywood or OSB. Any holes around plumbing wiring, etc., in the subfloor must be sealed.
Note, best practice in all climates is to connect the crawlspace or basement to the house by air sealing and insulating along the walls of the basement or crawlspace and providing air exchange between the house and the basement or crawlspace to condition that space. See the guide Air Sealed, Insulated Basements and the article Crawlspaces - Either In or Out by Joe Lstiburek (BSC 2020). This should especially be considered in homes located in hot humid climates and in cold climates where HVAC equipment and plumbing pipes will be located in the crawlspace (especially in cold climates) or if the basement may be used for living space, the crawlspace or basement should be insulated along the walls rather than in the ceiling to include these areas within the thermal envelope of the home. The thermal envelope consists of the continuous air barrier and thermal barrier/insulation which surround the home's walls, ceilings, and foundation. (See the guide Continuous Air Barrier in Exterior Walls for more information.) However, if HVAC equipment is not located in the basement and plumbing lines can be protected, it might be reasonable to establish the thermal and pressure boundary in the floor above the basement, rather than along the foundation walls.
How to Insulate and Air Seal the Floor above an Unconditioned Basement or Crawlspace
- Install a subfloor that can serve as a continuous air barrier between the crawlspace or basement and the house above. In most cases, this air barrier will be plywood or OSB floor sheathing. Install the subfloor sheathing panels as follows (APA 2011):
- Install subfloor in panel widths that align with the framing (typically 16-, 20-, or 24-inches on-center).
- Stagger subfloor panel end joints.
- Use tongue-and-groove subfloor panels or install blocking beneath panel joints.
- Apply construction adhesive or caulk at panel seams and between subfloor panel and framing members.
- Air seal the band and rim joists and any penetrations.
- Block off any open floor joists running from the crawlspace to under other conditioned parts of the home. Use a rigid air barrier material and seal the edges with spray foam. Batt insulation is not a good air barrier. See the guide Garage Rim/Band Joist Adjoining Conditioned Space for more information.
- Seal around any mechanical penetrations such as piping, wiring, or ducts. Use rigid foam or plywood plus spray foam for large holes, as shown in Figure 2 (EPA 2011).
- Seal the rim joists with rigid foam cut to fit and caulked or spray foamed at edges to seal in place (Figure 3). Rim joists can also be air sealed and insulated in one step using spray foam insulation. To qualify as an air barrier, open-cell or closed-cell foam must have a finished thickness greater than or equal to 5.5 inches or 1.5 inches, respectively (EPA 2011). Batt insulation can be added over spray foam or rigid foam but should not be installed alone as it is not an air barrier (Figure 4).
- Install insulation in the floor joists.
- Insulation should be fully "aligned" with the air barrier, i.e., in full contact with the subfloor above. There should be no gaps between the insulation and the sheathing above it like those seen in Figure 5.(EPA 2011).
- Batt insulation should fit tightly between the floor joists but avoid gaps, compressions, or voids. Batts should be fully lofted (not crammed or compressed into the space). Batts should be cut lengthwise to fit narrow joist bays, and split to fit neatly around electrical wiring running across joist bays. See the guide Insulation Installation Achieves RESNET Grade 1 for more on quality installation.
- Spray foam insulation can be used to insulate the floor above a crawlspace or basement as well as the band joist. Spray foam provides high R-value and a continuous air barrier in one labor-saving application. To qualify as an air barrier, open-cell spray foam must have a finished thickness ≥ 5.5 inches and closed-cell spray foam must have a finished thickness ≥ 1.5 inches (EPA 2011).
- Support the insulation. Install metal staves or mechanically fastened wire to keep batts in continuous contact with the subfloor above.
Blower door testing, conducted as part of whole-house energy performance testing, may help indicate whether air leakage at through the floor has been successfully sealed.
Infrared imaging. An infrared camera may also be used to determine air leakage through the floor over a garage if a sufficient temperature difference exists between the outdoors and the conditioned space above the garage. For best results, scan twice – first under static conditions before blower door testing has been conducted. This will allow the technician to evaluate the integrity of insulation behind the drywall, if the garage ceiling has been finished. Conduct a second scan with the blower door running in depressurization mode and the door between the house and the garage open. This will demonstrate the integrity of the air barrier, showing where air leakage has infiltrated the framing and seeped through insulation.
The infrared image below shows the rim joist area. Typically the darker areas indicate the areas that are the coldest, and represent the greatest amount of heat loss. Here, where the insulation meets the subfloor and the I-joists, you can see dark purple spots. These spots indicate heat loss or air leakage. These areas are where you want to be sure you air seal before you insulate.
Diagnostic smoke. With the blower door pressurizing, the garage door closed, and the door to the house open, check for air leaks in the floor and near the rim joist with a smoke pencil. A smoke trail moving away from the smoke pencil indicates a leak to the outdoors that should be sealed.
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.
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.
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.
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) Insulation levels shall meet the 2015 IECC and achieve Grade 1 installation, per RESNET standards. 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 12) Building envelope assemblies, including exterior walls and unvented attic assemblies (where used), shall comply with the relevant vapor retarder provisions of the 2015 International Residential Code (IRC).
Footnote 23) Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.
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.
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.
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.
Table N1188.8.131.52 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.
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.
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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.