Upgrade a crawlspace floor in an existing home by adding insulation as follows:
- Determine if the crawlspace floor surface is suitable for walking and storage.
- Install either rigid foam insulation under a thin slab or or closed-cell spray foam insulation over the crawlspace floor. If the crawlspace will be accessed frequently for storage, choose rigid foam and a concrete slab.
- If there is a history of flooding or water leakage in the crawlspace, provide proper drainage.
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.
Crawlspaces can be designed and constructed as mini-basements, part of the house and within the conditioned space. When crawlspaces are included within the thermal enclosure, they should be insulated and air sealed on their perimeters and should have a continuous sealed ground cover. In cold climates or in very low-energy-consuming buildings, installing insulation below on the floor of the crawlspace is also recommended. When a crawlspace floor is air sealed and insulated and the crawlspace walls are air sealed and insulated, the temperature and humidity conditions in the crawlspace become very stable and essentially identical to the interior of the building. See the guide Unvented Crawlspaces and Conditioned Basements for more information.
The building code does not specifically call for insulation at the crawlspace floor. Therefore, minimum thermal resistance (R-value) requirements for slabs should be followed. See the Climate tab for R-values.
If it is not possible to treat the crawlspace as a part of the house, such as in flood zones in coastal areas or in dry climates where it is not necessary, it is important to construct the house such that the crawlspace is isolated from the house, i.e., is outside of the home’s thermal boundary. Homes with uninsulated, unsealed crawlspaces should be treated like homes built on piers with thorough air sealing of the floor and floor joists and insulation installed on the underside of the floor in full contact with the floor.
Conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability, and energy consumption. These benefits are particularly evident when the crawlspace is used to locate HVAC equipment and ducting. Vented crawlspaces in humid climates have experienced serious moisture and mold problems, because rather than venting moisture out, the crawlspace have allowed hot, humid outside air into the crawlspace where it has caused condensation to form on the cooler floor joists, causing moisture issues that have cost builders and homeowners significant resources to repair. Despite the obvious problems with existing vented crawlspaces and the obvious benefits of conditioned crawlspaces, there is not a significant trend toward the construction of conditioned crawlspaces. One of the reasons typically cited by builders and designers is “the code does not allow me to build unvented crawlspaces.” This is both generally correct and misleading. The model codes do not allow the construction of “unvented” crawlspaces – except in very limited circumstances, but they do allow the construction of “conditioned” crawlspaces. The distinction is important and necessary. When the crawlspace is sealed, insulated, and provided with heated and cooled air, it is considered conditioned space. The space is part of the “interior” of the building and should be heated, cooled, and ventilated as part of the building’s heating, cooling, and ventilating strategy.
Whether or not the crawlspace is included within the conditioned space of the home, it must always have a ground cover that prevents evaporation of ground moisture into the crawlspace. There are many ways to provide a durable ground cover or liner. The option used depends on the resources available and the frequency of people entering the crawlspace to either store possessions or to maintain equipment. This ground cover must be continuous around piers and supports. (See the guides Capillary Break at Crawlspace Floor - Polyethylene Sheeting under Concrete Slab and Capillary Break at Crawlspace Floors - Polyethylene Lapped Up Walls and Piers or Secured in the Ground.)
If there are any moisture issues in the crawlspace or around the foundation, those must be taken care of before insulating the crawlspace floor. See the guide Water Management of Existing Crawlspace Floor for more information. If a sump pump is needed see the guide Drain or Sump Pump Installed in Basements or Crawlspaces.
Consider testing radon levels in the basement and home prior to the crawlspace insulating project. If a radon mitigation system is needed, install the radon vent pipe and stack before insulating the crawlspace floor.
Insulation and Thin Slab over Dirt/Gravel
The crawlspace floor assembly shown in Figure 1 provides a surface suitable for light storage. (This floor design is also suitable for slab on grade construction.)
How to Install Insulation and Thin Slab Over Dirt/Gravel
- Install drainage pipe and a sump pump that pumps to daylight away from the house or to the sewer system if there is a history of flooding or pooling water in the crawlspace.
- Spread a layer of gravel for leveling. Slope to drain if a drainage system needs to be installed. See Figure 1.
- Install radon mitigation system if needed.
- Install XPS rigid foam insulation.
- Install a continuous 6-mil polyethylene sheet with seams taped. See the guide Capillary Break at Crawlspace Floor - Polyethylene Sheeting under Concrete Slab for more information.
- Install a thin layer of concrete as a finished surface.
Closed-Cell Spray Foam over Dirt/Gravel
Closed-cell spray foam can be installed over a polyethylene vapor barrier directly onto the crawlspace floor as shown in Figure 2. This crawlspace floor assembly should not be used under a (future) concrete slab. This assembly is not appropriate in a crawlspace that needs to be accessed frequently.
How to Install Closed-Cell Spray Foam over Dirt/Gravel
- Install drainage pipe and a sump pump that pumps to daylight away from the house or to the sewer system if there is history of flooding or pooling water in the crawlspace.
- Level the existing ground cover (earth or gravel). Slope to drain if a drainage system needs to be installed.
- Install radon mitigation system if needed.
- Install a continuous 6-mil polyethylene sheet with seams taped.
- Install closed-cell spray foam over polyethylene. See Figure 2.
- Apply spray-on thermal/ignition barrier as required by code.
Groundwater Control for Crawlspaces
Keep rain water away from the foundation perimeter (See Figure 3).
Crawlspaces should have perimeter drainage just like a basement when the crawlspace ground level is below the ground level of the surrounding grade. If there is a history of flooding or water leakage in the crawlspace and there is no functioning perimeter drainage system, a drainage system (e.g., an interior drain or gravel layer connected to an interior sump pit) must be installed under this assembly. See Figure 4. Also see the guide Drain or Sump Pump Installed in Basements and Crawlspaces.) An interior sump pit must have an airtight and gasketed cover. (See the guide Gasketed/Sealed Sump Pump Covers.)
Level out the gravel/earth surface in the crawlspace for installation.
Install radon-mitigation pipe venting system if needed.
Perform any water needed water control measures before installing floor covering.
Overlap the seams of the polyethylene by at least 6 inches and secure with tape.
Install various assembly layers in a continuous manner around all penetrations such as piers and supports. Also lap up walls and around piers and secure with furring strips and fasteners, tape, or mastic.
The crawlspace floor assembly should be designed for a specific hygrothermal region, rain exposure zone, and interior 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.
The insulation levels should be based on the minimum requirements for vapor control in the current adopted building code and the minimum requirements for thermal control in the current energy code. Additional insulation can be added above these minimums to create high R-Value floor assemblies. The table below provides the minimum thermal resistance (R-value) requirements for slab floors specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b), based on climate zone.
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 requires that ceiling, wall, floor, and slab insulation levels meet or exceed those specified in the 2009 International Energy Conservation Code (IECC) with some alternatives and exceptions, and achieve Grade 1 installation per RESNET Standards (see 2009 and 2012 IECC Code Level Insulation – ENERGY STAR Requirements, Insulation Installation (RESNET Grade 1) and Insulation Installation (RESNET Grade 2). If the state or local residential building energy code requires higher insulation levels than those specified in the 2009 IECC, you must meet or exceed the locally mandated requirements.
National Rater Field Checklist
2. Fully-Aligned Air Barriers 7 - At each insulated location below, a complete air barrier is provided that is fully aligned as follows:
Ceilings: At interior or exterior horizontal surface of ceiling insulation in Climate Zones 1-3; at interior horizontal surface of ceiling insulation in Climate Zones 4-8. Also, at exterior vertical surface of ceiling insulation in all climate zones (e.g., using a wind baffle that extends to the full height of the insulation in every bay or a tabbed baffle in each bay with a soffit vent that prevents wind washing in adjacent bays). 8
Walls: At exterior vertical surface of wall insulation in all climate zones; also at interior vertical surface of wall insulation in Climate Zones 4-8.9
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
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 8) All insulated ceiling surfaces, regardless of slope (e.g., cathedral ceilings, tray ceilings, conditioned attic roof decks, flat ceilings, sloped ceilings), must meet the requirements for ceilings.
Footnote 9) All insulated vertical surfaces are considered walls (e.g., above and below grade exterior walls, knee walls) and must meet the air barrier requirements for walls. The following exceptions apply: air barriers recommended, but not required, in adiabatic walls in multifamily dwellings; and, in Climate Zones 4 through 8, an air barrier at the interior vertical surface of insulation is recommended but not required in basement walls or crawlspace walls. For the purpose of these exceptions, a basement or crawlspace is a space for which ≥ 40% of the total gross wall area is below-grade.
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.
National Water Management System Builder Requirements
1. Water-Managed Site and Foundation.
1.3 Capillary break beneath all slabs (e.g., slab on grade, basement slab) except crawlspace slabs using either: ≥ 6 mil polyethylene sheeting, lapped 6-12 in., or ≥ 1 in. extruded polystyrene insulation with taped joints.4, 5, 6
1.4 Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., & installed using one of the following:4, 5, 6
1.4.1 Placed beneath a concrete slab; OR,
1.4.2 Lapped up each wall or pier and fastened with furring strips or equivalent; OR,
1.4.3 Secured in the ground at the perimeter using stakes.
1.7 Sump pit cover mechanically attached with full gasket seal or equivalent.
Footnote 4) Not required in Dry (B) climates as shown in 2009 IECC Figure 301.1 and Table 301.1.
Footnote 5) Not required for raised pier foundations with no walls. To earn the ENERGY STAR, EPA recommends, but does not require, that radon-resistant features be included in homes built in EPA Radon Zones 1, 2 & 3. For more information, see www.epa.gov/indoorairplus.
Footnote 6) For an existing slab (e.g., in a home undergoing a gut rehabilitation), in lieu of a capillary break beneath the slab, a continuous and sealed Class I or Class II Vapor Retarder (per Footnote 7) is permitted to be installed on top of the entire slab. In such cases, up to 10% of the slab surface is permitted to be exempted from this requirement (e.g., for sill plates). In addition, for existing slabs in occupiable space, the Vapor Retarder shall be, or shall be protected by, a durable floor surface. If Class I Vapor Retarders are installed, they shall not be installed on the interior side of air permeable insulation or materials prone to moisture damage.
Footnote 7) Interior surface of an existing below-grade wall (e.g., in a home undergoing a gut rehab.) listed in Item 1.5a is permitted to be finished by:
- Installing a continuous and sealed drainage plane, capillary break, Class I Vapor Retarder (per Footnote 8) and air barrier that terminates into a foundation drainage system as specified in Item 1.8; OR
- If a drain tile is not required as specified in Footnote 9, adhering a capillary break and Class I Vapor Retarder (per Footnote 8) directly to the wall with the edges taped/sealed to make it continuous.
Note that no alternative compliance option is provided for existing below-grade wood-framed walls in Item 1.5b.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.
The DOE Zero Energy Ready Home Program is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy. Builders and remodelers who are performing retrofits on existing homes are welcome to seek certification for those homes through this voluntary program.
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 2, 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 1, Item 6) Certified under EPA Indoor airPLUS.
1.4 Basement and Crawlspace Insulation and Conditioned Air.
- Seal crawlspace and basement perimeter walls to prevent outside air infiltration.
- Insulate crawlspace and basement perimeter walls according to the prescriptive values determined by local code or R-5, whichever is greater.
- Provide conditioned air at a rate not less than 1 cfm per 50 sq. ft. of horizontal floor area. This can be achieved by a dedicated supply (2015 IRC section R408.3.2.2) or through crawl-space exhaust (2015 IRC section R408.3.2.1). However, if radon-resistant features are required (see Specification 2.1), do not use the crawlspace exhaust method.
See Indoor airPLUS Specifications for exceptions.
R401.3 (404.5.3 in 2009 IECC) Certificate (Mandatory).
Table R402.4.1.1 (402.4.2 in 2009 IECC) Air barrier and insulation installation.
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.
Section R302.10 Flame spread index and smoke-developed index for insulation.
Section R302.10.1 Insulation.
Section R316.5.4 Crawlspace.
Section R408.3 Unvented crawlspace.
Only the 2012 IRC includes Section R501.3 detailing Fire protection of floors.
Section N1101.14 (N1101.16 in 2012 IRC) Certificate (Mandatory).
Table N1220.127.116.11 (N1102.4.2 in 2009 IRC) Air barrier and insulation installation.
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.
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.
There are two levels of foundation insulation: high-efficiency insulation, which meets the 2015 International Energy Conservation Code, and ultra-efficient insulation, which is 25% more efficient than this national code. Using high-efficiency and ultra-efficient insulation, along with professional installation (e.g., no gaps, voids, compression, or misalignment with air barriers; complete air barriers; and minimal thermal bridging) creates conditioned spaces that require very little heating and cooling, along with, even comfort and quiet throughout the house.