Insulation for Existing Crawl Space Floors

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Rigid foam insulation and a thin slab were installed over the dirt and gravel of this sealed crawlspace
Rigid foam insulation and a thin slab were installed over the dirt and gravel of this sealed crawlspace
Scope

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 Certified Homes, and Indoor airPLUS.

Description

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 Concrete Slab over Polyethylene and Polyethylene Lapped Up Walls/Piers and 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.)

Rigid foam insulation, a polyethylene vapor barrier, and a thin concrete slab are installed over the dirt and gravel floor of a sealed crawlspace
Figure 1. Rigid foam insulation, a polyethylene vapor barrier, and a thin concrete slab are installed over the dirt and gravel floor of a sealed crawlspace.

How to Install Insulation and Thin Slab Over Dirt/Gravel

  1. 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.
  2. Spread a layer of gravel for leveling. Slope to drain if a drainage system needs to be installed. See Figure 1.
  3. Install radon mitigation system if needed.
  4. Install XPS rigid foam insulation.
  5. Install a continuous 6-mil polyethylene sheet with seams taped. See the guide Concrete Slab over Polyethylene for more information.
  6. 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.

Closed-cell spray foam is installed over the dirt and gravel floor of a sealed crawlspace
Figure 2. Closed-cell spray foam is installed over the dirt and gravel floor of a sealed crawlspace.

How to Install Closed-Cell Spray Foam over Dirt/Gravel

  1. 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.
  2. Level the existing ground cover (earth or gravel). Slope to drain if a drainage system needs to be installed.
  3. Install radon mitigation system if needed.
  4. Install a continuous 6-mil polyethylene sheet with seams taped.
  5. Install closed-cell spray foam over polyethylene. See Figure 2.
  6. 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.)

Gutters, downspouts, surface grading, and a polyethylene vapor barrier help keep moisture out of the crawlspace
Figure 3. Gutters, downspouts, surface grading, and a polyethylene vapor barrier help keep moisture out of the crawlspace.
Interior and exterior perimeter drains are wrapped in filter fabric and installed in gravel
Figure 4. Interior and exterior perimeter drains are wrapped in filter fabric and installed in gravel. Poly vapor retarder, caulking, waterproofing, grading away from house, and gutters are other water control measures to keep crawlspaces dry.

 

Ensuring Success

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.

Climate

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. 

Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18.
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)

 

Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)

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.

Minimum R-Value Requirements for Slab Insulation in the 2009 and 2012 IECC
Table 1. Minimum R-Value Requirements for Slab Insulation in the 2009 and 2012 IECC
CAD

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 Certified Homes, Version 3/3.1 (Rev. 09)

ENERGY STAR Certified 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 and Insulation Installation (RESNET Grade 1). 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. 

2. Fully-Aligned Air Barriers 6 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).7​
Walls: At exterior vertical surface of wall insulation in all climate zones; also at interior vertical surface of wall insulation in Climate Zones 4-8.8
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 11 & 12.10, 11, 12

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.3, 4, 5
1.4 Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., & installed using one of the following:3, 4, 5
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 pump covers mechanically attached with full gasket seal or equivalent.

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home

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.

EPA Indoor airPLUS (Revision 04)

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.  

2009 International Energy Conservation Code (IECC)

Table 402.4.2 Air barrier and insulation inspection component criteria.

2012, 2015, and 2018 IECC

R401.3 Certificate (Mandatory).

Table R402.4.1.1 Air barrier and insulation installation.

Retrofit:

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC). 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.)

2009 International Residential Code (IRC)

Section R302.10 Flame spread index and smoke developed index for insulation.

Section R302.10.1 Insulation.

Section R316.5.4 Crawlspace.

Section R322.2.2 Enclosed area below design floor elevation.

Section R408.3 Unvented crawlspace.

Table N1102.4.2 Air Barrier and Insulation Inspection.

2012 IRC

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.

Section R501.3 Fire protection of floors.

Section N1101.16 (R401.3) Certificate (Mandatory).

Table N1102.4.1.1 (R402.4.1.1) Air barrier and insulation installation.

2015 and 2018 IRC

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.

Section N1101.14 (R401.3) Certificate (Mandatory).

Table N1102.4.1.1 (R402.4.1.1) Air barrier and insulation installation.

Retrofit: 

2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). 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.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

More Info.

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.

References and Resources*
Author(s)
Pettit,
Neuhauser,
Gates
Organization(s)
Building Science Corporation
Publication Date
Description
Guidebook providing useful examples of high performance retrofit techniques for the building enclosure of wood frame residential construction in a cold and somewhat wet climate.
Author(s)
Lstiburek
Organization(s)
Building Science Corporation
Publication Date
Description
Report outlining how conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability and energy consumption.
Author(s)
Building Science Corporation
Organization(s)
Building Science Corporation
Publication Date
Description
Information sheet about groundwater control.
Author(s)
Building Science Corporation
Organization(s)
Building Science Corporation
Publication Date
Description
Information sheet about crawlspace insulation, including installation details.
*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.

Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team

Building Science Measures
Building Science-to-Sales Translator

High-R Foundation Insulation = High-Efficiency or Ultra-Efficient Foundation Insulation

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Technical Description

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.

High-Efficiency or Ultra-Efficient Foundation Insulation
Sales Message

High-efficiency foundation insulation helps provide added thermal protection. What this means to you is less wasted energy along with enhanced comfort and quiet. Knowing there is one opportunity to optimize performance during construction, wouldn’t you agree it’s a great opportunity to meet or exceed future codes?

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