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Insulation for Existing Crawl Space Floors

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 climate zones are shown on the map below, which is taken from Figure C301.1 of the 2012 IECC.

IECC climate zones
IECC Climate Zone Map

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

Training

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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 

[Guidance for Version 3.0, Rev 08 is coming soon.]

ENERGY STAR Certified Homes is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy and the U.S. Environmental Protection Agency. Builders and remodelers who are conducting retrofits are welcome to seek certification for existing homes through this voluntary program.

ENERGY STAR Certified Homes (Version 3, Rev. 07) requires that ceiling, wall, floor, and slab insulation levels meet or exceed those specified in the 2009 International Energy Conservation Code (IECC).

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. Some states have adopted the 2012 IECC. Visit the U.S. DOE Building Energy Codes Program to see what code has been adopted in each state. For states that have adopted the 2012 IECC or an equivalent code, EPA intends to implement the ENERGY STAR Certified Homes Version 3.1 National Program Requirements for homes permitted starting one year after state-level implementation of the 2012 IECC or an equivalent code. However, EPA will make a final determination of the implementation timeline on a state-by-state basis. Some states and regions of the country have ENERGY STAR requirements that differ from the national requirements. Visit ENERGY STAR’s Regional Specifications page for more information on those region-specific requirements.

The ENERGY STAR Thermal Enclosure System Rater Checklist (Ver 3, Rev 07) specifies:

3 Fully-Aligned Air Barriers. At each insulated location noted below, a complete air barrier shall be provided that is fully aligned with the insulation as follows:

  • At interior or exterior surface of ceilings in Climate Zones 1-3; at interior surface of ceilings in Climate Zones 4-8. Also, include barrier at interior edge of attic eave in all climate zones using a wind baffle that extends to the full height of the insulation. Include a baffle in every bay or a tabbed baffle in each bay with a soffit vent that will also prevent wind washing of insulation in adjacent bays
  • At exterior surface of walls in all climate zones; and also at interior surface of walls for Climate Zones 4-8 7
  • At interior surface of floors in all climate zones, including supports to ensure permanent contact and blocking at exposed edge

The ENERGY STAR Water Management Checklist, Water-Managed Site and Foundation specifies:

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” extruded polystyrene insulation with taped joints.

1.4 Capillary break at all crawlspace floors using ≥ 6 mil polyethylene sheeting, lapped 6-12 in., and installed using one of the following three options:

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.

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.

The DOE Zero Energy Ready Home program requires compliance with EPA's Indoor airPLUS program. Item 1.4 under moisture control requires that basements/crawlspaces are insulated, sealed and conditioned. 

The U.S. Department of Energy Zero Energy Ready Home National Program Requirements specify as a mandatory requirement (Exhibit 1, #2.2) that, for all labeled homes, whether prescriptive or performance path, ceiling, wall, floor, and slab insulation shall meet or exceed 2012 IECC levels. See the guide 2012 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.

2009 IECC

Table 402.4.2 Air barrier and insulation inspection component criteria.

20122015, and 2018 IECC

R401.3 Certificate (Mandatory).

Table R402.4.1.1 Air barrier and insulation installation.

Retrofit: 200920122015, and 2018 IECC

Section R101.4.3 (Section R501.1.1 in 2015 and 2018 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 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: 200920122015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 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.

Case Studies

None Available

References and Resources*

  1. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  2. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  3. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  4. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  5. Author(s): Lstiburek
    Organization(s): Building Science Corporation
    Publication Date: November, 2004

    Report outlining how conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability and energy consumption.

  6. Author(s): Building Science Corporation
    Organization(s): Building Science Corporation
    Publication Date: May, 2009

    Information sheet about crawlspace insulation, including installation details.

  7. Author(s): Building Science Corporation
    Organization(s): Building Science Corporation
    Publication Date: May, 2009

    Information sheet about groundwater control.

  8. Author(s): Pettit, Neuhauser, Gates
    Organization(s): Building Science Corporation
    Publication Date: July, 2013

    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.

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

Last Updated: 12/15/2015