Slab Edge Insulation

Scope Images
In cold climates, install slab edge insulation when pouring slab on grade foundations.
In cold climates, install slab edge insulation when pouring slab on grade foundations.
Scope

In cold climates, install slab edge insulation when constructing slab-on-grade foundations.

  • Install insulation along the edge of the slab for a slab-on-grade foundation to meet or exceed the insulation R-value required by code. 
  • Install from the top of the slab down to the depth required by code.
  • If the slab is poured separately from the exterior foundation wall and slab edge insulation is installed between the floor slab and the foundation wall, code permits that the top of the foam be cut at a 45-degree angle away from the exterior wall so that the upper edge is protected by concrete.

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

Poorly or incorrectly insulated foundation slabs can present several problems for homes, including energy loss, moisture control issues, and indoor air quality challenges. Energy loss through slabs is primarily a result of heat conducted outward through the perimeter of the slab and into the surrounding soil. Moisture can become an issue inside the house if the relative temperature difference between the slab and indoor air temperatures become too great and condensation or high localized relative humidity issues occur. With condensation, mold may have a chance to grow and create indoor air quality issues. While it is common practice to install insulation, specifically rigid insulation, during the construction of the slab, it is often installed incorrectly or incompletely. However, by properly installing insulation that extends to the top of the slab in either a monolithic slab with a grade beam or a slab independent of the foundation design, thermal bridging, moisture, and air quality issues can all be addressed and minimized (Ueno and Lstiburek 2012).

properly installed rigid insulation
Figure 1 - Properly installed rigid insulation. This image shows monolithic slab construction with properly installed rigid insulation that extends to the top of the slab and provides a complete thermal break. 

General Steps

To address the problems associated with thermal bridging and potential moisture issues that can occur with improperly insulated slab foundation systems, follow the steps below. The strategies for insulating the slab edge depend upon whether the slab on grade is:

  1. Monolithic with a grade beam
  2. Slabs independent of the foundation wall

For either design style it is important to review the plan for slab insulation with pest control and local building officials to ensure code compliance. Material selection is also important. Insulation levels should meet or exceed state requirements in accordance with the International Energy Conservation Code (IECC). Finally, only use insulation approved for below-grade use.

properly installed vapor barrier
Figure 2 - Properly installed vapor barrier. The polyethlene vapor barrier shown above is properly positioned to work in conjunction with the rigid foam insulation that will be installed on the exterior of the slab after the wood forms have been removed. 

Monolithic with a Grade Beam

When the slab is monolithic with a grade beam, the insulation must be installed to the exterior of the slab edge/grade beam and continue vertically to the bottom of the grade beam as shown below. Different regions may have different code requirements, so be sure to check the local codes to make sure this meets the requirements. The insulation material must be appropriate for ground contact. XPS, rigid fiberglass, and rock wool are examples of acceptable materials. The exterior insulation will need to be protected from impact damage during construction and, subsequently, the above-grade portion must be protected from UV and impact damage (BSC 2009).

General steps for installation of slab insulation in monolithic slabs with grade beam construction:

  1. Install rigid insulation on the exterior that extends to the bottom of the grade beam.
  2. Secure a protection board over the above-grade portion of the rigid insulation.
  3. Make sure the protection board is of non-water-sensitive material and coated to control absorption of water.
  4. Install a protective membrane adhered to the slab and wrapped over the top of the insulation.
  5. Ensure that insulation material is non-moisture-sensitive and not subject to degradation with ground contact.
monolithic slab with a grade beam
Figure 3 - Monolithic slab with a grade beam. The diagram above shows the monolithic slab with rigid insulation properly installed on the exterior of the slab and extending fully to the top of the concrete. 

Important notes if insulation is installed on the exterior of the slab (DOE 2000):

  • Install the insulation from the top of the slab to the bottom of the frost line unless a termite inspection gap is required.

  • Encapsulate or cover the exterior face of the insulation with a protective membrane to serve as a capillary break and to protect the insulation from termites.

  • Cover the above-grade portion of the insulation exposed to outside air using a stucco coating, pressure-treated wood, brick, or aluminum flashing. When covering insulation, be conscious of how to detect termites in areas prone to termite infestation. Some states in termite-prone areas have addressed this issue by requiring a termite inspection gap near the top of the slab insulation.


Independent of the Foundation Wall

When the slab is independent from a perimeter foundation wall, insulation may be installed either on the exterior of the foundation wall or between the foundation wall and the slab. In order for the slab to be independent of the foundation wall, a bond break is needed between the slab, which is supported on grade, and the foundation wall that supports the exterior wall structure and its loads. Insulation at the vertical slab edge and under the slab perimeter provides this bond break. The graphic shows an example of a slab on grade that is structurally and thermally isolated from the perimeter foundation wall. Limiting factors on the width of the slab edge insulation in this situation are determined by the attachment of floor finishes and the width of the foundation wall needed to support the wall structure.

slab independent of foundation wall
Figure 4 - Slab independent of foundation wall. The diagram above shows the proper placement of rigid insulation when the concrete slab is placed independent of the foundation wall.   

General steps for installation of slab insulation independent of the foundation wall:

  1. Make sure the slab is insulated vertically at the edge and horizontally at the perimeter or under the entire slab.

  2. Install the rigid insulation to provide a bond break between the slab and foundation wall.

  3. Ensure the protective membrane is adhered to both the slab and top of foundation wall.

Whenever insulation is used in contact with ground or near ground, appropriate insect control measures must be used (BSC 2009).

Ensuring Success

A successful measurement of slab edge insulation is the temperature of the slab in relation to the air temperature. In the thermal image shown below, the air temperature was 71° F. A close match between slab temperature and air temperatures reveals that there is a minimal chance of slab condensation or high localized relative humidity issues.

Well insulated slab
Figure 1 - Well-insulated slab. This thermal image shows the low relative temperature difference between the well-insulated slab and the air temperature.

 

Climate

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

Rater Field Checklist

Thermal Enclosure System.
3. Reduced Thermal Bridging.
3.2 For slabs on grade in CZ 4-8, 100% of slab edge insulated to ≥ R-5 at the depth specified by the 2009 IECC and aligned with the thermal boundary of the walls 14, 15

2009 - 2018 IECC and IRC Minimum Insulation Requirements

Install insulation in amounts that meet or exceed code-required levels for your climate zone. Please see this table for the minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, and 2018 IECC and IRC. 

2009 IECC

Section R402.2.8, Slab-on-grade floors. Slab insulation requirements: CZ 1-3: R-0; CZ 4-5: R-10, 2 ft; CZ 6-8: R-10, 4 ft. R-5 must be added to the requirement for heated slabs.  The insulation depth is to the depth of the footing or 2 feet, whichever is less in Climate Zones 1-3 (for heated slabs). The insulation must extend downward from the top of the slab on the inside or outside of the foundation wall. If located below grade, the insulation must extend the distance required by any combination of insulation installed vertically, under the slab or extending out from the building. If extending away from the building, the insulation must be protected by pavement or at least 10 inches of soil. The IECC doesn’t require slab edge insulation in locations deemed under very heavy termite infestation by the code official.

2012, 2015, and 2018 IECC

Section R402.2.9 (R402.2.10 in 2015 and 2018 IECC), Slab-on-grade floors. Slab insulation requirements: CZ 1-3: R-0; CZ 4-5: R-10, 2 ft; CZ 6-8: R-10, 4 ft. R-5 must be added to the requirement for heated slabs.  The insulation depth is to the depth of the footing or 2 feet, whichever is less in Climate Zones 1-3 (for heated slabs). The insulation must extend downward from the top of the slab on the inside or outside of the foundation wall. If located below grade, the insulation must extend the distance required by any combination of insulation installed vertically, under the slab or extending out from the building. If extending away from the building, the insulation must be protected by pavement or at least 10 inches of soil. The IECC doesn’t require slab edge insulation in locations deemed under very heavy termite infestation by the code official.

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)

 

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)

Rater Field Checklist

Thermal Enclosure System.
3. Reduced Thermal Bridging.
3.2 For slabs on grade in CZ 4-8, 100% of slab edge insulated to ≥ R-5 at the depth specified by the 2009 IECC and aligned with the thermal boundary of the walls 14, 15

Footnote 14) Consistent with the 2009 IECC, slab edge insulation is only required for slab-on-grade floors with a floor surface less than 12 inches below grade. Slab insulation shall extend to the top of the slab to provide a complete thermal break. If the top edge of the insulation is installed between the exterior wall and the edge of the interior slab, it shall be permitted to be cut at a 45-degree angle away from the exterior wall. Alternatively, the thermal break is permitted to be created using ≥ R-3 rigid insulation on top of an existing slab (e.g., in a home undergoing a gut rehabilitation). In such cases, up to 10% of the slab surface is permitted to not be insulated (e.g., for sleepers, for sill plates). Insulation installed on top of slab shall be covered by a durable floor surface (e.g., hardwood, tile, carpet).

Footnote 15) Where an insulated wall separates a garage, patio, porch, or other unconditioned space from the conditioned space of the house, slab insulation shall also be installed at this interface to provide a thermal break between the conditioned and unconditioned slab. Where specific details cannot meet this requirement, partners shall provide the detail to EPA to request an exemption prior to the home’s certification. EPA will compile exempted details and work with industry to develop feasible details for use in future revisions to the program. A list of currently exempted details is available at: energystar.gov/slabedge.

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 (Revision 07)

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.

2009 - 2021 IECC and IRC Minimum Insulation Requirements: 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.

2009 International Energy Conservation Code (IECC)

Section R402.2.8, Slab-on-grade floors. Slab insulation requirements: CZ 1-3: R-0; CZ 4-5: R-10, 2 ft; CZ 6-8: R-10, 4 ft. R-5 must be added to the requirement for heated slabs.  The insulation depth is to the depth of the footing or 2 feet, whichever is less in Climate Zones 1-3 (for heated slabs). The insulation must extend downward from the top of the slab on the inside or outside of the foundation wall. If located below grade, the insulation must extend the distance required by any combination of insulation installed vertically, under the slab or extending out from the building. If extending away from the building, the insulation must be protected by pavement or at least 10 inches of soil. The IECC doesn’t require slab edge insulation in locations deemed under very heavy termite infestation by the code official.

2012, 2015, and 2018 IECC

Section R402.2.9 (R402.2.10 in 2015 and 2018 IECC), Slab-on-grade floors. Slab insulation requirements: CZ 1-3: R-0; CZ 4-5: R-10, 2 ft; CZ 6-8: R-10, 4 ft. R-5 must be added to the requirement for heated slabs.  The insulation depth is to the depth of the footing or 2 feet, whichever is less in Climate Zones 1-3 (for heated slabs). The insulation must extend downward from the top of the slab on the inside or outside of the foundation wall. If located below grade, the insulation must extend the distance required by any combination of insulation installed vertically, under the slab or extending out from the building. If extending away from the building, the insulation must be protected by pavement or at least 10 inches of soil. The IECC doesn’t require slab edge insulation in locations deemed under very heavy termite infestation by the code official.

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.
Figure 1. 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. (Source: 2021 IECC).

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 N1102.2.8, Slab-on-grade floors. Slab insulation requirements: CZ 1-3: R-0; CZ 4-5: R-10, 2 ft; CZ 6-8: R-10, 4 ft. R-5 must be added to the requirement for heated slabs.  The insulation depth is to the depth of the footing or 2 feet, whichever is less in Climate Zones 1-3 (for heated slabs). The insulation must extend downward from the top of the slab on the inside or outside of the foundation wall. If located below grade, the insulation must extend the distance required by any combination of insulation installed vertically, under the slab or extending out from the building. If extending away from the building, the insulation must be protected by pavement or at least 10 inches of soil. The IECC doesn’t require slab edge insulation in locations deemed under very heavy termite infestation by the code official.

2012, 2015, and 2018 IRC

Section N1102.2.9 (N1102.2.10 in 2015 and 2018 IRC), Slab-on-grade floors. Slab insulation requirements: CZ 1-3: R-0; CZ 4-5: R-10, 2 ft; CZ 6-8: R-10, 4 ft. R-5 must be added to the requirement for heated slabs. The insulation depth is to the depth of the footing or 2 feet, whichever is less in Climate Zones 1-3 (for heated slabs). The insulation must extend downward from the top of the slab on the inside or outside of the foundation wall. If located below grade, the insulation must extend the distance required by any combination of insulation installed vertically, under the slab or extending out from the building. If extending away from the building, the insulation must be protected by pavement or at least 10 inches of soil. The IECC doesn’t require slab edge insulation in locations deemed under very heavy termite infestation by the code official.

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)
U.S. Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.
*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 Measures
Building Science-to-Sales Translator

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

Image(s)
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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