Slab Edge Insulation

Please Register or Login to Provide Feedback.

Climate

ENERGY STAR Version 3, (Rev. 07)

Thermal Enclosure Checklist, Reduced Thermal Bridging. For slabs on grade in CZ 4 and higher, 100% of slab edge insulated to >= R-5 at the depth specified by the 2009 IECC and aligned with thermal boundary of the walls. 

2009 IECC

Section 402.2.8, 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). 

2009 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).* 

2012 IECC

Section R402.2.9, 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).

2012 IRC

Section N1102.2.9, 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).

climate zone map

International Energy Conservation Code (IECC) Climate Regions

Please Register or Login to Provide Feedback.

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.  Reference

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.  Reference

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 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.  Reference

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.  Reference 

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. REF Icon

Scope

For slabs on grade in CZ 4 and higher, 100% of slab edge insulated to ≥ R-5 at the depth specified by the 2009 IECC and aligned with thermal boundary of the walls

Reduced Thermal Bridging

For slabs on grade in Climate Zone 4 and higher, 100% of the slab edge is insulated to >= R-5 at the depth specified by the 2009 IECC and aligned with thermal boundary of the walls.

  1. Install slab edge insulation to extend to the top of the slab so it provides a complete thermal break.

ENERGY STAR Notes:

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.

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 ENERGY STAR to request an exemption prior to the home’s qualification. ENERGY STAR will compile exempted details and work with industry to develop feasible details for use in future revisions to the program. 

ENERGY STAR Notes for Existing Homes:

Alternative to slab edge insulation, 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).

Training

Right and Wrong Images

Presentations

None Available

Videos

None Available

CAD Images

Compliance

ENERGY STAR Version 3, (Rev. 07)

Thermal Enclosure Checklist, Reduced Thermal Bridging. For slabs on grade in Climate Zone 4 and higher, 100% of the slab edge is insulated to >= R-5 at the depth specified by the 2009 IECC and aligned with thermal boundary of the walls. 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. 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 ENERGY STAR to request an exemption prior to the home’s qualification. ENERGY STAR will compile exempted details and work with industry to develop feasible details for use in future revisions to the program. Alternative to slab edge insulation, 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).

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3. Ceiling, wall, floor, and slab insulation shall meet or exceed 2012 IECC levels and achieve Grade 1 installation, per RESNET standards.

2009 IECC

Section 402.2.8, 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.*

2009 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 IECC

Section R402.2.9, 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 IRC

Section N1102.2.9, 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.*

*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.

More Info.

Case Studies

None Available

References and Resources*

  1. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  2. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  3. Author(s): Ueno, Lstiburek
    Organization(s): BSC
    Publication Date: May 2012

    Document providing information on basement insulation, air sealing and water management retrofits.

  4. Author(s): BSC
    Organization(s): BSC
    Publication Date: May 2009

    Information sheet about insulating slabs.

  5. Author(s): Southface Energy Institute
    Organization(s): DOE
    Publication Date: December 2000

    Brochure describing insulation approaches for homes with slab-on-grade floors.

  6. Author(s): EPA
    Organization(s): EPA
    Publication Date: October 2011

    Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.

Last Updated: 08/15/2013

Mobile Field Kit

The Building America Field Kit allows you to save items to your profile for review or use on-site.