Rigid Foam Insulation Installed over Existing Foundation Slabs

Scope Images
Retrofit of an existing basement slab by adding dimple plastic mat, rigid foam insulation, and a floating subfloor
Retrofit of an existing basement slab by adding dimple plastic mat, rigid foam insulation, and a floating subfloor
Scope

Insulate an existing slab floor by adding rigid foam insulation over the slab then installing a floating subfloor as follows:

  • Treat any significant or persistent site water management issues that are contributing to basement water management issues first.
  • Inspect the existing slab for cracks, holes, and penetrations.
  • Clean the slab prior to beginning any repairs or retrofit work.
  • Seal the cracks, holes, and penetrations.
  • Install negative side vapor barrier.
  • Install rigid foam insulation.
  • Install sleepers if needed.
  • Install a floating subfloor.

For more information, see the U.S. Department of Energy’s Standard Work Specifications regarding slab foundations.

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

If the existing basement slab is uninsulated, there are two options for adding insulation. One option is to remove the existing slab, excavate below, install insulation, then pour a new slab along with taking other appropriate moisture protection steps. This option is rarely considered feasible due to cost and other considerations. The other option is to install rigid foam insulation and water control measures on top of the existing slab. A floating subfloor is then installed either directly over the rigid foam insulation (Figures 1 and 2) or over furring strips (Figure 3).

When the existing slab is covered with rigid foam insulation, steps must be taken to manage moisture. One option includes providing a water drainage space underneath the insulation to allow water to be drained over the existing slab to a sump or other drainage system. This drainage space is completely isolated from the home by vapor-impermeable and air-impermeable materials. Figure 1 shows rigid insulation installed over an existing slab using polyethylene dimple mat as the drainage layer between the insulation and the existing slab. The other option (Figures 2 and 3) requires installation of a perimeter drain system tied into a sump or other drainage system. For more on sump pump installation, see the guide Drain or Sump Pump Installed in Basements or Crawlspaces.

When performing basement retrofits, consider testing for radon. If radon levels warrant installation of a passive or active radon mitigation system it can be installed before insulation, vapor and water barrier layers are installed over the slab. See the guide Vertical Radon Ventilation Pipe for more information on radon testing and mitigation.

Head height in the basement should be verified prior to beginning retrofit work in order to comply with the current adopted building code, as this retrofit measure will raise the floor height. Where it is not possible to add insulation over the existing basement slab due to insufficient head height in the basement, at a minimum, the slab should be patched and sealed to the foundation wall for soil gas control and epoxy paint should be applied to the surface for moisture transfer control. For more information on this approach see Water Management of Existing Basement Floor.

Insulation with Floating Floor over Existing Slab with Drainage Layer between Insulation and Existing Slab

A drainage mesh or dimple plastic mat is installed over the existing slab then covered with rigid foam and a floating subfloor (see Figure 1). The above-slab drainage layer must be connected to an interior sump pit. The interior sump pit must have an air tight and gasketed cover that is designed to provide the dual function of keeping soil gasses out and preventing ground water from entering the basement. (See the guide Gasketed/Sealed Sump Pump Covers.)

Through-assembly fasteners should not be used because these would puncture the air and vapor control layers and could potentially admit soil gas that is collected in the air space below (in the drainage mesh or dimple mat).

The existing basement slab is retrofitted by installing a dimple plastic drainage mat, rigid foam insulation, and a floating subfloor
Figure 1. The existing basement slab is retrofitted by installing a dimple plastic drainage mat, rigid foam insulation, and a floating subfloor.

How to Install a Drainage Layer and Insulation with Floating Floor over an Existing Slab

  1. If significant or persistent water issues exist in the basement, treat those issues prior to sealing the slab.
  2. Consider testing radon levels in the basement and home prior to the sealing project. If a radon stack is needed, install it before urethane sealant and epoxy paint are applied.
  3. Inspect the existing slab for cracks, holes, and penetrations. Locate the areas that need to be addressed.
  4. Remove any debris or dust particles from the slab prior to applying urethane sealant to ensure proper adherence of the sealant.
  5. Apply a generous and continuous bead of urethane sealant to any cracks, holes, around any penetrations, and at wall-to-slab joints to prevent pest and soil gas (including radon) entry.
  6. Install a 6-mil polyethylene vapor barrier with seams taped or a high-density polyethylene dimple mat with seams taped (see Figure 1).
  7. Install rigid foam insulation (XPS or EPS Type II) with seams taped.
  8. Install a floating subfloor. Do not fasten the subfloor through the floor assembly.

Insulation with Floating Floor Directly Over Existing Slab

Instead of drainage mat, the existing slab can be coated with two coats of epoxy paint, then rigid foam can be installed directly over the epoxy layer (see Figure 2). Use of this slab assembly requires a sub-slab drainage system such as an interior perimeter drain.

The existing slab is retrofitted by coating with epoxy paint, then installing rigid foam insulation and a floating subfloor
Figure 2. The existing slab is retrofitted by coating with epoxy paint, then installing rigid foam insulation and a floating subfloor.

How to Install Insulation with Floating Floor Directly over an Existing Slab

  1. If significant or persistent water issues exist in the basement, treat those issues prior to sealing the slab.
  2. Consider testing radon levels in the basement and home prior to the sealing project. If a radon stack is needed, install it before urethane sealant and epoxy paint are applied.
  3. Inspect the existing slab for cracks, holes, and penetrations. Locate the areas that need to be addressed.
  4. Remove any debris or dust particles from the slab prior to applying urethane sealant to ensure proper adherence of the sealant.
  5. Apply a generous and continuous bead of urethane sealant to any cracks, holes, around any penetrations, and at wall-to-slab joints to prevent pest and soil gas entry
  6. Apply two coats of epoxy paint over the existing slab. The area should be clear of dust and debris.
  7. Install rigid foam insulation (XPS or EPS Type II) with seams taped (see Figure 2).
  8. Install a floating subfloor. Do not fasten the subfloor through the floor assembly.

Insulation with Sleepers and Floating Floor over Existing Slab with Drainage Layer between Insulation and Existing Slab

A third option is to install unfaced rigid foam over an existing slab that has been coated with epoxy paint, then to install 1x4 sleepers for the plywood subfloor to sit on (see Figure 3). Use of this slab assembly requires a sub-slab drainage system such as an interior perimeter drain.

The existing slab is retrofitted with epoxy paint, sleepers, and subfloor
Figure 3. The existing slab is retrofitted with epoxy paint, sleepers, and subfloor.

How to Install Insulation with Sleepers and Floating Floor Directly Over an Existing Slab

  1. If significant or persistent water issues exist in the basement, treat those issues prior to sealing the slab.
  2. Consider testing radon levels in the basement and home prior to the sealing project. If a radon stack is needed, install it before urethane sealant and epoxy paint are applied.
  3. Inspect the existing slab for cracks, holes, and penetrations. Locate the areas that need to be addressed.
  4. Remove any debris or dust particles from the slab prior to applying urethane sealant to ensure proper adherence of the sealant.
  5. Apply a generous and continuous bead of urethane sealant to any cracks, holes, around any penetrations, and at wall-to-slab joints to prevent pest and soil gas entry.
  6. Apply two coats of epoxy paint over the existing slab. The area should be clear of dust and debris.
  7. Install rigid foam insulation (XPS or EPS Type II) with the seams taped (see Figure 3).
  8. Install 1x4 furring strips at 16 inches o.c. and attach them to the concrete slab through the rigid foam insulation.
  9. Install the plywood subfloor. Fasten the subfloor to the furring strips.
Ensuring Success

Apply urethane sealant and epoxy paint to a clean, dust-free surface.

Visually verify the quality of the sealant application and paint application for continuous adhesion and coverage before proceeding to the next steps.

Seal any penetrations in the slab, such as around sump pumps and plumbing and vent pipes.

Climate

The slab 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. Some states have adopted the 2012 or 2015 IECC. Visit the U.S. DOE Building Energy Codes Program to see what code has been adopted in each state.

Rater Design Review Checklist

3. High-Performance Insulation.
3.1 Specified ceiling, wall, floor, and slab insulation levels comply with one of the following options:
3.1.1 Meets or exceeds 2009 IECC levels4, 5, 6 OR;
3.1.2 Achieves ≤ 133% of the total UA resulting from the U-factors in 2009 IECC Table 402.1.3, per guidance in Footnote 4d, AND specified home infiltration does not exceed the following:5, 6

  • 3 ACH50 in CZs 1, 2
  • 2.5 ACH50 in CZs 3, 4
  • 2 ACH50 in CZs 5, 6, 7
  • 1.5 ACH50 in CZ 8

Rater Field Checklist

Thermal Enclosure System
1. High-Performance Fenestration & Insulation.
1.3 All insulation achieves Grade I install. per ANSI / RESNET / ICC Std. 301. Alternatives in Footnote 4.4, 5

Footnote 4) Two alternatives are provided: a) Grade II cavity insulation is permitted to be used for assemblies that contain a layer of continuous, air impermeable insulation ≥ R-3 in Climate Zones 1 to 4, ≥ R-5 in Climate Zones 5 to 8; b) Grade II batts are permitted to be used in floors if they 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 Grade I is the compression caused by the excess insulation.

Foontote 5) Ensure compliance with this requirement using the version of ANSI / RESNET / ICC Std. 301 utilized by RESNET for HERS ratings. 

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.
1.7 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 (Revision 07)

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.

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 402 Building thermal envelope.

Table 402.1.1 Insulation and fenestration requirements by component.

Section 402.4 Air leakage (Mandatory).

Section 402.4.1 Building thermal envelope.

Section 402.4.2 Air sealing and insulation.

2012 IECC

Section R402 Building thermal envelope.

Table R402.1.1 Insulation and fenestration requirements by component.

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building thermal envelope.

2015 and 2018 IECC

Section R402 Building thermal envelope.

Table R402.1.2 Insulation and fenestration requirements by component.

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building thermal envelope.

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 R317 Protection of wood and wood based products against decay.

Section R317.1 Location required.

Section R318 Protection against subterranean termites.

Section R318.4 Foam plastic protection.

N1102 Building thermal envelope

Table N1102.1 Insulation and fenestration requirements by component.

Section N1102.4 Air leakage.

Section N1102.4.1 Building thermal envelope.

Section N1102.4.2 Air sealing and insulation.

2012 IRC

Section R317 Protection of wood and wood based products against decay.

Section R317.1 Location required.

Section R318 Protection against subterranean termites.

Section R318.4 Foam plastic protection.

Section N1102 Building thermal envelope.

Table N1102.1.1 (R402.1.1) Insulation and fenestration requirements by component.

Section N1102.4 (R402.4) Air leakage (Mandatory).

Section N1102.4.1 (R402.4.1) Building thermal envelope.

2015 and 2018 IRC

Section R317 Protection of wood and wood based products against decay.

Section R317.1 Location required.

Section R318 Protection against subterranean termites.

Section R318.4 Foam plastic protection.

Section N1102 Building thermal envelope.

Table N1102.1.2 (R402.1.2) Insulation and fenestration requirements by component.

Section N1102.4 (R402.4) Air leakage (Mandatory).

Section N1102.4.1 (R402.4.1) Building thermal envelope.

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
Information guide describing issues with concrete floors, in multiple climate zones.
*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

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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