Retrofit a basement floor or slab to reduce moisture issues, 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 and apply epoxy paint.
This option is intended as a low-cost/low-disruption method to address moisture transmission from basement slab floors. There are alternate measures to perform this retrofit (e.g., adding topside rigid insulation), but they are all more extensive and will reduce available height in the basement (see Rigid Foam Insulation Installed over Existing Foundation Slabs).
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 Single-Family New Homes, and Indoor airPLUS.
When retrofitting a basement, one simple and low-cost technique for reducing dampness in the basement is to seal and damp proof the basement floor with an epoxy paint. Soil gas entry and moisture transfer from the ground into the basement can be reduced by patching any cracks in the slab, sealing the seam between the slab and the foundation wall, and sealing the slab surface with epoxy paint.
Note, sealing penetrations and epoxy paint alone will not prevent dampness if poor site grading, lack of gutters, high water table, or other factors are causing serious water intrusion issues at the site. For more on water management of the site, see BA-1015: Bulk Water Control Methods for Foundations.
Also see the following guides:
- Final Grade Slopes Away from Foundation
- Damp Proof Exterior Surface Below Grade Walls
- Drain or Sump Pump Installed in Basements and Crawlspaces
- Gutters and Downspouts
When retrofitting a basement that will be part of the conditioned space of the envelope, it may be possible to insulate the basement floor either by (a) removing the existing slab, installing insulation, and casting a new slab, or (b) installing insulation over an existing slab. See the guide Rigid Foam Insulation Installed over Existing Foundation Slabs for more about these options.
If space or budget constraints make insulating the slab unfeasible, the slab can still be sealed, as described in this guide, to improve moisture control in the basement. If the existing slab has continuous insulation and/or has a polyethylene vapor barrier below the slab, then only sealing of penetrations is required. In these cases, retrofit of a coating on the basement floor slab surface is not required.
If sealing the basement floor is part of an overall basement retrofit that includes insulating the walls, see the guide Rigid Foam Board Interior Insulation for Existing Foundation Walls for air sealing and insulating recommendations.
When performing basement retrofits, consider testing for radon. If radon levels warrant installation of a passive or active radon mitigation system, it should 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.
Uninsulated (or Existing Insulated) Slab Assembly
The epoxy paint is applied directly to the cleaned, patched concrete slab surface (see Figure 1). Water-sensitive materials should not be placed in direct contact with the slab. For instance, all wood wall sill plates should be pressure treated and/or separated from the slab with a capillary break. All interior gypsum board should be held off the slab surface.
How to Water Manage an Uninsulated Slab
- If significant or persistent water issues exist in the basement, treat those issues prior to sealing the slab.
- Consider testing radon levels in the basement and home prior to the sealing project. If a radon stack is needed, it could be installed before urethane sealant and epoxy paint are applied to avoid additional steps in the project.
- Inspect the existing slab for cracks, holes, and penetrations. Locate the areas that need to be addressed.
- Remove any debris or dust particles from the slab prior to applying urethane sealant and epoxy paint to ensure proper adherence of the products.
- Apply a generous and continuous bead of urethane sealant to any cracks, holes, around any penetrations, and at the wall-to-slab joints to prevent pest and soil gas entry.
- Apply two coats of epoxy paint over the existing slab per manufacturer’s recommendations (see Figure 1).
- Recommend that the homeowner consider the use of a dehumidifier in the basement during humid months.
Consider performing a short-term or a long-term radon test prior to commencing any retrofit work and after the project has been completed. For information on radon testing visit the Environmental Protection Agency (EPA)'s Radon website. Also see the guide Vertical Radon Ventilation Pipe for more information on radon testing and mitigation.
Apply urethane sealant and epoxy paint to a clean, dust-free surface. The urethane sealant must be compatible with a concrete substrate. The epoxy paint must be compatible with a high pH environment (see BSI-003: Concrete Floor Problems). The epoxy paint must be applied per the manufacturer’s directions, including surface preparation (removal of oil, grease, laitance, and curing compounds) and temperature/relative humidity requirements.
Upon completion of the work, visually verify the quality of the seal and paint for continuous adhesion and coverage.
Seal any penetrations in the slab, such as sump pumps and plumbing and vent pipes.
No climate specific information applies.
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 Single-Family New Homes, Version 3/3.1 (Rev. 11)
National Water Management System Builder Requirements
1. Water-Managed Site and Foundation.
1.7 Sump pit cover mechanically attached with full gasket seal or equivalent.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable 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 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, 2012, 2015, 2018, and 2021 International Energy Conservation Code (IECC)
Section R402.4 Air leakage (Mandatory).
Retrofit: 2009, 2012, 2015, 2018, and 2021 IECC
Section R101.4.3 (in 2009 and 2012). 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.)
Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.
2009, 2012, 2015, 2018, and 2021 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 N1102.4 Air leakage (Mandatory).
Retrofit: 2009, 2012, 2015, 2018, and 2021 IRC
Section R102.7.1 Additions, alterations, or repairs. 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 the requirements of this code, unless otherwise stated. (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.
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.
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
Foundation Capillary Break over Aggregate = Foundation Floor Water Barrier
Without an effective moisture barrier, water can penetrate the pores of a concrete foundation slab, which can potentially lead to moisture-related issues. To prevent moisture from wicking up into the concrete slab and foundation walls, a water-proof layer is installed under the concrete consisting of 6-mil plastic sheeting overlapped and taped at the seams or soil-contact-rated rigid foam taped at all seams. Under this barrier is a 4-inch layer of gravel to help moisture drain away. In crawlspaces, plastic sheeting should installed over the dirt floor.