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

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, it could be installed before urethane sealant and epoxy paint are applied to avoid additional steps in the project.
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 and epoxy paint to ensure proper adherence of the products.
5. 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.
6. Apply two coats of epoxy paint over the existing slab per manufacturer’s recommendations (see Figure 1).
7. Recommend that the homeowner consider the use of a dehumidifier in the basement during humid months.