Basement slabs are solid and strong, but they aren’t armor for a home. Concrete has the ability to wick up water through capillary action. Capillary action is the tendency of water, because of its molecular structure, to defy gravity and move upward through small spaces (capillaries). Water is able to rise through the soil, moving upward between soil particles and into the slab.
Water intrusion through the slab can do some serious damage, especially if it’s a finished basement. The floor finish can act as a barrier, trapping water in the slab. Under carpet, the trapped water raises the humidity and increases the potential for mold and allergens. Under hard surfaces, water is less likely to evaporate; instead, it condenses on the underlying surface, degrading latex adhesives, discoloring grouts, and warping wood floors.
The best way to prevent water vapor from entering the slab is to place a 10 mil polyethylene vapor barrier on the gravel layer. The vapor barrier also protects the foundation from water intrusion if the water table rises through the gravel layer during times of heavy rainfall or snowmelt.
Follow these strategies to ensure an effective vapor barrier installation.
Install the vapor barrier correctly
Make sure the vapor barrier fully covers the foundation footprint and doesn't have any tears or holes. Overlap all seams in the vapor barrier at least 12", and seal the seams with 3" construction tape. Extend the vapor barrier 3" to 6" up the foundation wall, and then fix and seal it to the wall with construction tape or adhesive.
Seal the vapor barrier around penetrations
Before placing the foundation concrete, properly seal the vapor barrier around penetrations, including plumbing and electrical stubs. When applying the vapor barrier to the foundation footprint, cut it to fit around the penetration. Cut out a 4'x4' piece of vapor barrier material. Make an X-shaped incision in the 4'x4' square that's just large enough to fit tightly around the plumbing or electrical stub. Place the square over the penetration; the square should lie flat on the vapor barrier. Then, tape the incision around the penetration.
Create a capillary break at the footings
If the footings are in contact with groundwater, they can wick up water from the soil. The water can then move into the foundation walls, usually showing up as dampness around the base of the foundation wall, especially at the corners. Keep the footings from transferring water to the foundation walls by installing a capillary break. One method is to leave enough extra vapor barrier material to extend the sheet across the top of the footings, as well as between the wall and slab. The preferred alternative is to spray an elastomeric asphalt coating on top of the footings.
Want to learn more?
Water intrusion through the slab can be a serious problem. Check out our companion article on preventing moisture damage in basements. You can also find more Best Practices® for water management in BuildIQ University's online training courses, Water Management: Basements & Crawlspaces, and Foundations: Basements & Crawlspaces.
