If I Seal My Concrete Floor, Do I Need a Vapor Barrier?

Concrete and Moisture

• Whether it's necessary to use a vapor barrier if you've already sealed your concrete floor to control moisture depends on several factors. Concrete floors that sit directly on soil will be much more prone to moisture problems than those with a gravel or rock layer between the concrete and soil. Similarly, if the soil around your concrete floor is sloped away from the home and if your home uses rain gutters that slope away, moisture problems are less likely. In the absence of these and other moisture control measures around your concrete floor, though, you likely need a vapor barrier in addition to a sealant for complete moisture control.

Vapor Barriers

• Vapor barriers for concrete floors typically are inexpensive and easy to install. A layer of polyethylene film, polyethylene-coated kraft paper or glass-reinforced waterproof paper installed on the underside of the concrete floor between the concrete and the rock and gravel layer provides substantial moisture control. When combined with a commercially sold concrete sealant applied to the concrete surface and allowed to dry per the manufacturer's directions, it can go a long way in preventing capillary moisture migrating up through the slab.

Installation Tips

• As noted, vapor barriers are typically installed on the underside of the concrete slab, between the concrete and the rock and gravel layer above the soil. While inexpensive, polyethylene film is punctured more easily than other, more durable materials. Whichever material you choose for your vapor barrier, use an 8-mm or thicker layer and don't puncture the material during installation. Any punctures or rips in the vapor barrier can compromise its effectiveness in controlling moisture.

Other Concrete Moisture Control Tips

• If moisture is a particularly persistent problem in your area or using a concrete sealant and vapor barrier together doesn't provide sufficient moisture control, opt for insulating concrete--a special type of concrete mixture designed for moisture control. Insulating concrete prevents moisture buildup on the slab. Insulating the floor above the slab and adding indoor moisture control strategies such as ventilation, repairing water leaks and reducing indoor relative humidity can also help prevent moisture problems indoors.