My Basement Slab Is Sweating

What Causes Slab Sweating?

• Sweating slab syndrome, as it is often referred to, is caused by either one condition or a combination of conditions. The main culprit is the dew point. A basement slab is much cooler than the ambient air temperature in the basement, especially when the interior is heated. This causes whatever humidity is present to become condensed and form on the coolest, lowest spot -- like a basement floor. Outside air can also enter the basement through windows and doors, which also affects the interior air temperature.

Water Vapor

• Another cause of sweating slab syndrome is water vapor rising through the basement slab. Concrete appears to be solid, but is in fact a porous material. It will always allow some water vapor to escape through the surface, which is measured as the water vapor emission rate (WVER). The accepted WVER for a basement slab is between 3 to 5 pounds per 1,000 square feet, which amounts to 3 pints per day for a basement slab that is 31 feet by 31 feet in size. Basements will generally provide enough evaporation at this rate to prevent sweating slab syndrome.

Vapor Barriers

• A reputable concrete contractor will install a vapor barrier or retarder beneath the basement slab before pouring the concrete. This is generally a polyethylene sheet of at least 10 mil in thickness that covers the entire footprint of the basement slab. This vapor barrier will keep groundwater from seeping or “wicking” through the concrete, thus preventing the basement slab from sweating and also protecting any floor coverings you install on the slab.

Sweating Slab Prevention Tips

• It is a good idea to keep your basement ventilated by occasionally opening basement doors and windows, if you have them, or using fans and de-humidifiers to remove moisture from the air. You should also use a vapor barrier or retarder before installing floor coverings such as carpeting or laminate wood flooring. Sealants are also available to apply to the basement slab to help control wicking of substrate moisture.