Why is installing a concrete vapor barrier or moisture barrier necessary under concrete? What are the benefits? Installing a vapor barrier under concrete slab is one of the most critical steps in construction used to avoid indoor air quality issues by preventing radon and other volatile organic compounds from entering the home. Concrete is naturally porous which leads to moisture migration, which in turn can lead to poor air quality, mold, mildew and fungi. A concrete floor vapor barrier can prevent these problems. A certain amount of moisture will dissipate from the concrete as it dries, but once it does the moisture movement should cease as long as there is no other source of water.
The soil directly beneath the concrete slab contains a great deal of moisture and can even reach a relative humidity level of 100% regardless of where the water table is. The best way to eliminate this unwanted water source is to install a concrete vapor barrier. Preventing water from entering the concrete from below can also speed up the concrete’s curing time, which is another great reason to utilize a vapor barrier under concrete slab.
It is essential to install a concrete vapor barrier when the space above is going to be finished. Certain adhesives used when installing floor coverings are sensitive to moisture. The vapor barrier under concrete slab prevents moisture from damaging these adhesives. Excess moisture can also delaminate wood flooring and cause distortion and discoloration of other types of floor covering. Omitting a vapor barrier for concrete can even void the warranty of floor coverings that specifically ask for such a barrier.
Another important reason to install a concrete slab vapor barrier is to protect sensitive equipment such as computers, wood furniture, and anything else that comes in contact with the concrete. Even if the space above the slab is not initially intended for such uses, it is an excellent idea to add the vapor barrier for concrete during construction anyway. Learn about how to perform a moisture test for concrete.
According to ASTM E 1745 there is a distinction between a concrete vapor barrier and a vapor retarder. A true concrete slab vapor barrier has 0.0 perms per square foot per hour. Typically 10mil low-density polyethylene with a permeance of less than .3 US perms is used. This technically is referred to as a vapor retarder. EcoFoil’s Under Concrete Insulation is an excellent alternative to 10mil polyethylene because it has a permeability of <0.02 perms (ASTM 96) and is 5/16” thick, which is more than thirty times thicker. Another added bonus of using EcoFoil is that it increases the efficiency of your under slab insulation by almost R-4. The inclusion of a vapor barrier concrete floor can also contribute to several LEED credits when the structure in construction is seeking certification as a U.S. Green Building Council new project.