Vapor barriers: nuisance or necessity? Where a moisture-sensitive floor covering or coating is planned, plan to take the ground out of play.

The subject of vapor barriers or retarders beneath concrete slabs on grade has long been controversial. Some justifiably argue that slabs placed in direct contact with a vapor barrier or retarder are more susceptible to curling and other slab problems than those cast on a granular base. They

Others have experienced or witnessed the devastating effect of moisture on modern floor coverings, adhesives, coatings, and a building's environment. They will justifiably argue that a vapor barrier or retarder beneath the slab can be an absolute necessity.

Not surprisingly, reaching consensus on this subject has been difficult. Both sides raise genuine arguments that simply cannot be dismissed. There is, however, a single answer to the title question and it is yes, a vapor barrier can be both a nuisance and a necessity.

To better understand why moisture in concrete slabs has become such a problem, we must examine the sources of slab moisture, how moisture moves, and how it can adversely affect flooring materials, adhesives, and coatings.

Where does the moisture come from?

Free water within the concrete itself is the first source of moisture that challenges a floor covering or coating.

To produce concrete of a workable consistency, more water is added to the mixture than that which merely satisfies chemical hydration of the cement. After the slab is placed, finished, and cured, some of this additional water-of-convenience must leave the slab in order for the concrete to reach the moisture requirements of the floor covering, adhesive, or coating. Most manufacturers of flooring materials currently require, before the product is installed, that the moisture emission rate from the concrete not exceed 3.0 pounds, or in some cases 5.0 pounds, of water per 1000 square feet in 24 hours.

To understand how challenging it is to comply with these requirements, consider that a 4-inch-thick, 0.50 w/c concrete slab placed at a 4-inch slump can contain between 1600 and 1700 pounds of non-chemically bound water in a 1000-square-foot area.

How quickly moisture is lost from a slab depends on the water-to-cement ratio, density of the concrete finish, ambient conditions above the slab, and moisture below the slab. The following table summarizes the time necessary for laboratory cast, 4-inch-thick concrete samples to reach the commonly required moisture emission limit of 3.0 lbs/1000 sq ft/24 hrs when exposed to 73[degrees] F and 50% relative humidity.