The Problem:  Water Vapor Emission is an often overlooked, misdiagnosed, and misunderstood problem, and it can lead to damaged flooring on new and existing floors.

The Cause:  Water can migrate upward and through concrete substrates in the form of water vapor, and then condense and accumulate in the space between a concrete slab and the flooring.  Concrete slabs-on-grade are especially vulnerable to this phenomenon, but elevated slabs can experience this problem too.  Most flooring is adhered to the concrete substrate with a glue or adhesive, and all adhesives have a limit as to how well they can "stick" the flooring to the concrete under these conditions.  The accumulated water interferes with most adhesives and soon the flooring is loose and can shift, float, bunch up or cause gaps in flooring.  Additionally, the water can damage the flooring material itself, especially if it is made from natural or water-absorbent materials.  The end result is a damaged floor and a significant risk of tripping or slip hazards for those who use the building.

The Solution(s):  Most of the solutions to this Water Vapor Emission problem focus on barriers to prevent or limit the amount of water vapor that can migrate to the space between the flooring and the concrete where it interferes with the adhesive.  Many projects use a "belt and suspenders" approach by using more than one of these solutions at the same time to tackle this problem.  Some of the solutions include:

1. Improved under-slab vapor barrier - This is a vapor barrier that goes above and beyond the typical 6 mil visqueen used for years for slab-on-grade construction.  New products are 10 mil or greater in thickness, with heat-welded seams and heat-welded boots clamped at all floor penetrations resulting in an water-tight seal.  Many product specifications of this type require smoke testing and/or certified installers to ensure quality installation.  The end product is often as water-tight as any single-ply roofing system.
2. Improved concrete add-mixtures - There are a number of specially formulated concrete add-mixtures that can be added at the concrete batch plant that limit water vapor migration through a concrete slab.  
3. Water Vapor Emission Control Sealers (new concrete) - These products are normally applied to green (new) concrete 1 to 2 days after a concrete pour by certified installers.  These product are typically applied by spray or by roll and squeegee, requiring 1 to 2 days to cure.  These products also often come with multi-year warranties that include the cost of flooring replacement, and are some of the most effective solutions to the Vapor Emission Control problem.
4. Water Vapor Emission Control Sealers (existing concrete slabs) - These products are very similar (or often identical) to the Vapor Emission Control Sealers for new slabs described above, but there is an important (and expensive) difference.  Most product manufacturers require that any existing concrete slab be bead- or shot-blasted prior to applying the Sealer product.  This extra step ensures that any existing flooring adhesives or other deleterious material is removed and does not hinder the performance of the Vapor Emission Control product, and that the sealer can properly adhere to the concrete surface.  Unfortunately, this extra bead-blasting step requires additional equipment (e.g.: a bead blaster) and labor, which are often equal to the labor and material cost to install the new flooring itself.  Fortunately, the same multi-year warranties normally apply to the sealer for existing concrete slabs if these steps are followed.

The Lesson:  Make sure to evaluate your project to determine if Water Vapor Emission is a potential risk for any flooring work, explore your options and make sure the project documents outline and reflect your strategy to mitigate this risk.