Moisture-related problems with floor covering and coating installations over concrete floor slabs have been a serious and extremely costly issue for many years. While moisture has always been a concern for flooring installations, such problems became far more common over the past 25 years.

Much has been written about where potentially damaging moisture comes from, how moisture migrates, how long it takes concrete to dry, and how moisture acting either alone, or as an initiator, can lead to serious flooring and coating problems. The focus of this article is not to go back over the same ground but rather to discuss the evolution of strategies used to mitigate unacceptably high slab moisture conditions and how today’s approaches are reliably bringing an end to such problems.

The evolution of moisture mitigation

The need for vapor retarders and their location below concrete floor slabs had for many years been one of great debate and controversy. Those whose primary focus was the behavior of the slab itself cited a number of valid concerns that led to recommendations to sandwich a layer of granular fill between the vapor retarder and the underside of the slab. That approach did help to mitigate many of the concrete-related concerns. However, flooring failures directly related to that detail ultimately led design, construction, and flooring professionals to agree that to provide the highest level of protection to moisture-sensitive flooring materials and products, the vapor retarder/barrier needed to be placed directly in contact with the underside of the slab. In ACI 302.1R-15; “Guide to Concrete Floor and Slab Construction,” the vapor retarder/barrier decision-guiding flow chart supports this position.

Over the years we’ve also come to understand and appreciate that the relative humidity in the ground below a structure will approximate 100% regardless of where in the country a project is being built or how far the water table is below the structure. With the surface of the ground covered by a structure, the water table can be hundreds of feet below the surface of the earth and the relative humidity beneath the building will still approximate 100%.

Too often, those involved with projects designed and constructed in arid regions of the country mistakenly did not include below-slab moisture protection on the belief that moisture from the ground would not be an issue. Many of these projects and the participants, have paid a serious price for this error in judgment. With mandates from organizations within the flooring industry, and from the manufacturers of flooring materials, the installation of an effective, low-permeance vapor retarder placed directly below the slab is not optional when a moisture-sensitive flooring material is being installed over a concrete slab on ground. With what we now know about moisture, moisture migration and its potential affects on flooring installations, and products, in the litigious world in which we live, it would be a serious error for a design team, owner, or contractor to omit installing an ASTM E1745 Class A-compliant vapor retarder directly below the slab.

Standing up to traffic

Over the past 20 years significant improvements to vapor retarder materials have resulted in products with after-conditioning permeance levels of 0.01 perms or lower. The best of these newer generation materials are also capable of standing up to the direct traffic of transport and placement equipment over the material. Using an extremely low-permeance vapor retarder positioned directly below the slab is an important step in making moisture-related flooring and coating problems a thing of the past.

At the time when the frequency of moisture-related flooring problems was beginning to increase it was generally assumed that a concrete floor slab on a new project would dry naturally to an acceptable level. Little attention was given to the slab’s moisture condition until late-stage testing of the slab determined that the moisture levels were unacceptable. By that time the interior walls were up, many trades were at work, and no provision had been made for what would be done to mitigate a high slab-moisture condition or who would pay for it. Disputes over responsibility and cost began to escalate.

As unacceptable slab moisture conditions became more common, the potential need for a moisture mitigation strategy on many projects became part of the thought process during design and a mitigation system was bid as a contingency item. With this approach, if the slabs had not dried to an acceptable level, the system to mitigate the condition had already been selected, bid out, and funds appropriated.

If, however, the slabs had dried to an acceptable level, the mitigation system was not installed and the money was not spent. With the contingency approach the decision whether the mitigation system was needed was in the 23rd hour, such that when mitigation was deemed necessary, implementation was difficult, disruptive, and costly.

While selecting a mitigation system, and bidding it as a contingency item made sense, and is still an approach some follow today, the number of projects where the slabs did not dry naturally to an acceptable level and mitigation was needed ultimately led many design firms, contractors, and owners to view moisture mitigation not as a contingency but as a necessity. The search then became how to implement a moisture mitigation strategy at the most opportune time and at the lowest cost.

Early-stage approaches

Historically, even when moisture mitigation was deemed necessary and included in the project budget from the beginning, it was a process undertaken at the 23rd hour when interior walls were framed and other trades were at work. Installing a mitigation system at that point was not only disruptive to the project schedule and other trades, but surface preparation and application was challenging. It was the most expensive means of mitigating moisture in a concrete floor slab.

The search for easier, more cost-effective solutions to mitigating slab moisture on new construction projects has led to early-stage approaches such as rapid-drying concrete and early application of two-part, no-limit, hybrid epoxy coatings that today are helping to reliably make moisture-related flooring problems a thing of the past.

Early-stage approaches address the issue of slab moisture and floor leveling when the building is wide open and activity from other trades is minimal. Not only does this approach help lower costs, but it also minimizes disruption of the project schedule and the activity of other trades that comes with late-stage implementation. The ability to mitigate moisture and level the slabs shortly after they are placed provides the lowest possible cost and disruption to the project schedule and other activity.

Today, the use of new materials and approaches is helping bring the era of moisture-related flooring failures to an end.

Peter Craig is an independent concrete floor consultant with the firm Concrete Constructives. He is one of the nation’s foremost experts on moisture-related slab issues. Visit for more information.

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