In my travels, I am constantly seeing concrete moisture problems due to the lack of a moisture retarder. I’ve been given a lot of reasons (excuses?) for this: “The water table was thought to be plenty deep.”  “We are in an arid climate and we don’t need one.” “We use a capillary break and don’t need one.” “We never have specified one.”  “We don’t know about ASTM F-710 requirements” and so on.

Let’s look at a few of these “reasons” in more depth.


Water Tables

Many people think that unless the water table is close to the surface it is not a concern. Not true. A concrete slab can, if the fines content of the soil is high enough, draw water as a liquid as deep as 20 feet, and at three to four times that depth as a vapor.

There are those who think a 6-to-8-inch layer of gravel or crushed rock will work as a capillary break. True, it will stop the capillary rise of water as a liquid, but the capillary break also allows for the conversion of water to vapor, which will enter the slab. A vapor retarder will control this.

Arid Climates

Many architects and contractors believe that if they are in an arid climate they will not have moisture problems. To the contrary: some of the worst moisture problems I have encountered have been in the desert states.

Moisture vapor drive is controlled by temperature and humidity. The higher the temperature and the lower the humidity, the faster the moisture vapor drives, and just because there is a limited amount of annual rainfall doesn’t mean the moisture isn’t still there beneath the slab in the soil. 

You must also take into account artificial sources of moisture like irrigation and sprinkler systems; without a vapor retarder there is no stopping the diffusion of moisture to the underside of the slab.

Lack of Knowledge

It is amazing how many times I reference the ASTM F-710 “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring” only to see a shocked look in the eyes of the architect, the general contractor or both.

Whether they don’t know – or don’t wantto know – remains to be seen, but section X1.6.1 states that “The installation of a permanent, effective moisture vapor retarder, as described in Specification E 1745, is recommended under all on- or below-grade concrete floors. The use of such a moisture vapor retarder, provided its integrity has not been compromised, reduces potential severity of moisture vapor penetration. Every concrete floor slab on- or below-grade to receive resilient flooring should have a moisture retarder (often improperly called a vapor barrier) installed below the slab.”

Value Engineering

There are a lot of products on the market marketed as vapor retarders, and as too often happens, the majority head for the cheapest product they can find. And a lot of the time, the membrane selected just does not suffice as a vapor-retarder membrane.

So what does it take to be an effective vapor retarder?

Durability– a vapor retarder must be able to withstand the rigorous demands placed on it throughout the construction process. Things like the installation of wire mesh or re-bar, workers walking over the membrane and the occasional vehicle, like a cement truck, driving over it.

Resistanceto degradation– many inexpensive plastics exposed to ultraviolet light for a short period of time will start to degrade, and that degradation will continue long after the slab is placed. I once witnessed a section of slab being removed, and the 6-mil vapor retarder looked like an assortment of black potato chips.

Low permeability– the membrane should have a low water-vapor permeability of 0.3 perms (1 perm = 1 grain/h · ft2 in. · Hg (inch·pound)). Many of the plastic membranes used for vapor retarders are recycled plastics, and when a plastic is recycled, the permeability increases. That is why the really good plastic membrane manufacturer will use only virgin plastics.

Installation– just throwing down a sheet of plastic does not suffice as an installation of the vapor retarder. The foundation wall should be flashed and all protrusions should be flashed and taped. All seams should be overlapped and taped to prevent moisture diffusion to the slab.

Blotter Layer

The blotter layer used to be deployed on a regular basis; however, in April 2001 the American Concrete Institute (ACI) changed the specification which eliminated the layer of granular fill between the slab and the vapor retarder on slabs to receive moisture-sensitive flooring materials.

There were several reasons for this action, the main one being that the water in the concrete mix would migrate down into the blotter layer, and getting the water out is difficult and extremely slow. It will extend the drying time of the slab by months and, with fast-track construction, delays are not acceptable. And you know who gets blamed for the delays! 

The elimination of the blotter layer has affected the concrete industry as well. They do not like working directly on a vapor retarder; the set of the concrete is slower, the concrete requires more attention to curing practices and slab curl is a much larger issue. Many contractors, in an attempt to keep the concrete placement people happy, will use the blotter layer, giving no concern to what the effect it has on the flooring industry.

There is still a learning curve for general contractors. We need to do our part in educating them.

Most resilient flooring manufacturers will not recommend their products over a slab that does not have a vapor retarder. Even some concrete moisture remediation companies will not recommend their products over a slab without a vapor retarder.

Whatever reason an architect or contractor gives for not using a vapor retarder, it is absurd. An open slab is nothing more than an invitation for a flooring failure. As an industry, we have to stand firm on this subject.

Moisture failures are driving more and more end users away from flooring toward stained concrete and polished concrete, products that see very limited effects from moisture issues. Our industry cannot afford to assume this blame.