In my travels throughout North America I am constantly running into concrete moisture problems that could have been easily avoided. Almost always I find that for some reason, the folks in charge decided not to use a moisture retarder. When I ask why this vital step was skipped I get a myriad of reasons: We thought the water table was plenty deep…We are in an arid climate…We used a capillary break…It was never specified…and on and on. Then there are those who tried to address the issue by using a vapor retarder that was nothing more than a cheap piece of plastic. And those who were convinced it would be enough to include a layer of granular fill (blotter layer) between the vapor retarder and the bottom of the slab.
The lack of knowledge coupled with the need for speed has been the undoing of too many resilient floors. Often those in charge of specifying floors conclude that the resilient flooring material (or even worse the flooring contractor!) is to blame. So let’s look at some of the common excuses and misconceptions concerning the need to protect a concrete slab with a good sturdy barrier against moisture:
We looked at the water table and decided we don’t need it.People who should know better often think that unless the water table is close to the surface it cannot possibly undermine the flooring. Not true. In some circumstances a concrete slab will draw water (in liquid form) from as deep as twenty feet. In vapor form it can be three to four times that depth. There are those who think a six to eight inch layer of gravel or crushed rock will work as a capillary break. While it may stop the water as a liquid, it will not prevent water vapor from entering (and compromising) the slab. What is needed is a vapor retarder.
We are in the middle of a scorching hot desert; moisture is the least of our problems.Many architects and contractors believe an arid climate means everything is dry as a bone. Quite the contrary. Some of the worst moisture problems I’ve seen have been in desert states. Why? It’s simple physics: Moisture vapor is controlled by temperature and humidity. The higher the temperature and the lower the humidity, the faster the moisture vapor drives. Lack of rainfall does not mean you don’t have moisture in the soil beneath the slab. And don’t forget the artificial sources of moisture found in arid climates: Irrigation and sprinkler systems are common in these areas. Without a vapor retarder there is nothing to stop them from diffusing moisture to the underside of the slab.
Nobody told us we needed to protect the floor against moisture in the slab.Yes, there are people who will actually say that. I am amazed how many times an architect or general contractor is shocked when I point to information that is readily available to them. I refer them to “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring” (ASTM F-71, section X1.6.1). It could not be more clear: “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.” And if that is not clear enough how about this: “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.” If you are professional there is no excuse for this lack of knowledge.
We are on a tight budget.For many in the construction industry there is constant pressure to find the cheapest product available. We hear the term “value engineering” and the need to trim expenses every step of the way. So, understandably, there are many products on the market that are thought to be acceptable for use as a vapor retarder. You may save money upfront but you will be installing material that is not up to the task.
Another consideration is the absence of a blotter layer. While these were once used on a regular basis, in April, 2001 the American Concrete Institute (ACI) changed the specification and 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 change. First, the water in the concrete mix would migrate down into the blotter layer. Getting it out is difficult and can extend the drying time of the slab by months. With fast track construction there is zero tolerance for delays-and we all know who gets blamed for the delays!
The elimination of the blotter layer has affected those working in the concrete trade too. They do not like working directly on a vapor retarder. If the set of the concrete is slower it requires more attention to curing practices, so slab curl becomes a much larger issue. Too often contractors eager to keep the concrete placement people happy will use the blotter layer, oblivious to its effect on the flooring contractor. It is still a learning curve for many general contractors. We in the flooring industry need to do our part in educating them.
Bottomline: 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. Still we find architects and contractors who refuse to use a vapor retarder. This is absurd. An open slab is an invitation for a flooring disaster.
We in the flooring industry must stand firm on this subject. Moisture failures are driving more and more purchase decisions. Instead of a nice resilient floor they simply go with stained or polished concrete. It may not look nearly as nice but to their way of thinking it is a sure way to avoid moisture issues. Our industry should confront this mindset. And just as important we cannot afford to assume the blame for poor decisions made by others.
Picking Your Membrane: Questions to askIt is essential that you protect your floor (and your work) from moisture problems by selecting the right type of vapor retarder. When making the selection ask the following questions:
Is it tough enough? Durability should be common sense. You need vapor retarder that can stand up to the rigorous demands placed on it during construction. Ask yourself: What will happen to this material when wire mesh or re-bar is installed on it or when workers walk on it or the occasional vehicle (a cement truck perhaps) drives over it?
Will it resist degradation? Many inexpensive plastics exposed to ultra-violet light for even a short period of time will immediately start to decay. The process will continue long after the slab is in place. I once witnessed the removal of a section of slab to find that the 6-mil. vapor retarder was now tattered. It looked like an assortment of blackened potato chips.
What about permeance (the number of grains of water vapor that pass through a square foot of material per hour)? The membrane should have a low water vapor permeance (0.3 perms. is desired). I know everyone wants to be “Green” these days, but the fact is when plastic is recycled the permanence increases. That is why the really good plastic membrane manufacturers will use only virgin material.
How will it be installed? Just throwing down a sheet of plastic is not enough if you want a real 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. The extra time this takes is well worth it. – R.T.