Vapor Retarders: Friend or Foe?
The concrete contractor has little love for retarders because he knows that, when one is specified under his concrete slab on grade, that vapor retarder also prevents the slab from curing evenly. To elaborate, the free water is retained by the retarder under the slab, which keeps the bottom of the slab moist while the top dries out faster. This dynamic causes shrinking tensions that produce a curl in the concrete.
It’s been said, although I personally have not seen it done, that a cement contractor often will deliberately puncture the vapor retarder to allow the free water to pass through and thus allow the slab to cure — from top to bottom — at a relatively equal rate. This, according to the contractor’s rationale, will help keep the slab from curling.
Unfortunately, the flooring contractor may encounter moisture transmission problems if the retarder has been punctured, or he may experience difficulties if there is lateral movement of water from outside sources such as rain, irrigation, poor grading, and so forth. This lateral water movement can be accelerated by the presence of a vapor retarder which, in effect, acts like a saucer to hold the water under the slab. The sand above the retarder becomes saturated and the water sits there — dumb, fat and wet — until it begins moving upward in the form of vapor. Lateral movement of water is not contained or blocked by the retarder because, when they are installed, vapor retarders are not wrapped upward at the margins of the foundation.
A study published by Bruce A. Suprenant and Ward R. Malisch determined that 5/8-inch stake holes in a vapor retarder could contribute to a slab vapor emission rate of 3 lbs. of moisture. That’s potentially enough to cause localized flooring failures.
The American Concrete Institute (ACI) provides guidelines for the use of vapor retarders under concrete slabs. ACI 302 recommends that a 3-inch-thick sand layer be placed over the vapor retarder before the concrete is poured. However, some contractors suggest that a 1/2- to 1-inch sand layer be used, because a 3-inch layer is more likely to be displaced during concrete placement and contribute to “pumping.”
Others recommend placing the concrete directly on top of the vapor retarder. But this may lead to slab curling — which is undesirable because curling can lead to cracking.
Placement of vapor retarders can cause floor covering problems. In one development, the specifier opted to place a vapor retarder under the kitchen area, but nowhere else. Obviously, that specification provided no value at all. Also, garage floors typically are left out of the vapor retarder spec, which virtually assures a flooring problem if the garage is converted into a bedroom or other type of living space.
Another question that frequently surfaces is whether or not polyethylene vapor retarders decrease or promote slab cracking. Laboratory tests indicate that a vapor retarder that’s in contact with the underside of the slab does appear to promote cracking. This, then, is the rationale for recommending that 3 inches of sand be placed on top of the retarder. Slab cracking, of course, can lead to water intrusion problems.
In diagnosing flooring problems, core drilling may not yield the results you require, in terms of detecting the presence of a vapor retarder. In fact, the problem may stem from moisture intrusion due to stake holes, intentional ruptures of the retarder and/or lateral intrusion to due to irrigation, landscaping and so on.