A slab was poured about 8 months ago in the Phoenix area. Soils reports don’t indicate any groundwater. Recent vapor transmission tests have come back at 6 lbs, both before and after the AC was turned on (about 4 weeks ago). A under slab vapor retarder was not used, per the request of the Architect. Any suggestions as to why the vapor transmission could still be so high?
Hi Robin,
I always inform my architects that a vapor retarder needs to be directly under the slab. The blotter layer under the slab acts as a reservoir for moisture. The Subbase or Blotter layer is usually heavily watered and compacted before they pour the wet concrete. All this moisture takes a long time to dry. Even in the hot Southwest As you are aware Moisture can move through concrete slabs as water vapor or liquid water. Especial Concrete that has too high of a water content or to high of a water to cement ratio, this makes it easier to work for the concrete finishers, but leaves a lot of voids for vapor and moisture to travel. According to Howard m. Kanares Book Concrete Floors and Moisture The relative humidity in Subgrade soil or installed subbase will be close to 100%. The difference in relative humidity above and below a slab drives the moisture from the higher RH below the slab toward the lower RH in the Building.
I believe a mistake was made to eliminate the vapor barrier.
Mitch: Well, maybe, but that doesn’t help me!
Rich - I lost your call - fax # is 702-450-9865. Thanks for your help.
how large is the slab? perhaps you could drill vents into the slab and even run pvc pipe with a power fan to allow the escape of moisture. In the northeast, it is a typical way to vent the slab for radon gas.
Here’s one possible reason: The dry climate has nothing to do with it. The vapor drive (from that of higher concentration of moisture to lower) is still present, since the moisture is coming from BELOW the slab (i.e. in the ground). This is a simple and irreversible law of physics.
Some would even say the dry climate makes the vapor drive even stronger. Having the AC on may not help either. One way to check: what is the prevailing RH of the area, versus that of the closed in (and air-conditioned) building? Pretty close, I suspect.
And the absence of groundwater is not necessarily a valid reason to omit the vapor retarder. The RH of the ground is still 100%. So the vapor drive is still present, and that moisture DEFINITELY works its way into the (lower RH) interior space.
Another possibility: Was normal-weight aggregate concrete used, or lightweight aggregate? Lightweight aggregate takes twice as long to dry out (that’s DRY, not cure) as normal-weight.
Another: What was the water-cement ratio of the concrete mix? Probably should not have been higher than 0.48.
Note: For a 4-inch slab, with a W/C ratio of 0.60, the concrete will take about 360 days to reach 3# / 1000 sf. Double that time for LW concrete.
Another: Was the slab exposed to moisture (rain, water leaks, wetting down for cleaning purposes, etc.) prior to building dry-in? Key point to remember - once the slab gets wet, the drying process starts all over.
Do some more checking; I’m sure the other readers would be curious to see how this all works out (and what the architect’s reasoning or justification for leaving out the vapor retarder was).
Good luck!
I could not agree more with the second sentence of David Combs reply, “The dry climate has nothing to do with it” For years and years I have called on the New Orleans area where architects have told me that they do not have a water problem and no need for a vapor retarder or vapor barrier is needed. My first trip back since the hurricane has opened up their eyes to the water problems.
What David said also is correct about vapor drive and the need to calulate it. W. R. Meadows has a piece of literature “Controlling Moisture Movement in Buildings” # MM 03- that can be downloaded by going to www.wrmeadows.com and going to What’s New, click there and move down to New Literature, click on and then scan down to # MM 03 and you can download the Brochure in PDF format, or request your local salesman to bring it by to you. This brochure will have the Tables set up for Vapor Pressure for Various Temperatures and Relative Humidites already calulated for you. It does the simple and irreversible law of physics for you. Just know the inside temperatures and the outside temperature and relative humidities of each and you can see it in pounds per inch. It also shows water vapor permeability of various building materials that will show you why you got the reading you got when your ground cores showed no water present. The WVT rating is still one of the most important things you need to look at in materials. I repete " the WVT rating or perms is still one of the most important things you need to look at in materials"
Meadows has a vapor-retarder, Perminator with a perm rating of 0.018 that meets ASTM E 1745 class A, in 15 mil thick size, put it down in a 20,000sf building and in 30 days you will have around 15 gallons of water moving through that structure, but put down their PMPC Premoulded Membrane with Plastmatic Core which has a perm rating of 0.0011 that meets ASTM E 1993-98, in that same 20,000sf building in a 30 day period you will have about 8 oz. of water or 1/2 Coke can moving through the structure. Thats 3- 5 gal. pails against a half full coke can! That is why many floor manufactures recommend it before issuing a warranty or guarantee.
You might tell the architect who left out even 10 mil poly, Meadows has a nice flip chart presentation to show why there is a need in a dry climate like Phoenix and an area like New Orleans which is actually setting below sea level or a humid area like Houston, there is a need for protection, even in a cold area like New York or Buffalo,all need a vapor retarder or true vapor barrier to be used.
I was told recently that Arizona has more flooring failures than any other state. They said it was because of the vapor drive issue. The highly air conditioned interiors in the hot climate do tend to suck moisture in from the warmer earth below the building. Even a highly air conditioned upper floor room can pull moisture from a warmer room below. The best bets are to make sure that the water/cement ratio is at the level required. A vapor barrier is good below a slab as long as there is no wet (rained on) sand placed above the barrier. You can also add products to the concrete mix that will decrease the ability of vapor to pass through the slab.
I suggest that you view this presentation by Ken Bondy (http://www.kenbondy.com/images/ProfessionalArticles/VETTalkCGEA.pdf)
Mr Bondy suggests that the amount of moisture measured is in excess of the ability of the concrete to transmit and that the source of the moisture is mostly from the concrete slab itself. In addition the test results can vary wildly depending on the finness of the calcium grains used. An interesting read.
The end result is that your problem may have more to do with the criteria and the test procedure than with the application of flooring in most cases.
any architect who leaves a vapor barrier out from below a slab should have their license yanked. all the chemistry quoted above is an issue, and the vapor drive will not stop; a moisture coating will mitigate vapor transmission into the space, but you’re going to end up with a damp slab no matter what.
although we no longer use sand above the vapor barrier, even wet sand will dry out eventually as the vapor drive pulls the moisture out through the slab, so even that isn’t necessarily a problem.
the additives to concrete only work if the slab doesn’t crack – and a differentially heated slab will probably crack. Also, there are the control joints that will transmit moisture through the slab as well.
let us know when the law suits for flooring failures start…
Deep in Armstrongs installation literature is a statement about using a vapor barrier below concrete floors with their vinyl flooring. I found this out investigating a 5 year old elementary school with mold growing on the back of the vinyl and in the carpet. Cost the architect’s insurance company 3/4 mil to install new floor.
Never delete the vapor retarder and in this day and age you had better be specifing something more than just plain 10 mil poly. To quote the attorney’s “Mold is Gold”.
For what it is worth, ACI has published a flow chart for determining where, or if, a vapor retarder is required. [See the document “ADDENDUM GUIDE FOR CONCRETE FLOOR AND SLAB CONSTRUCTION (302.1R-96)Vapor Retarder Location”] This can be downloaded from the ACI website.
But I’m not sure this is really answering Robin’s original problem. There are liquid products that can be applied to slab on grade that are supposed to lower the rate of vapor transmission. I have never specified or used one, but have seen the literature offered by various floor covering manufacturers. That might be the easy solution. Anyone have any experience with those products?
Since 10 mill polyethylene, with laps sealed and taped, and penetrations sealed, meets ACI requirements, that may be adequate for most installations, depending on ground water conditions, slope of site, recent court decisions, and a few other variables.
One might want to be sure to specify all the moisture testing recommended by CRI, regardless of the floor covering material being installed, including measuring moisture of surfaces in accordance with ASTM D1864, performing anhydrous calcium chloride test in accordance with ASTM F1869, and by using an electronic moisture meter in accordance with ASTM D4263, as well as testing for alkalinity condition of concrete.
Refer to ACI documentation (which numbers are not at my fingertips) for their latest recommendations, which include omission of the “blotter layer” (commonly sand), use of a minimum 15 mil thckness vapor retarder, meeting or exceeding ASTM E1745 Class A, with puncture resistance exceeding 2000 g per ASTM D1709 Method A. Install per ASTM E1683 and manufacturer.
There are a number of manufacturers of membranes meeting these standards listed on this website under 03-0850. Stego has an excellent education series addressing these issues in detail.
The prevalence of this problem, even in dry climates, is well-documented. One of the sources of moisture vapor is the so-called “water of convenience”, which is the water contained in the concrete mix in excess of that required for full hydration, which then must dissipate somewhere, usually into the conditioned space. Refer to the ACI documents for full recommendations on the maximum W/C ratios for mix designs, use of plasticizing admixtures to reduce the need for water in the mix and other mix design and concrete placement issues.
Even these precautions may not eliminate the problem in damp climates, and use of one of the topical vapor control coatings found on this website under 09-6105 may be a prudent and inexpensive measure (at least in comparison to replacement of the floor). It may be prudent to consider use of a coating before installing a second floor which may also fail.
While advocacy of the loss of license may be an extreme response to a flooring failure, the economic loss from the experience may amount to the same thing. Sympathy may be more in order than censure, after all. Everyone is ignorant, just on different subjects.
Fortunately there is ample design assistance available to prevent this problem on future work.
Hope this helps.
I would like to hear other’s solution to Robin’s dilema. I dont hear anyone else offering suggestuons on a resolution??
USG’s Levelrock Floor Underlayments MIGHT provide a solution. Find a rep near you; I couldn’t find references to it on USG’s web site, but that doesn’t mean it’s not there.
http://www.gypsumsolutions.com/brand.asp?brand=Levelrock
Here is a link to the information on the USG site for LevelRock brand family.
Here is an interesting article:
Reducing Bond Failures Caused by Moisture Vapor Transmission
In addition to dealing with “how to do the slab right in the first place”, the article also mentions remedial solutions:
"There are several companies that offer warranted treatments to the surface that are aimed at reducing or eliminating the problem. These solutions, however, are quite costly. Floor coating manufacturers are also offering treatments to go under their systems for prevention of bond failure. Some treatments that have shown promise are:
"The sure-fire remedy and repair is to use a breathable system, which allows the passage of moisture vapor without interfacing with the bond. These systems are typically some form of modified cementitious material.
"The use of penetrating primers and hardeners, which reduce the rate of moisture transmission, are effective if the initial transmission rates are not excessively high. In these cases, as in all scenarios, testing along the way is important. The three-pound per one thousand square feet per twenty four-hour figure is the goal.
“Semipermeable membranes are being used to reduce the moisture rate below the three pounds. Again, these are usually modified cementitious materials applied in multiple coats. Once applied to a thickness that produces an acceptable transmission rate, the manufacturers coating/flooring system can be applied.”
As mentioned in my previous post, I have seen literature on some of these systems, but have never used them.
There are several companies, as noted, above, that do remedial work on wet slabs, and will guarantee the results. They are very expensive. I wrote a spec. for a slab that failed badly a few years ago, fortunately didn’t cause me any trouble, except to warn me that this can be an unexpected problem.
Wet slabs can be caused by more than just omitting vapor barriers. Even with one, if the construction period is fast, the weather is bad, the roof is put on before the slab is dry, the slab is in a depression, ie. a depressed altar area in a church, and all of the above together, the slab may never dry out. In this instance, a large thinset tile was the selected finish floor. The contractor paid $$$ to use one of the remedial products, after much finger pointing, and eventually the floor was installed, and I think, OK. At least I haven’t heard more about it.
It caused me to include a section on water vapor transmission and remediation in every spec. I do. Most architects are delighted with it, and only on one developer-driven, i.e. lowest cost kind, project have I ever been asked to remove it.
It includes in one section: moisture testing, remedial work if needed, and releveling with Ardex or something like it.
Jo, et al. Please don’t rely on moisture testing using the conventional dome test. In recent years, the dome tests have been disregarded as inappropriate in common applications. Please refer to the site listed earlier in this thread for the following articles:
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http://www.kenbondy.com/images/ProfessionalArticles/MoistureDomeTests1.pdf
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http://www.kenbondy.com/images/ProfessionalArticles/VETTalkCGEA.pdf
The authors of this report are widely considered to be among the most sought after experts in the field of concrete and slab design.
Disclosure: I am acquaintances with both, and related to one of them.