Slab on grade vapor barrier performance

Archive of Older Topics Archive - Specifications Discussions #4
1

I have been asked to idenify the appropirate vapor barrier performance to comply with ACI 302.2R-06 that will satisfy flooring manufacturer’s requirements. There would be many flooring manufacturers to contact in this regard - the common element is that almost all of them require the floor to pass testing at moisture content below 3 lb.per 1000 s.f.per 24 hour. The ASTM 302.2R-06 that I was given identifies:
It should be determined whether a vapor retarder with 0.1 perm rating…is sufficient protection for flooring material to be installed. If not a vapor barrier with a perm rating oof 0.01 or less…should be specified.

The above is from a synopsis of “Updated standards and expert recommendations” given to me by our structural engineering staff.

I expect that if I start contacting floor manufacturers, they are going to tell me that they don’t care how we achieve it - they just want the end result to test our per their requirements.

In our office, we typically specify the vapor barrier to be directly below the slab with the sand cushion below. We have encountered projects where we have had difficulty meeting the moisture requirements and have had to go to costly remediation. We believe that a primary part of the problem is insufficient building enclosure and/or failure to wait long enough for the concrete to dry out. Although we have also encountered moisture problems when these two concerns did not appear to be part of the problem.

Are there any floor manufacturer reps out there or underslab vapor barrier reps who may be able to address this. Comments from anyone with input on this concern will be welcome.

2

You also need to look at ACI 302.1R

You might want to contact Stego Industries
They are at the forefront of this

There is also a CSI Green Sheet put out by the Phoenix, AZ CSI Chapter that may offer some insight

3

Might look at A-1030 and B-1030 docs on this site–http://www.conspectusinc.com/publications.htm

And might try Tom Dudick, who is a very vocal manufacturer [his own company] of floor coatings-- www.dudick.com or tdudick@dudick.com

4

Ditto Richard’s suggestion on Stego.

What have you used in the past? If you’re meeting just the code minimum (6-mil poly) you’re just wasting money.

5

Dale, if you happen to be in the LA area, there is a presentation TODAY at lunch on exactly this topic, presented by the ACI Southern California Chapter . Presentation includes:

Moisture in Floors Presentation will include the following:
California construction defect litigation past and present, particularly residential, involving concrete slab cracking and the presence of moisture related to the suitability for flooring installations; Senate Bill 800 and what it means for the stakeholders in the concrete industry, especially suppliers, subcontractors, and design professionals; degree of acceptance for, initial development of, changes in, and current uses for vapor emission and relative humidity tests of concrete before and after installation of flooring; the changing authority and requirements of flooring standards; discussions of concrete provided within resilient flooring guidelines. The changing risks and potential duties for those who design, specify, order, install and supply concrete especially foundations and interior slabs.

Location: Luminarias Restaurant & Banquets 3500 West Ramona Boulevard, Monterey Park, CA 91754

For a good contact at Stego, try Mike McCarthy at (949) 412-3444 or mikemccarthy@stegoindustries.com
(Mike is also a local CSI chapter Board Member)

There are some excellent articles on moisture migration and related topics pertaining to concrete on Ken Bondy’s website: http://kenbondy.com/professional.htm

6

I am always amazed at how quick the responses come. Thank you for the info, I will be following up on the leads provided. Unfortunately I am not in the LA area - a couple thousand of miles away. We also recently had a seminar/meeting on slab moisture and curl problems at our local Building Envelope Council. The question reqarding specific perm ratings for the vapor barrier had not come up and did not get addressed there.

7

I have a chart that I would be willing to share (jpjordan@jordanconsultants.com). Although Stego almost invented this category of product, there are several manufacturers with products that are more durable and more impermeable.

Although a good vapor retarder installed under a concrete slab on grade may prevent ground water in a vapor state from penetrating through the slab, the amount of water in the concrete may be problematic for floor coverings. This can be an issue for concrete floors over steel form deck since the moisture can’t escape through the bottom.

8

Peter is correct. I have seen as many floor finish failures from slab moisture issues in raised slabs as I have seen in slab on grade. You certainly need a good vapor barrier under the slab on grade, but you also need to do moisture testing of the cured slab and be prepared with a remedial surface-applied moisture control product.

I always enjoy relating that my first personal experience with a flooring failure was about 20 years ago when raised dot Pirelli rubber flooring started blistering like crazy on a high-end hotel restoration. The point of bringing this up was that particular concrete floor was several levels above grade and had been poured in 1908, more than 80 years before. Plenty of time to “cure”. But enough moisture got into that concrete during the construction phase to delaminate the rubber flooring.

9

We specify .45 water/cement ratio for all floor slab concrete, whether it’s on grade or elevated deck.

10

The vapor barrier will have little to no impact on the time it takes to pass the moisture content test. Essentially the moisture being measured has to do with the moisture in the upper part of the slab. Transmission of moisture from the subgrade should be essentially nill in this short term.

It has been reported that this 3 pound number may be difficult if not impossible to meet in many cases. In such circumstances the flooring manufacturer can deny all responsibility for the finished flooring.

Recommendation to use concrete with a low water cement ratio and heat the space and provide normal air changes before performing the test. Also use the test devices with the courses sized salt particles acceptable under the standard to give you the lowest values.

11

With respect to Ron’s comment:
Are current spec is for a 15 mil vapor barrier with several manufactures listed including some of those identified here. It is specified to comply with ASTM E 1745-95, Class A.1.
I did also receive the chart from Peter - Thank you.

12

The issue of vapor emission and alkalinity in slabs on grade is like “alchemy” (a medieval chemical science and speculative philosphy aiming to achieve the transmission of base metals into gold). Everyone has a solution but no one has produced the gold.

However, there are several things you can do to minimize the problem:

  1. any 15-mil vapor retarder
  2. no sand layer between the retarder and the concrete!!!
  3. water/cement ratio of 0.45 to 0.48
  4. continuous water cure for 7 to 10 days

In addition, you can ventilate the space above the slab to reduce the relative humidity.

13

Or you could dig up some of the old high-VOC adhesives!

14

Responding to the original question regarding the requirements for the sub slab vapor retarder, I believe ASTM E1745 (Standard Specification for Water Vapor Retarders Used In Contact With Soil or Granular Fill Under Concrete Slabs) calls for a product with a permeance of .1 or less.
This specification is also called out in F710 (Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring). Is that the type of information you were looking for?

15

David said:
However, there are several things you can do to minimize the problem:

  1. any 15-mil vapor retarder
  2. no sand layer between the retarder and the concrete!!!
  3. water/cement ratio of 0.45 to 0.48
  4. continuous water cure for 7 to 10 days

And those are excellent points!
But I advise quite a bit more to increase the chances your slab will be ready for moisture sensitive flooring.
5. Limit the cement in the mix. Less cement means less water. If you can remove 100 pounds of cement by using a well graded aggregate and decreasing the required compressive strength (many slabs are coming in at 6,000psi and above) you can eliminate 50 pounds of water per yard of concrete.
6. Limit wet curing to 3-5 days. This is sufficient to achieve a hard cap on the concrete, yet it will begin the drying process earlier, and retain some of the porosity in the surface which promotes drying.
7. Minimize recurring wettings. Each time the slab is re-wet, the drying process is impacted significantly. Don’t let sprinkler fitters flood the floors, or drywall mudders mix their products on drying slabs.
8. Install RH probes early and keep all trades posted as to how you are coming along toward your goal. I have seen a real team spirit form when trades understand the goal and see the active pursuit of a dry slab.

16

Thank you JD. This is the type of info that I was looking for. I will check out F710.

17

It seems that many flooring manufacturers are also moving away from re-emulsifiable acrylic adhesives in favor of epoxies and urethanes depending on application requirements. Still, as Dave and JD pointed out, if the water isn’t there it isn’t going to cause the problem.
Oh, allow for the thickness of the concrete. An 8 inch thick concrete deck is going to take twice as long to dry as a 4 inch deck.
Do not permit the Installer to heat the space to ‘dry’ it out. The heat will drive the moisture deeper into the concrete until the heat source is removed, one reason why RH testing is preferred to calcium chloride.
Good luck.

18

I’m with David Lorenzini on this subject of moisture mitigation. In fact, we share a client and have been fighting ignorance on this subject for literally years.

The thickness of the vapor barrier under the slab on grade concrete is irrelevent. What is relevent is the performance of the vapor barrier. That is, how many perms (i.e., how much moisture vapor) will pass through the barrier. Roughly speaking, the perm rating should be 0.01. My understanding is that you can get this by using SaranWrap (“or equal”). But it isn’t durable enough for construction use. THEREFORE, a vapor retarder of some thickness (8 mils, 10 mils, 15 mils?) get’s specified. The 10 mil thickness is commonly specified because that’s what’s in the soils report’s recommendations: the infamous 10 mil Visqueen. (Is unreinforced polyethylene Visqueen made and sold in the US anymore?)

So, the vapor barrier stops moisture coming out from the soil under the building. But then there’s the residual water in the concrete. Water in concrete GOOD for hydrating portland cement. Water in concrete BAD after hydrating all done. Gotta get rid of residual water in concrete or stop it from getting to slab surface.

My understanding, after attending the seminars by SINAK Corporation at several CSI “Conventions”, is that it isn’t water that causes flooring adhesive failure but what’s carried in the water: alkali. Alkali attacks the adhesive and causes it to stop bonding. Oh, and water provides a medium for growing mold, which can be a big NO-NO too.

So, concrete gets tested before flooring gets installed. If minimizing moisture coming out of the concrete is the goal, expressed in vapor pressure (pounds per 1000 square feet of area in 24 hr period), then spec low value. 3 pounds is a nice low number but 5 pounds might be more reasonable.

My understanding is that 5 pounds is the goal for adhesively-applied floor coverings. 3 pounds is the goal for coatings and products like epoxy terrazo. Getting 5 pounds is fairly common but takes serious work. Getting 3 pounds is difficult. Independent specifiers in Orange County, CA had a presentation by the president of Koester, a manufacturer of high-end moisture mitigation and other concrete remediation products. He’s a true expert and he stated that 3 pounds is nearly impossible to achieve.

But say the tests get done and the moisture vapor pressure is acceptable … today. In weeks following, moisture comes up from inside the concrete. Remember, only the upper surface was tested. There’s LOTS of water left in the concrete. Also remember that there was 2-inches of sand over top of the vapor barrier and it was soaked with water before the concrete was placed because the structural engineer insisted on it so the slab wouldn’t curl. The structural engineer is responsible for slab curling but not flooring failures. The gobs of water in the concrete works its way to the surface over weeks and weeks or even years and years of time.

Is it a wonder that any adhered flooring or floor coating doesn’t fail?

So … here come the snake oil peddlars. “Our product is guaranteed (sort of) to not allow moisture vapor greater than (3) (5) pounds per 1000 square feet in 24 hours (as long as the concrete doesn’t crack or get gouged during construction).” AND you get to save lots and lots of money compared to the cost of moist curing the concrete for 10 or 14 days. And you (contractor and/or CM) get a defined risk because they now have a set price for “moisture mitigation,” especially compared to the high price of the undefined scope of topical remediation when the moisture vapor pressure gets tested just before the flooring is installed and fails. And if after you have used the topical treatment (snake oil, in my opinion) and there’s a failure, it isn’t the fault of the contractor or CM. They only followed the spec or deductive change order that the Architect-of-Record (LEED AP and all) said was okey-dokey.

One final thought about the old, elevated concrete floor slab failing the moisture vapor test. A conscientious flooring contractor might have cleaned the floor slab before the flooring installation and might have even flooded the floor when washing it. This would send the moisture vapor pressure sky-high temporarily. I’ve heard of contractors who have an allowance in the contract for moisture vapor remediation purposely wetting the concrete surface to make it test high.

After all this, I have to say I don’t know what to do except specify 0.45 water:cement ratio, 14 day moist cure by sheet method and test the concrete floor before installing the flooring. If the test fails, fix it the way the flooring manufacturer says to do it so the flooring adhesive or coating won’t fail.

The Contractor and/or CM have control over this problem. They built the floor slab. Let them use their superior knowledge and experience to figure out how to fix their problem.

19

The thickness of the slab should not have a negative impact on the “drying” time. A couple of realities for dry slabs are the water that impacts the test is found near the surface of the slab and the interior of dry concrete will typically be saturated even many years afrter the slab has dried

20

I tend to agree with Mr. Regener. I would say that when you build on a swamp (like in Houston) you probably will have more of a problem with ongoning vapor transmission; however, I believe that much of the initial problem is with unhydrated water in the concrete.

I would also like emphasize that thickness doesn’t have much to do with perm rating. 10 mil polyethylene only has a perm rating of 0.1400, far above the 0.01 Mr. Regener recommends. Stego’s 15 mil product has a perm rating of 0.0084. There are, however, at least two 10 mil products on the market with a lower perm rating and better puncture resistance.

There are some reps out there who are claiming that a product with a perm rating of 0.01 or less can be called a “vapor barrier”; I have yet to see any standard (ASTM, ANSI, etc.) cited for this statement. It is my suspicion that when the perm ratings get below 0.02, the difference this makes in the field is negligible, but I don’t know that for a fact, and the 0.02 cutoff line is purely arbitrary.