Permeance is a measure of how quickly (or slowly) water vapor moves through a material, such as a bread bag or plastic kitchen wrap or a vapor barrier beneath a concrete slab. Permeance is one of the essential properties of a vapor barrier—that’s why it’s called a vapor barrier. When selecting these products for beneath a concrete slab, the decision is often based on this one essential property.
Upfront, we need to make it clear that the research this article is reporting on was funded by a vapor barrier manufacturer, Stego Industries. We believe the permeance values for the various materials that are reported in the study are unbiased and fair (see Blind Study below). Discussions with representatives from a competing vapor barrier manufacturer, W.R. Meadows, however, point out that there are two test methods that can be used to measure permeance and the two methods can yield different results making direct comparison of the test results problematic.
Meadows has released a white paper partially in response to this study, “Under-Slab Vapor Barriers/Retarders: Perm Ratings and Puncture Resistance—Striking the Right Balance for Optimum Performance” (go.hw.net/SlabVapor), arguing that puncture resistance is at least as important as permeance. Puncture resistance is indeed important, since punctures in the vapor barrier, or poorly sealed seams and edges, will allow a significant amount of moisture to pass through and into the slab. We will discuss puncture resistance and how susceptible vapor barriers are to punctures during construction.
First let’s cover the issue of terminology: Is it a vapor barrier or a vapor retarder? ACI 302.1R-05, “Guide for Concrete Floor and Slab Construction,” repeats what’s in a now outdated version of ASTM E 1745, “Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs,” that a vapor retarder should have a permeance below 0.3 perms as determined by ASTM E 96, “Standard Test Methods for Water Vapor Transmission of Materials.” ACI 302.1 goes on to state that a true vapor barrier should have a permeance of 0.00, which is impossible.
ACI 302.2R-06, “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials,” states that the specifier should decide if a 0.3 perm retarder is sufficient protection and recommends a “vapor barrier with a perm rating of 0.01 or less” beneath slabs covered with low-permeance flooring materials.
ASTM E 1745 currently specifies the maximum permeance rating of a vapor retarder to be 0.1 perms and that it be measured by either E 96 or F 1249, “Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor.”
“ASTM E 1745 considers 0.1 perms to be acceptable as a vapor retarder,” says moisture expert Peter Craig, of Concrete Constructives. “I have investigated several flooring failures where a 0.1 perm vapor retarder did not adequately protect a low-permeance flooring material. I believe that there are many types of flooring installations that need greater protection than a 0.1 perm vapor retarder material can provide and I continue my efforts to help establish a dividing point between materials that can be considered as vapor barriers as opposed to vapor retarders”
The permeance study, “ Report of Water Vapor Permeation Testing of Construction Vapor Barrier Materials,” was conducted by Dr. Kay Cooksey, Department of Packaging Science, Clemson University (see sidebar, Blind Study). Permeance values were measured for 17 different vapor barrier/retarder materials using the ASTM F 1249 test method. The results reported show what might be considered a fairly narrow range in permeance values (see table at right for a selection of results and go online to see the complete results and a comparison to published values). Note that all materials tested comply with E 1745 but only two are below the ACI 302.2 recommendation of 0.01.
Stego has argued that the results should be judged by comparison to the permeance value claimed by the manufacturer in its published literature. By that measure, some materials are close to, or better than, the published values while some measured values are as much as 2344 percent higher than reported in the literature.
Russell Snow, Building Science Specialist, W. R. Meadows of Canada, however, notes that some manufacturers may report permeance values as determined using the ASTM E 96 test method. “These two tests produce different results,” he says, “and there is not a good correlation between the two. It’s like apples and oranges; to compare the results is not fair.”
Which test is one to believe? “I have always considered the E 96 test method to be the “gold standard” for determining the water vapor transmission rate (WVTR) of a vapor retarder material,” says Craig. “However, I have come to appreciate that the F 1249 test method can provide very comparable results to E 96 and is in almost all cases the decided choice for testing very low-permeance materials.”
A quick check of the published product data sheet for the material with the highest variation between results, however, indicates that the permeance test was performed according to F 1249, the same test used in the Cooksey study. To see all of the comparisons, read the complete report at www.vaporbarrierpermeancestudy.com.
The other properties of vapor retarders/barriers specified by ASTM E 1745 are tensile strength and puncture resistance. When a vapor barrier/retarder is placed on a gravel sub-base and driven on by a laser screed, puncture resistance is indeed critical. That’s why ACI 302.1 “strongly recommends” a minimum thickness of 10 mils. That is probably acceptable for residential work, but most experts recommend a 15-mil thickness for commercial construction. ACI 302.1 also recommends that all seams overlap 6 inches and that joints and penetrations be sealed.
ASTM E 1745 specifies the required puncture resistance according to Class A, B, or C materials. Class A materials must have a minimum puncture resistance of 2200 grams. All of the 15-mil vapor barriers tested in the Stego study advertise that they pass this specification.
Meadows notes in their white paper that some materials with slightly higher permeance ratings “tend to have higher tensile strength and greater puncture resistance.” Meadows says the puncture resistance of its Perminator product is 4300 grams; Stego Wrap lists puncture resistance at 2326 grams—still above the ASTM requirement. “We agree that puncture resistance is a critical factor during installation,” says Stego’s engineering director Joe Marks. “That’s why we have engineered our films to stand up to rigorous construction traffic.”
Peter Craig agrees. “I have learned from polymer chemists that currently with plastic films one must choose a main objective. You either design a material to achieve the lowest permeance possible, or you design it for strength. Gain on one and you lose on the other. I personally side with companies that produce vapor retarder materials of proven adequate strength (ASTM E 1745 Class A) but make the protective aspect of their material the primary focus. I do put value on strength and puncture resistance but once the concrete hits the ground these properties have served their purpose. In my opinion and experience, it is long-term resistance to decay and low permeance that are the most important properties of a vapor retarder over time.”
Stego undertook this study when it saw advertised permeance results from other manufacturers that it felt were unreasonable. Stego knew, however, that any permeance values for competing products that it measured in its own lab would not be believable to contractors and specifiers. “We wanted a way to make the results bullet-proof,” says Bret Houck, Stego’s director of business development.
So it developed a plan to fund a blind study and selected a researcher at Clemson University’s Department of Packaging Science, who is an expert in “permeation of materials,” Professor Kay Cooksey. Joe Marks at Stego organized the study and made all contacts under an alias, Mike Joseph Smith. “At the time of writing this report,” writes Cooksey, “I do not know the real identity of Mike Joseph Smith or his affiliation.” All payments were made via a third party and there was no direct communication between Stego and anyone at Clemson.
Cooksey was instructed to test the permeance of 17 different products being marketed as under-slab vapor barriers. She obtained these materials directly from construction supply houses, including the Stego materials. “It was important that materials were procured without indicating that testing would be performed,” Cooksey writes, “therefore I used the name of a family friend’s business when materials were ordered.” Telling them that the materials were for a project at her home, vendors shipped them to the university or Cooksey picked them up herself.
Two duplicate samples of the various materials were then cut from inner, undamaged parts of the rolls. These were then coded (so as not to indicate the manufacturer) and shipped to an independent laboratory (Mocon, Minneapolis, Minn.) that specializes in permeance testing. The testing lab did not even know what industry these materials were from. Permeance was tested in accordance with ASTM F 1249, “Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor,” at 23° C and 50 percent relative humidity, as required by ASTM E 1745.
Mocon’s results were sent to Cooksey who then reported them to Mike Joseph Smith. At no time before reporting the results did she know who had funded this study. She made no attempt to analyze or interpret the results. One can argue with certain aspects of this study, but that these permeance results are unbiased seems pretty “bullet-proof.” You can review the complete report at www.vaporbarrierpermeancestudy.com.