ASTM PERM Testing

So I’ve been talking standards these last few weeks—ASTM and ANSI, etc. I noted in the ASTM post that these standards are often old and outdated. It is very important we regularly check our choice of and reference to standards. This is an issue of liability and marketing both. As an example, I want to look at PERM testing.

I was researching the standard the NWFA uses to evaluate moisture barriers, which is specified as “ASTM E96 Method A.” You know how I always say that the devil is in the details? Well, when I pulled up the actual standard, I found that there is no Method A. The language in some later revision had changed it to “Procedure A.” All the references the industry has been using are technically incorrect. (And you just KNOW some lawyer is going to make a fuss someday over the difference between “Method” & “Procedure.”)

The standard was created over 50 years ago, in 1953, and has been modified several times since then—the first modification coming just a few years after it was developed when the first testing method was found to be flawed. Alternative testing standards have also been created since then and apparently, there’s quite a bit of variety possible in how you rate a moisture barrier.

Here’s what the NWFA wants in a moisture barrier:

When tested in accordance with ASTM E96 Method A.
1. Class I 0.1 perm or less.
2. Class II 0.1 perm less than or equal to 1.0 perm.
3. Class III 1.0 perm less than or equal to 10 perm.

Ok, that’s not particularly helpful. What’s a Perm? Apparently that depends on where you live. As it happens, I have a colleague looking into the perm rating of a vapor barrier we were handling. He was told by the supplier that the rating was this:

Water vapor permeability approx. 14 g/m²/d
Water impermeability approx. 7 m / water column

He went hunting to try to figure out what that meant and found these definitions:

US perm
The US perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.

1 US perm = 0.659045 metric perms
≈ 57.2135 ng·s−1·m−2·Pa−1

Metric perm
The metric perm (not an SI unit) is defined as 1 gram of water vapor per day, per square meter, per millimeter of mercury.

1 metric perm = 1.51735 US perms
≈ 86.8127 ng·s−1·m−2·Pa−1

He wanted to convert the numbers given, but since the supplier of the barrier didn’t specify an ASTM, we don’t know if that US perm was developed through the same procedure as the metric perm and therefore if (converted) it meets the NWFA standard. Because remember, the NWFA specified an ASTM which means following a very specific testing protocol.

So, what’s the take away, beyond math and standards are confusing and give me a headache? First, always specify your standard when giving data/preparing marketing material. Numbers without a reference don’t help. Second, check to see what standard is needed by the customer—the US flooring customer needs something that matches the NWFA specified standard.

(By the way, if anyone can explain all this math, I’ve got a salesman who is very confused.)

See, for most of us, that’s really the bottom line with standards: we want a yes/no answer. We’re told we need to “meet X” and if a product does meet X, that’s all we feel we need to understand. We don’t want to understand the math or the testing—we have confidence that professionals who understand this stuff worked it all out assume that X is actually a meaningful number as well.

Numbers can be a form of green washing—throw enough statistics and figures and formulas at us, and our eyes glaze over and we’ll just nod and accept it all. But we can’t always do that—we need to dig into the details sometimes because things do change and if we don’t check, we might find out that it was the wrong method, um, make that procedure, all along.

(Btw, the NWFA is looking into updating their verbiage in the next release of their specs… I think they might also want to look at the other standards available and see if a more modern one is appropriate. Just because it’s always been done one way doesn’t mean there isn’t a better one now.)

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