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Is the permeance of Foamular 1/4" fanfold insulation really only .75 perms?

As found in the product properties provided by Owens Corning (See chart in link below)


They give a perm rating of .75. Looking at the product, it appears to be perforated with a number of small holes. I'm considering using this as an air barrier material for a knee wall, but concerned with the low permeance. Is the permeance rated on a per inch basis?

Owens corning provides data showing permeance of .75 for 1/4" and 3/8" fanfold, and 1.1 for half inch, 1 inch, and 2" foamular. I thought it varied based upon the thickness of the product?

Asked by Rick Van Handel
Posted Jun 26, 2014 3:57 PM ET


6 Answers

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The document you linked to lists counterintuitive vapor permeance numbers. In general, as a material gets thinner, it's vapor permeance should rise; thicker samples of the same material should have a lower vapor permeance. So the table that shows a vapor permeance of 0.75 perm for 1/4 inch XPS and a vapor permeance of 1.1 perm for 1 inch XPS is counterintuitive.

Elsewhere, an Owens Corning document states, "Actual water vapor permeance data decreases as thickness increases."

A perm rating of 1.1 perm for a 1-inch-thick sample of XPS is the rating that shows up in many tables.

Of course, it's possible that the 1/4-inch fan-fold material includes a plastic coating that lowers the product's permeance.

Answered by Martin Holladay, GBA Advisor
Posted Jun 26, 2014 4:10 PM ET
Edited Jun 26, 2014 4:12 PM ET.


Most of fan-fold XPS products have thin polyolefin facers for vapor control. Pactiv even has a low-E aluminized plastic facer option that runs about 1.3 perm. These facers typically have vapor permeance in the high end of Class-II or the low end of Class-III level. Since the facer is the limiting factor for the vapor diffusion, it's insensitive to thickness until the foam is thick enough. On it's own sans-facer 1/4" XPS runs about 5 perms, 3/8" runs about 3 perms, but with 0.75 perm facers either will run about 0.75 perms, or very slightly tighter..

Perforated versions usually are (intentionally) MUCH higher permeance and the spec sheets usually don't have ASTM E96 test ratings, since they're designed to be high-perm. Whether that's 50 perms or 150 perms, it doesn't really matter much.

Answered by Dana Dorsett
Posted Jun 26, 2014 4:32 PM ET


Thanks Martin and Dan for the help.

The material I saw at the big box does appear to have a plastic like facing as one side is more shiny than the other, but it also has the small holes regularly spaced.

It sure would be nice if manufacturers would print the product specs on it.

Is there a way that a average Joe can test the permeance of a product?

Do you think this is an acceptable air barrier product if it's within the .75 perm-5 perm range?

Answered by Rick Van Handel
Posted Jun 26, 2014 5:35 PM ET


Q. "Do you think this is an acceptable air barrier product if it's within the .75 perm-5 perm range?"

A. You are confusing air leakage with vapor permeance. A product can be a good air barrier but still be vapor-permeable (e.g. gypsum drywall). A product can be a lousy air barrier but still have a low vapor permeance (e.g. vapor retarder paint).

Answered by Martin Holladay, GBA Advisor
Posted Jun 26, 2014 5:44 PM ET


It's nearly impossible to get fanfold to work properly in that application. Use polyiso with a minimum thickness of 3/4".

Answered by Jesse Smith
Posted Jun 26, 2014 7:01 PM ET


I agree that sealing fan-fold is a bit awkward & unreliable.

But if using exterior polyiso sheathing the thickness required for dew-point control inside the wall cavities varies a lot by climate. As a true vapor barrier there is zero drying toward the exterior, and if the average winter temp at the structural sheathing is below 40F there is potential for some serious moisture loading of the sheathing, or even wetting of fiber insulation in the wall cavities. And if there is a pre-existing interior side polyethylene vapor barrier you absolutely SHOULD NOT use foil faced polyiso on the exterior.

Answered by Dana Dorsett
Posted Jun 27, 2014 3:27 PM ET
Edited Jun 27, 2014 3:27 PM ET.

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