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Polyethylene as an air barrier in very cold wall assembly

Myrtleboone | Posted in GBA Pro Help on

My question pertains to the use of a polyethylene air barrier (6 mil) within a double wall assembly (and ceiling) in a very cold climate. My concerns when we came up with this wall were obviously wall performance, related to moisture buildup and drying, as well as keeping the build relatively inexpensive both in materials/labor while at the same time providing me with a system provides high R-value while decreasing air leakage.

My designer who I trust and who has many years of success with double wall assemblies in cold climates, has provided me with the following assembly from out to in: siding, vapor permeable housewrap with drainage plane behind siding, wall sheathing, load-bearing 2×6 outer wall with Roxul insulation, 3″ XPS foam board between walls, poly air barrier to be fastened to the inner side of the XPS, then a non load-bearing 2×4 inner wall with Roxul, and finished with painted drywall.

Based on my knowledge, the air barrier is placed deep enough into the wall to not be affected by dewpoint, thus not having a condensation issue. If rain would get into my assembly from the exterior, my understanding is that it can dry to the outside, and any moisture on the inside of the poly could dry to the interior (having to move through 3.5″ of Roxul and painted drywall). My question is with XPS, which is quick to erect, provides nice R-value, and allows for a firm surface to attach the poly air barrier layer. Could moisture easily form between the poly and foam (there is ~R-40 separating the exterior to this layer). If the poly could be easily attached to another surface such as Roxul to fill the void between walls, I would do it. Just not sure how.

Any thoughts? Thanks in advance.

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  1. user-1140531 | | #1


    How would you attach the poly to the XPS?

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Could moisture easily form between the poly and foam?"

    A. No.

    This type of double-stud wall -- with rigid foam in the middle of the wall -- is a little unusual, but there is no reason the assembly won't work.

    You could simplify the wall by using foil-faced polyiso instead of XPS. Polyiso is more environmentally friendly, and the foil facing is easier to tape than XPS. Once the polyiso seams are taped, you can use the polyiso as your air barrier -- and you can skip the polyethylene.

  3. Myrtleboone | | #3


    Thanks for your response. Yes, not including poly layer would be much less labor intensive than taping polyiso seams. I have read that polyiso will decrease in r-value as it ages (as compared to XPS which increases). Is there any truth to this? Please see link below.

    Thanks for your help.

  4. GBA Editor
    Martin Holladay | | #4

    It's true that as the blowing agent in polyiso slowly escapes and is replaced by air, the R-value drops. But at its worst, after about 50 years, it will be in the same range as XPS.

  5. Myrtleboone | | #5

    Is there any long term issues with off gassing from the polyiso?

  6. GBA Editor
    Martin Holladay | | #6

    By definition, everything offgases, which is why bloodhounds can find you if you try to run away.

    Polyiso uses pentane, a hydrocarbon, as a blowing agent. It escapes extremely slowly, over many decades. If you own a car and visit a gas station, you probably breathe in more hydrocarbon fumes every time you fill your gas tank than you'll get from the polyiso insulation in your walls over many decades -- especially since there is a layer of drywall between you and the polyiso.

  7. Expert Member
    Dana Dorsett | | #7

    At 50 years 1.5lb foil faced polyiso will still be better than R5.5/inch @ 75F whereas that R5/inch 1.5lb XPS will has also been losing it's (much more environmentally destructive) blowing agent, and run about R4.3/inch. (Roofing polyiso is typically 2lbs density and uses facers permeable to pentane, and is thus labeled at it's fully-depleted value of ~R5.5-R5.7/inch.)

    When used as exterior sheathing in a cold environment the performance of fresh XPS & iso at the same thickness & density are pretty much the same when the temp at mid-thickness on the foam is at -20C/-4F. (The reasons why XPS gains a bit of R performance at lower temp and polyiso loses a bit are complex and I couldn't possibly do it justice here, but the derating/uprating curves are well documented.) The temp at which it reached rough-equivalence doesn't change much over the lifecycle of the products either. But when the temperature at the mid-point of the foam is significantly warmer than -20C (as it will be, if in the center of the overall insulation layer, polyiso will outperform XPS.

  8. Myrtleboone | | #8

    Great info. Thanks everyone.

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