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Community and Q&A

Plastic or smart vapor barrier in zone 6?

bobhol | Posted in General Questions on

My builder is planning on using a normal 6 mil vapor barrier on my new build …the wall structure is R-22 fiberglass with 5 inches of EPS foam on the outside ….the building inspector says that is code …

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  1. GBA Editor
    Martin Holladay | | #1

    Ideally, your wall assembly needs to be able to dry in at least one direction. With 5 inches of exterior foam, you aren't going to get much drying toward the exterior. That means that your wall assembly needs to be able to dry to the interior.

    If your building inspector insists that 6 mil polyethylene is required, your building inspector doesn't understand the relevant underlying building science principles. However, it's usually not a good idea to tell that to your building inspector. The easiest route is just to install a smart vapor retarder like MemBrain -- even though it's a waste of money -- because that way the inspector can see that you unrolled a product and stapled it up.

    If the building inspector insists on 6 mil poly, and won't allow MemBrain, it's time to dispute the case or go over his head and speak to a supervisor.

  2. wjrobinson | | #2

    Should not say this, but, you could install plastic, get inspected, remove plastic and then drywall. I never had an inspector check for plastic after drywall....

    but, buy the membrane it is good stuff so say some, one head ache in the past at that point.

  3. bobhol | | #3

    Thank you gentleman confirmed my understanding of the situation so I will take the appropriate steps .If I have to install membrain ,so be it .It will be done correctly...regards,Bob

  4. davidmeiland | | #4

    I'd just to what AJ suggests if you can't persuade the inspector. Put the plastic up with few staples so that it can be reused for something else.

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