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Community and Q&A

Interior vapor barrier with EPS under vinyl exterior

Navalav8r | Posted in Energy Efficiency and Durability on

I’m remodeling a kitchen in Omaha, zone 5, and am ready to insulate the stud wall. Here is my concern: the owner recently had vinyl siding applied. As part of the project, the contractor installed 3/8″ EPS under the vinyl. The EPS has a plastic film on one side and a foil film on the other. The owner said this was to “eliminate” air infiltration.

So, should I still install a 4 mil vapor barrier on the interior after I insulate the wall? By doing so will the poly create a “wet wall” with the EPS exterior layer?

Dick Ryker

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  1. GBA Editor
    Martin Holladay | | #1

    Installing thin rigid foam with a plastic or foil facer on the exterior of a wall is usually a bad idea. You don't want an exterior vapor barrier unless your foam is quite thick -- thick enough to keep your wall sheathing above the dew point during the winter.

    Fortunately, most fanfold foam underlayment products (the type installed by vinyl siding contractors) aren't tight vapor barriers. In some cases, the facings on these products are deliberately perforated to make the products more vapor-permeable.

    However, in your case it's impossible to know the vapor permeance of the product that was installed, unless the siding installer gives you the brand name of the product.

    Some examples can be found on the web:
    3/8 inch Foamular fanfold insulation has a vapor permeance of 0.75 perm.

    3/8 inch Green Guard XP series fanfold insulation has a vapor permeance of 1.3 perm.

    These products aren't quite as tight as aluminum foil (0.05 perm) or 6 mil polyethylene (0.06 perm).

    To answer your question: in your climate zone, you never want to install interior polyethylene on your wall. Building codes do not require interior vapor barriers; all they require is a vapor retarder. So skip the polyethylene and install vapor-retarder paint. For more information on this issue, see Do I Need a Vapor Retarder?

  2. Expert Member
    Dana Dorsett | | #2

    With most perforated fan-fold the perforations are large and dead-obvious. Even on a 2" or 4" grid that dramatically affects the average vapor permeance of the sheet.

    R-Tech's 3/8" foil-faced fan-fold siding underlayment appears to be a perforated type, and they only specify it to be under 1.0 perms (a class-II vapor retarder). Were not perforated it would always be under 0.1 perms, and a true vapor barrier due to the very low permeance of the facers:

    Martin has it right- with sub-1-perm material on the exterior, the last thing you should install is a polyethylene vapor barrier on the interior. The typical 6 mil sheet goods is typically ~0.05 perms give or take, and even 4-mil polyethylene runs ~0.08 perms, a too-tight Class-I vapor retarder.

    Vapor barrier paint runs about 0.5 perms, which would work just fine, and it's relatively cheap. If you want to go one better 2 mil nylon sheeting is a variable permeance "smart" vapor retarder- fairly vapor open when the proximate air has high humidity (60-70% RH+), but is about at tight as vapor barrier paint when the air is dry (under 35%RH). That way it slows wintertime moisture migration, but doesn't impede drying toward the interior nearly as much as 0.5 perm paint would.

    It's installed pretty much in the same way as poly sheeting, but runs about 2x the material price. (Menards carries Certainteed's "MemBrain" 2-mil nylon vapor retarder on their web-store, but I'd be surprised it's in the local stores in Omaha. Scuttlebutt has it some of their upper midwest stores are starting to carry it- it's worth asking. Otherwise you'd have to either buy it through distributors catering to the construction trades, or online.)

  3. Navalav8r | | #3

    Thank you for your answers, they are very helpful

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