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Community and Q&A

Interior vapor barrier for old house?

user-5454807 | Posted in GBA Pro Help on

I have a hundred plus year old building (originally single-family, converted to three-family) on Long Island (Zone 4A) that has a mix of 2×4 framing (balloon & other), with 1″ nominal (minimum 3/4″, to 7/8″) tongue & grove wood sheathing underneath clapboard that’s covered with aluminum siding.
We’re going to remove the clapboard & aluminum sidings, keep the sheathing and cover it with Tyvek, 1″ foam (polyiso or XPS) and vinyl siding.

On the inside we’re insulating with Roxul Comfort Batts (R-15). Do we need to use an interior vapor barrier? If so, (I would prefer to use a smart vapor barrier if they are effective) which brand?

Thanks in advance.

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 4A you don't need or want a true vapor barrier anywhere in the stackup. With the R5-ish exterior foam the average winter temp at the sheathing has HUGE dew point margin from interior moisture drives.

    Polyiso is greener (due to far less damaging blowing agents) and easier to air seal than XPS (due to the foil facer, which can be reliably sealed with temperature rated foil tapes), and the foil facers would give it another modest performance boost due to the low-E foil in combination with the air spaces between the vinyl siding and the foil facer.

    It's not really necessary in your stackup & climate, but Certainteed MemBrain (2-mil nylon) is cheap (compared to the imports) & effective.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana.

    For more information on vapor barriers, see this article: Do I Need a Vapor Retarder?

    For more information on why walls with exterior rigid foam need to be vapor open on the interior, see this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  3. user-5454807 | | #3

    Just to clarify, although Dana assumed correctly, that's 1 inch exterior foam - not 1 foot.

  4. user-5454807 | | #4

    I was leaning toward polyiso and will definitely use it. A further question: should we use single sided or dual sided foil faced boards?

    If single sided, the foil side should face out?

    Thank you again gentlemen.

  5. GBA Editor
    Martin Holladay | | #5

    It doesn't make any difference whether you purchase polyiso with foil on one side or foil on two sides.

    You won't get any thermal advantage from foil unless the foil faces an air space.

    If you decide to install vertical furring strips to create a rainscreen gap, then the foil should face the furring strips.

    If you won't be installing furring strips, but plan to attach the vinyl siding directly to the foam, you'll have a slight benefit by facing the foil out -- since there is a little bit of air behind vinyl siding.

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