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Open-cell spray foam wall insulation

NEplumber | Posted in Green Building Techniques on

Hi there,

I was doing consulting on a heating system in a new home this morning and brought up insulation as a subject of the heat loss calculation.
It was mentioned that they are going to be spraying open cell six inches thick in the wall bays. The house has 2 inches of styrofoam for the stucco on the outside. Next they have areas where there is a layer of tar paper or some wrap and then wood. Other areas the brick structure was retained.
My concern is specifically with the open cell foam not having a vapor barrier. The rest of the cladding isnt up to me, and they have no intentions of changing it. I can sway them in the choice of spray insulation, and I am being paid to research it for the home owner.

Thanks in advance,

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  1. GBA Editor
    Martin Holladay | | #1

    We can't answer your question thoroughly unless you tell us your geographical location or climate zone.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    A 2x6 wall has only a 5.5" cavity depth, not 6". Most half-pound density open cell foam would have an R-value of ~R20 @ 5.5".

    Styrofoam is a trademarked name for Dow's XPS, with a labeled R-value of R5 per inch of thickness. May people use it interchangeably for EPS, which has a labeled R-value of R4.2 per inch of thickness if the density is 1.5lbs per cubic foot or higher, or R3.9/inch for 1-lb EPS. The blowing agents for XPS that give it a higher R/inch dissipate over the lifecycle of a house, and it will eventually be R4.2/inch, just like mid density EPS.

    So let's assume R4.2/inch, or R8.4 for the rigid foam, and R20 for the cavity fill. At that ratio you have sufficient dew point control at the structural sheathing to exceed IRC prescriptives for US climate zones 1-5, but would need something more vapor retardent than standard latex paint on gypsum board for climate zones 6 & up. Even it if were low-density EPS and R7.8, it would still (barely) exceed the IRC prescriptive minimums for zone 5 & lower that would allow standard latex paint as the interior side vapor retarder.

    On the portion with no rigid foam, as long as there is a properly vented cavity between the stucco or brick and sheathing it's also going to work fine for zones 1-5, but not 6 & up. The brick veneer should have weep holes along the bottom courses, and corresponding vents to the exterior (or a cavity open to a vented attic), and similarly stucco needs weep screeds at the bottom and venting at the top to promote convective purging of the air in the cavity, replaced by outdoor air.

  3. NEplumber | | #3

    Thank you Martin for the response. The home is located in NY southern part of the state.
    I read your writings often, thanks for all the informational and educational posts.
    Where can i read up more about calculating the dewpoint behind the open cell foam? And what would be the pros-cons of going closed cell?


  4. GBA Editor
    Martin Holladay | | #4

    It sounds like your house is either in Climate Zone 5 or Climate Zone 6; see the map on this page to determine your climate zone.

    To determine whether 2 inches of exterior rigid foam is adequate for a 2x6 wall, we would need to know:
    (a) what type of rigid foam you are talking about (so that we can determine the foam's R-value), and
    (b) whether you are in Zone 5 or Zone 6.

    For more on this issue, see this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "My concern is specifically with the open cell foam not having a vapor barrier."

    A. As the article I just linked to explains, a wall with exterior rigid foam should never have an interior vapor barrier. This type of wall is designed to dry to the interior.

    Q. "Where can i read up more about calculating the dew point behind the open cell foam?"

    A. If you want to learn more on the topic, read this article: Are Dew-Point Calculations Really Necessary? However, you may not need to make these dew point calculations. All you really need to do is understand the principles in the article I mentioned earlier (Calculating the Minimum Thickness of Rigid Foam Sheathing) and to consult the table in that article.

    Q. "What would be the pros-cons of going closed-cell?"

    A. If your wall has exterior rigid foam, it's better to choose open-cell spray foam, not closed-cell, because open-cell spray foam allows the wall sheathing to dry to the interior. (Open-cell spray foam is much more vapor-permeable than closed-cell spray foam.) For more information on these issues, see:

    All About Vapor Diffusion

    How to Design a Wall

    -- Martin Holladay

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