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Community and Q&A

How does flash and batt impact mositure migration?

Bruce Chyka | Posted in Energy Efficiency and Durability on

Does this practice (flash and batt) work with single wall construction where the foam is sprayed against housewrap?

Some builders in the KC metro area use single wall construction. To meet efficiency expectations they are wrapping the home, spraying 1/2″ to 1″ closed cell foam and filling the cavity with batt insulation. Does this cause issues with moisture migration?

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  1. Expert Member
    Michael Maines | | #1

    Bruce, is there structural sheathing between the housewrap and sprayed foam layers? Flash and batt can work fine but I would be concerned about air leaks developing from cracked foam if there is no structural sheathing. You're in climate zone 4 so no interior vapor retarder is required, which also means that the thickness of foam does not matter for you.

    Check out Martin's blog if you haven't already:

  2. Bruce Chyka | | #2

    There is no sheathing its called single wall construction. They wrap the studs on the outside and flash the foam against it from the inside.

  3. GBA Editor
    Martin Holladay | | #3

    I've never heard of this method, and it sounds unlikely. The housewrap would end up bumpy and uneven, making the installation of siding impossible.

    Are you sure there isn't any sheathing? How are these walls braced?

  4. Bruce Chyka | | #4

    Its true the sided wall compsition is correct; siding-house wrap-studs-flash sprayfoam ~ 1/2"- R-19 batt insulation in the cavity - sheetrock. There are many homes built this way in the area. The walls are braced using interior walls and simpson ties & other bracing. The front of this home is stucco with OSB sheathing felt and house wrap. One section of the home has exposed insulation to the basement that the homeowner says is wet. I think its moisture from the poured walls and slab. The home has a measure ACHn of .14. My recommendation was to install a dehumidifier and look at ventilation to get moisture out of the home. My concern is the sprayfoam has interacted with the house wrap forming an impermiable barrier to moisture flow. What say you?

  5. Expert Member
    Michael Maines | | #5

    Bruce, yes, closed cell spray foam will block most water vapor transmission, housewrap or no housewrap. That's usually considered an advantage of closed cell foam.

    Any tight home should have some sort of ventilation system. You're now talking about a specific home; what is your role in the project?

  6. Bruce Chyka | | #6

    I did a performance audit on the home.

  7. GBA Editor
    Martin Holladay | | #7

    You say that the front of this home has OSB sheathing. I assume that you are implying that the other sides of the house have no OSB?

    If I understand correctly, and three sides of the house have stucco with no sheathing, then my guess is that the stucco was installed first, and then the spray-foam contractor came back to install the spray foam from the interior.

    First of all, I hope that the stucco contractor didn't install stucco directly over housewrap. Stucco requires at least two layers of WRB, and the WRB closest to the stucco shouldn't be housewrap -- it should be asphalt felt. Ideally, there should be an air gap behind the stucco, but few stucco installers include one.

    Flash-and-batt won't really work with 1/2 inch of spray foam. You need at least 1 inch of closed-cell spray foam, and 2 inches is better. You wrote, "My concern is the spray foam has interacted with the housewrap forming an impermeable barrier to moisture flow." In fact, closed-cell spray foam has a very low vapor permeance, and if it is installed properly, it is close to a vapor barrier. So I don't quite understand your point -- vapor impermeance and closed-cell spray foam go hand in hand.

    Stucco is a problematic type of siding, and it's easy for stucco to trap moisture. The source of the moisture is usually rain; but if the stucco dries slowly -- which it does -- the damp stucco can saturate the underlying sheathing or even the studs.

  8. Bruce Chyka | | #8

    The only side with stucco is the front. The other three sides of the home are wood framed and wood sided as described previously.


  9. John Brooks | | #9

    I thought Bruce was saying that the sides and rear of the home are siding.
    and the front (the hollywood side) is stucco

    edit to posted while I was posting

  10. GBA Editor
    Martin Holladay | | #10

    OK -- we're gradually getting a clearer idea of the construction of this house.

    As I understand it, there is only one area of concern (unless you care to provide more information). You wrote, "One section of the home has exposed insulation to the basement that the homeowner says is wet."

    If the basement has half-high walls with stud framing on the top half, and if these framed areas are insulated with exposed fiberglass batts, then it's no surprise that the stud bays or sheathing are wet. This is a typical situation. The warm, moist interior air easily passes through the fiberglass batts, and the moisture condenses on the first cold surface that it encounters.

    The best solution is to remove the fiberglass batts and throw them away. Once the stud bays and sheathing are dry, install some type of insulation that isn't air-permeable (spray foam or rigid foam). Then protect the foam with gypsum drywall that has been installed with attention to airtightness.

  11. Bruce Chyka | | #11

    If that is the case is there an issue with the walls covered with sheetrock? I offered to conduct a humidity/temperature study of enclosed walls to see if there is the same problem but have not heard back from the builder.

    Thanks for sticking with me on this :)

  12. GBA Editor
    Martin Holladay | | #12

    There is usually less airflow through walls finished with drywall than basement walls with exposed fiberglass batts. So it's easy to have problems in the basement walls, even if there are no comparable problems in the walls with drywall.

    However, the only way to be sure is to open up the wall and look. (I don't trust moisture meters very much -- unless you know what you are doing, it's easy to jump to conclusions based on a moisture meter reading.)

    Without more information, I wouldn't say you have much evidence that there is a problem in other walls.

  13. Expert Member
    Michael Maines | | #13

    Bruce, have you or the homeowners ruled out leaking doors or windows?

  14. Bruce Chyka | | #14

    Micheal this is a new home and as far as I know there are no leaking windows. The area where moisture was located is a corner of the basement with 1/2 high stud walls.

    Thanks all for your help.

  15. Expert Member
    Michael Maines | | #15

    Bruce, it's just that we once built a new house with spray foam insulation and a very similar issue of localized wet walls in the basement.

    After we spent a LOT of time investigating and measuring humidity, hiring energy auditors, etc. it turned out to be leaking windows, to everyones' surprise, but we didn't know that until we removed the interior trim and observed water coming in during a storm. Concrete walls give off a lot of water, but will also absorb a lot of water without evidence. The stud-framed half walls will show a leak much sooner.

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