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Open-cell spray foam for flash & batt?

sanleera | Posted in Green Building Techniques on

We are currently doing a green design review for a potential townhouse infill project in the Buffalo NY area. For the exterior wall the current assembly is brick, air space, tyvek, 3/8″ plywood, R-21 fiberglass batt foil-backed, 1/2″ gyp board, painted. We’re thinking of flash & batt of open cell spray foam and cellulose. (Not really flash and BATT, but not sure what to call it! Flash & Stuff?) Our reasoning is that the open cell has some ability to allow moisture to move to the exterior but provides air sealing. We would like to use a combination to minimize the amount of spray foam from a sustainability aspect. Thoughts? Thanks!

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  1. jklingel | | #1

    Why not caulk and tape the plywood for an air sealer? Use the airtight drywall approach on the sheet rock, too. I'm not a pro, but I see no reason why that would not work. Search for the Sunrise House here; Thorsten uses plywood for his air barrier. BTW: Why 3/8" ply? Is that code compliant?

  2. sanleera | | #2

    Thanks John, that is a good solution to consider. Sorry about the 3/8", a typo.

  3. jbmoyer | | #3


    If you want to build a green home, avoid petrochemical foam whenever possible.

    Realize open cell spray foam has the same R-value as cellulose (open cell- R3.6/inch, cellulose- R3.8/inch).

    As long as you establish an air barrier (as John K. suggests) you can use a less expensive, less environmentally destructive, fibrous insulation material such as cellulose.

    Furthermore, you must do something about the thermal bridging in your wall. I recommend using a double wall to reduce the heat loss/gain through the framing.

  4. homedesign | | #4

    I agree with John Klingel & Brett Moyer
    Not-So-Foamy is moving in the right direction

  5. wjrobinson | | #5

    To reduce our use of fossil fuels, we all should lead by example and be moving away from owning 3 vehicles each that we pump several hundred gallons of fuel into a year along with hundreds of KWs of electricity fueled by coal.

    Personally I think making long lasting products out of oil is better for the planet then driving my pick up.

    I hope to own a solar panel system someday large enough to fuel my home and an all electric work vehicle.

    Foam is a great building material. And yes so is cellulose. The gas station is the problem, burning fossil fuel is the waste of a resource better used for making useful products IMO.

  6. sanleera | | #6

    Re: Thermal bridging and a double wall. This is a project being subsidized and subject to the design standards of the lender. We're been asked to review the preliminary drawings and recommend green strategies within the current design. We'd love to recommend strategies to take the project to net zero, but realistically will be thrilled if we can get advanced framing into the spec and vinyl windows OUT!
    With respect to the stud thermal bridging, is there a recommendation for insulated sheathing in place of the 5/8" plywood?

  7. GBA Editor
    Martin Holladay | | #7

    1. First of all, open-cell foam cannot be used in a flash-and-batt or flash-and-cellulose job, because a thin (1 to 2 in.) layer of open-cell foam is not an air barrier. If you are installing a thin (1 to 2 in.) layer of spray foam as an air barrier, it has to be closed-cell foam, not open-cell foam.

    2. Second, you proposed wall assembly is at risk of developing problems from inward solar vapor drive. If the building is air conditioned during the summer, there can easily be problems from condensation accumulating at the foil facing of your proposed foil-faced batts. For more information on this problem -- and ways to avoid it -- see When Sunshine Drives Moisture Into Walls.

  8. sanleera | | #8

    Thank you for the spray foam clarification. I'm sure you've written that dozens of times! This site is an invaluable resource to us for reminders/clarifications such as this. With regard to the foil-faced insulation, I was surprised to see it on the drawings. The article you reference will aid our explanation on why not to use it. In my earlier post about insulated sheathing, I meant SIS. But thinking and researching it further, I'm concerned with strength, as we're advocating Advanced Framing.

  9. user-992104 | | #9

    Can anyone provide comparative quotes for installed cellulose compared to open cell spray foam? I live in upstate NY (zone 6A) and all the quotes I have received show the cost is the same?

  10. GBA Editor
    Martin Holladay | | #10

    All price quotes from insulation contractors are local. There is no such thing as national pricing for insulation installation.

    Your local prices are your local prices. They are what they are.

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