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Spray foam

user-481260 | Posted in Green Building Techniques on

Is there any literature discussing the application of closed-cell spray foam applied to a fiberglass or Roxul batt?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not aware to any articles that directly address that question. It would be helpful if you could describe the situation you want to address. Are you insulating a wall? An attic floor? Something else?

  2. user-481260 | | #2

    Hi Martin,
    I am conceptualizing an adaptation to the traditional double stud wall by replacing the 6mm poly vapour barrier with 1.5inchs of close cell spray foam.

    traditional double stud wall:
    2x4(batt) - 3.5" gap (batt) - 1/2ply wood - 6mm poly vapour barrier - 2x4(batt)

    adapted double stud wall concept:
    2x4(roxul batt) - 4.5" gap (3" roxul Comfortboard* + 1.5 close cell spray foam) - 2x4(roxul batt)

    I am interested in knowing if spray foam can be effectively sprayed on roxul Comfortboard and stand up to the rigourous pressure exerted on a vapour barrier. I have selected roxul because it is more rigid then a fiberglass batt and as a result will provide better support for the spray foam. I have not include the construction method and would like to focus on my question.
    Thanks for your help.


  3. GBA Editor
    Martin Holladay | | #3

    I think that it should be possible to install spray foam against the mineral wool -- assuming, of course, that you can find a spray foam installer who is willing to do it.

    However, I think you may be putting the spray foam on the wrong side of the wall assembly, due to a mistaken (and old-fashioned) idea about the need for an interior vapor retarder. Most building scientists would advise you to install the foam on the exterior side of the wall, following the "flash and batt" method. If you search the GBA website for "flash and batt," you will find lots of information on the topic.

    To learn why you don't need to worry about an interior vapor retarder, I suggest that you read the following three articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  4. user-481260 | | #4

    interesting articles - it cleared some misnomers I had.
    I did notice in "Do I Need A Vapor Retarder?" that one of the exception to the rule is in extreme cold climates. I live in the Canadian sub-arctic were we get long cold winters. With this information, does a vapor barrier make more sense?

  5. GBA Editor
    Martin Holladay | | #5

    Yes. In your climate, there is more of a justification for an interior vapor barrier than in warmer climates.

    So your proposed assembly might be appropriate for your climate.

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