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Insulation application

jrkd2015 | Posted in Energy Efficiency and Durability on

I am going to be remediating closed cell SPF in the addition of my home (zone 6) due to it being misapplied.  All of the outside walls and upper level roof deck has it applied.  I had the roof deck sprayed on top of plastic baffles to allow the roof to breathe.  The rafters are only 2×4 so the entire 3.5″ was filled with SPF.  The exterior walls (2×6) were SPF at 3″.  However, this is going to be removed.  I do not wish to have SPF again.  I plan to use bonded logic denim insulation every where.   The wall covering material will be T&G pine board.

Question #1:  how can I accomplish the same vapor barrier as the SPF with not using it?  

Question #2:  Could I use the denim insulation in the rafter bays then apply a smart vapor retarder like CertainTeed MemBrane?  Is there a need for an air gap between the denim and the plywood?  What about the walls?

Thanks for any advice on this matter!

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  1. MattJF | | #1

    What is wrong with your spray foam?

    Membrane will be a good vapor barrier, that is easy. Are you thinking about an air barrier instead (they are different things, though occasionally the same physical component). You can put up air tight drywall and then T&G using the drywall as your air barrier.

    If you are putting insulation in rafter bays, read up on insulating a cathedral ceiling.

    Fiberglass or rockwool are likely a better option for batt insulation.

  2. Expert Member
    Peter Yost | | #2

    Hi jrkd -

    1. Denim insulation is air permeable; you need a dedicated air control layer.

    2. The CertainTeed smart vapor retarder is called Membrain. It's use is restricted to Climate Zones 4 and above.


    1. Expert Member
      Dana Dorsett | | #3

      >2. The CertainTeed smart vapor retarder is called Membrain. It's use is restricted to Climate Zones 4 and above.

      Shouldn't that read "... restricted to Climate Zones 4 and LOWER." when talking about to an unvented rafter bay?

      In zone 6 you may be able to get away with a tighter-than-MemBrain smart vapor retarder such as Intello Plus, but even then you're kind of on your own.

      For the t & g pine wall interiors in zone 6 it makes sense to use MemBrain under air-tight drywall behind the t & g. It may not look like it, but t & g pine leaks air like a tennis racquet, and with fiber insulation in the cavities it need to be fully air tight as well as vapor retardent on the interior.

  3. jrkd2015 | | #4

    Thanks for the reply's. The SPF is off gassing and has been tested by two separate labs. So I am concerned about using it again.

    To make the drywall airtight, does it have to be completely finished (i.e seam taped, mud and two coats of latex paint)?

    Option #2: what about putting accuvent type baffles back in and then the denim insulation? If so, would I then use the CertainTeed Membrane? This would then be a vented roof.

  4. Expert Member
    Peter Yost | | #5

    Smart vapor retarders (on assembly interiors) are used in cold and mixed climates only (see for examples, Intello applications: and Membrain applications:

    You should review the Airtight Drywall Approach (ADA) to use it as an interior air barrier:

  5. krom | | #6

    Surprised that no one has said anything about the roof only being 2x4 and there is no way to get close to code min insulation levels

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