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Flash & batt insulation in Climate Zone 5

user-5594283 | Posted in Energy Efficiency and Durability on

I would like to insulate a 2×6 wall cavity with a combination of either EPS or XPS and Roxul Mineral Wool – Can someone please let me know what thickness of FOAM would provide the best R-Value?

Mark Kozlowski
Nelson, BC

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think that you are describing the flash-and-batt approach; I think that you are describing the cut-and-cobble approach.

    These two articles will clarify the difference between the two approaches:

    Flash-and-Batt Insulation

    Cut-and-Cobble Insulation

    If you want to combine a layer of rigid foam with mineral wool insulation, it's best to install the rigid foam on the exterior side of the wall sheathing (as a continuous layer). For more information on this approach, see How to Install Rigid Foam Sheathing.

    Q. "Can someone please let me know what thickness of foam would provide the best R-Value?"

    A. I'm not sure what the "best" R-value is. At a minimum, you have to meet the requirements of your local building code.

    -- Martin Holladay

  2. user-2310254 | | #2

    GBA has several articles on flash and batt, such as

    Is this new construction or a renovation? How is the wall constructed from the exterior to the interior?

  3. user-5594283 | | #3

    New Construction - Insulaing from the interior cavity - log home 2nd floor loft. Exterior loft : From Outside working inward: 1x10" Cedar Board and Batten - Roxul Comfort Board 80 with Coravent 1x3" furring strips - Taped Tyvek - Exterior plywood sheathing - still need to insulate the interior 2x6" interior cavity - would like to achieve best air sealing (Foam Panel - Cannot sprayfoam) Need best combination ratio/thickness for foam/Roxul comfort batt to maximize insulation value.
    Thanks, Mark

  4. Dana1 | | #4

    US codes for zone 5 require a minimum of R20 total in the walls if it's all thermally bridged by the framing. In your proposed cut'n'cobble approach with 2" foam and 3.5" fluff that can be maximized using 2" of polyisocyanurate (R12 labeled-R, R10.5 or so derated for climate & stackup), and R15 rock wool or fiberglass. The center-cavity R would be better than R25, but the net impact on whole wall performance is not 20% higher, or even 10% higher, since the thermal bridging of the framing has not changed.

    If you uses EPS or XPS the foam would be about R8.4, rock wool R15, for a total of R23.5. Despite the higher labeled-R performance of XPS, that benefit is temporary, and it will decay to about the same as EPS within the lifecycle of a house, as it's climate damaging HFC blowing agents escape. Thermally bridged by the framing the whole-wall difference between using R12 polyiso and R8.4 EPS is pretty negligible.

    Saving the foam for the exterior you would do much better with R23 rock wool in the caviteis, and 1.5" of continuous polyiso on the exterior (or 2" of EPS), which is enough in that climate to not need an interior vapor barrier, and improves the whole-assembly thermal performance by about 50% (reduction of heat flow of about 1/3) compared to the cut'n'cobbled 2" foam/3.5" rock wool approach.

  5. GBA Editor
    Martin Holladay | | #5

    From your latest post (Comment #3), it sounds like you want to put the mineral wool on the exterior side of the wall sheathing (a type of continuous exterior insulation). This will work well.

    If you take this approach, it doesn't make a lot of sense to install rigid foam insulation between the studs. Instead, you should create an air barrier at the layer of the plywood sheathing (by taping the plywood seams with Zip System tape or Siga Wigluv tape). Then you can install some type of fluffy insulation between the studs -- dense-packed cellulose would be ideal, but fiberglass batts, if carefully installed, would also work.

    -- Martin Holladay

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