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Community and Q&A

Extruded polystyrene

Rolf Reiss | Posted in Energy Efficiency and Durability on

I am interested in using your “cut and cobble” method for insulating the walls of my addition. I live in zone 5 and plan to use 2″ of extruded polystyrene and 4 1/2″ of dense pack cellulose. Are there any negative effects in using readily available uncoated extruded polystyrene with a perm rating of 1. ? The coated extruded poly seems to have a perm rating of .1 but I cannot locate any ( Dow is no longer making it).

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Are there any negative effects in using readily available uncoated extruded polystyrene with a perm rating of 1?"

    A. No, other than the usual drawbacks to the cut-and-cobble approach.

  2. Rolf Reiss | | #2

    Q. By usual drawbacks are you referring to the fact that studs and headers are not insulated?

  3. GBA Editor
    Martin Holladay | | #3

    The answer to your question can be found in an article called Cut-and-Cobble Insulation. In that article, I wrote:

    "Cut-and-cobble has some disadvantages:
    To address thermal bridging, it’s always better to put the rigid foam on the exterior side of the sheathing. However, in an existing house, homeowners are rarely willing to demolish their siding just to install a layer of rigid foam.
    This insulation method is very time-consuming. If you want to do a good job, it will take longer than any other insulation method.
    Wood framing expands and contracts with changing humidity levels, raising the possibility that attempts to seal the perimeter of the rigid foam (whether with caulk, spray foam, or tape) will fail over time. Anecdotal evidence suggest that this danger is real, especially for cut-and-cobble cathedral ceilings."

  4. Rolf Reiss | | #4

    I just sat down and read your insightful " cut and cobble" article as well as the responses. Thank you for passing it along to me. My project is new construction with all of the windows, doors, trim and cornerboards installed. After being shocked by the cost of sprayed icynene I had to alter my plans to the cut and cobble option. Being new construction I can use my table saw to cut the polystyrene and create reasonably tight fits between the wall studs. Do you have any recommendations for caulk that would be likely to adhere to both the polystyrene and the studs over the long haul. Silicone? urethane?
    Thanks again

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