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Interior rigid insulation ok?

user-6323668 | Posted in Green Building Techniques on

I am adding a small room to my house in zone 5.  I want to do it as simply and cheaply as possible while still being energy efficient.  My plan is 2×6 walls,  2×8 rafters, cathedral ceiling with high heel, ventilated roof.   Densepack cellulose in the cavities and 2″ polyiso rigid insulation on the inside of the walls and ceiling to prevent thermal bridging.  Cover the polyiso with drywall.  I don’t want to put the polyiso on the outside because it adds labor to the exterior siding and roofing.  Does this make sense?  Should the polyiso be foil backed?

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  1. Expert Member


    Yes it's doable. The complications are:
    - Attachment of electrical boxes, drywall, and trim.
    - Future attachment of cabinetry.
    - Unlike exterior foam it is not continuous at partitions, and at the floor system.
    - You need to frame corners and partitions differently fo compensate for the depth of the foam.
    - It doesn't keep your sheathing dry the way exterior foam does.

    Have you considered a Mooney wall to limit thermal bridging instead?

    Your description of the roof framing is a bit confusing. 2" x 8" rafters won't give you a high-heel, and if they are vented, the amount of insulation you will get into the cavities is disproportionately small compared to the walls.
    You may have thought this through and just not mentioned it but if you are including a ridge beam, rather than rafter-ties, that can interfere with venting and create a large thermal-bridge if it is flush.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Courtney.
    I think your idea that installing the foam inside is somehow less complicated than outside is misguided. Not only are there all the complications that Malcolm mentioned, but an outward drying wall should certainly have a vented rainscreen which introduces many of the same details as the exterior foam.

    I'm also confused about your roof assembly for the same reason as Malcolm.

  3. Expert Member
    BILL WICHERS | | #3

    2X8 rafters allow for only surprisingly short spans with typical snow load numbers too. I have a cathedral ceiling in my house made with 2x10s over a max allowable span and they flex enough to crack the paint at the seams between sheets of drywall.

    Putting the rigid foam on the exterior has advantages that interior foam does not. I think you’re thinking exterior foam is more difficult because of the need to fasten the siding through the foam, but you’re probably forgetting that putting the foam on the interior increases the complexity for all the same reasons, but for electrical, trim, and everything else on the interior. I’d try to find a way to put the rigid foam on the exterior myself.


  4. user-6323668 | | #4

    Thanks for all the ideas. Back to discussions with my builder!

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