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1″ of Rigid vs. closed cell foam

GBA Editor | Posted in Energy Efficiency and Durability on

I am working on a small house in a zone5 coastal area and have been going back and forth b/w 5″ of closed cell in a 2×6 stud wall or 1″ of exterior rigid and dense pack cellulose.

The house is clad in cedar shingles, so any rigid over about 1 1/2″ starts to complicate shingle install, casing etc.

These seem to be the 2 most reasonable options. For various reasons, we are not considering going w/ a double wall system or rigid insulation over 2″ in thickness.

Any thoughts…

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  1. Chris | | #1

    Saw the informative discussion below...but would be glad to hear additional thoughts/ opinions. I know there are plenty out there....

  2. user-626934 | | #2

    2x6, dense pack cellulose w/ 1" exterior insulation by a mile.

  3. Chris | | #3

    Thanks...Would you install building felt or other WRB b/w rigid and shingles?
    Also, Dow recommends sheathing over rigid before installing shingles, but I'm not sure that is necessary with just 1" of rigid. What do you think?

  4. Riversong | | #4

    Of those two options, I agree with John S that the exterior foam for thermal break and the cellulose in the interior is a far better option.

    That puts the petrochemical plastic on the outside where it will have little effect on indoor environmental quality (e.g. negative ions), and allow the hygroscopic cellulose to act as fire retardant, insect and rodent retardant, sound suppressor, and moisture buffer (as long as there is no interior vapor barrier).

    However, if your goal is a somewhat breatheable envelope with a thermal break, then you could also consider cross-hatched interior framing (horizontal 2x2s) with additional dense-pack cellulose and no plastic foam.

  5. Chris | | #5

    Thanks..any thoughts on Question#3?

  6. GBA Editor
    Martin Holladay | | #6

    It looks like you have moved your question about cedar shingles over foam to a new page by starting a new thread on the topic. To reduce confusion, any answers to your query on that topic should go on the new page:

  7. JR | | #7

    If putting horizontal 2-bys allows for a thermal break, is it acceptable to put a layer of rigid on the interior studs to accomplish the same goal?

  8. GBA Editor
    Martin Holladay | | #8

    Yes. But then your rigid foam can't address heat loss at the rim joists.

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