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“Perfect Wall” Construction Details

mapnerd | Posted in Green Building Techniques on

I have found myself reading Lstiburek’s “Perfect Wall” article many times over lately. I’m looking for reasons why I should consider another wall structure for my new home. I’m looking specifically at Figure 9 in his article “The Residential Wall”.

I do have a few questions about some of the construction details, though. I’m not a builder or framer, so pardon the lack of knowledge some of my questions may reveal:

  • In order to create a drainage/air cavity between the siding and the rigid foam insulation, would I use furring strips (1×4)?
  • Would furring strips also be the best way of hanging the siding (wood lap or fiber cement) if I have 2″+ of rigid foam?
  • If we use a housewrap under the foam sheathing, is it installed with construction adhesive? Or screwed together to the OSB underneath the housewrap?
  • I also have some concern about adding additional structural support for our south-facing wall. The house has a single-story passive solar design (single shed roof facing North), so the South wall is tall (just over 20′) with a lot of windows. How would I add additional structural support on that wall within the wall system described above? I’d like to arm myself with some ideas before I sit down with our architect.

    Many thanks for your time and opinions.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,

    Q. In order to create a drainage/air cavity between the siding and the rigid foam insulation, would I use furring strips (1x4)?

    A. Yes.

    Q. Would furring strips also be the best way of hanging the siding (wood lap or fiber cement) if I have 2"+ of rigid foam?

    A. Yes, although you should strongly consider the use of thicker foam.

    Q. If we use a housewrap under the foam sheathing, is it installed with construction adhesive? Or screwed together to the OSB underneath the housewrap?

    A. The housewrap is usually installed with staples or button-cap nails.

  2. GBA Editor
    Martin Holladay | | #2

    Michael,
    Q. How would I add additional structural support on that wall within the wall system described above?

    A. A bracing plan for such a wall is best prepared by an engineer.

  3. mapnerd | | #3

    Martin,

    How should I determine how much foam to use? I know at a minimum I want to add enough foam to keep the interior side of the foam warmer than the dewpoint of the inside of my home. How then do I calculate how much more foam to add before I reach the law of diminishing returns? I'm estimating around R18-R22 for the blown-in cellulose between the 2x6 stud cavities. If I use PIR for my exterior foam, I think that gives me in the neighborhood of R6.5-R7.2 per inch. I guess 3" of PIR should get me close to an R-40 wall? Of course, I haven't added any R-value for the other components. Is there a "best practice" or general rule-of-thumb for determining the point at which adding more foam doesn't make sense?

  4. Brett Moyer | | #4
  5. GBA Editor
    Martin Holladay | | #5

    Michael,
    The usual question: What's your climate? Where do you live?

  6. mapnerd | | #6

    I'm in Omaha NE (cold climate).

  7. GBA Editor
    Martin Holladay | | #7

    Michael,
    You are in Climate Zone 5. For a 2x6 wall in your climate, exterior foam sheathing should have a minimum R-value of R-7.5, according to Table N1102.5.1 in the 2007 Supplement to the International Residential Code (IRC). So 2 inches of rigid foam (any variety) is adequate.

    With cellulose between your studs and 2 inches of polyisocyanurate insulation, you get a wall with a total R-value of about R-33. That's certainly very respectable, although not quite R-40.

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