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Will I have a dew point problem in the wall cavity?

user-1141733 | Posted in Energy Efficiency and Durability on

I’m a contractor living in Southwestern NH and designed an insulation envelope for a house. From inside to the outside it consists off 1/2″ plaster, 2×4 with 3-1/2″ R-13 blown-in cellulose, 1/2″ CDX plywood and a 5-1/2″ polyurethane SIPS R-32. The SIPS has 1/2″ blue board, 4-1/2″ of urethane foam and 1/2″ OSB. All seams in the panel will be drilled and foamed.

Does the cellulose move the dew point into the 2×4 cavity?

The total R value of the wall is R-45.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    James,
    Q. "Does the cellulose move the dew point into the 2x4 cavity?"

    A. No. You have no dew point concerns.

    But talk about a complicated wall assembly! How did you come up with that one? Is this a retrofit job on an existing house with 2x4 walls, or did you come up with that complicated wall for a new house?

  2. user-1141733 | | #2

    It's a new house. I didn't think it was that complicated given that I see designs calling for foam boards to be applied over the house and tape the seams. All I'm doing is Substituting the polyiso foam boards with a urethane SIP that already has exterior sheathing.
    My goal is to eliminate any possibiltiy of moisture in the wall and give a space for electical and any other stuff that goes in the walls without compromising the insulation integrity of the SIPS.
    I did come up with this method but I have experience with SIPS.

  3. GBA Editor
    Martin Holladay | | #3

    James,
    Fair enough. Your plan will work -- you're good to go as far as I'm concerned.

  4. user-1141733 | | #4

    Thank You for your response.
    The reason I'm concerned is I did a remodel and my insulation sub put 2" of closed cell foam against the sheathing and filled the cavity with 3" of blown-in cellulose which was acceptable with the building inspector.
    But I had an area that I built a 2x4 wall to run plumbing in for a soapstone sink in front of the exterior wall. My insulator netted and fill the everything with cellulose. I had to open it up to get to the plumbing and the cellulose was damp part way in the cavity. Someone told me that the extra cellulose aloud the cold to come into cavity further creating the dew point. When I did this work it was winter and the temperature was around 15 degress at night for a week or more.

  5. GBA Editor
    Martin Holladay | | #5

    James,
    When you install a thin layer of closed-cell foam, the installation gets riskier and riskier the more insulation you put on the interior side of the foam.

    However, if you add additional insulation on the exterior side of the foam -- presumably, some additional foam -- the installation gets less and less risky.

    Exterior insulation is always safer than interior insulation.

  6. user-1141733 | | #6

    Thanks Martin.

    I don't mean to obsess about this but once the wall cavity is enclosed we have no idea whats going on in there and I've come across some bad situations.

    The amount of closed cell ureathane insulation installed on the outside 4-1/2" was a guess on my part. It would seem to me that there should be a formula to calculate the amount of insulation to be installed on the outside depending on your climate zone and heating degree days. At best if not a formula that can be easily understood than a chart would be handy.
    Say for every inch of foam installed on the outside your allowed X number of inches in the wall cavity before condensation is created.

    I'm going at this as a conscience builder but it would be nice to know that there's some type of science behind these decisions.

  7. GBA Editor
    Martin Holladay | | #7

    James,
    Here's the rule of thumb long used in Canada: At least 2/3 of the R-value of the total wall assembly should be on the exterior side of the vapor-retarding layer, and no more than 1/3 of the R-value should be on the interior side. (This rule is used for walls with polyethylene sheeting, not rigid foam.)

    For specific recommendations on the use of rigid foam sheathing on 2x4 and 2x6 walls, broken down by climate zone, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  8. user-1141733 | | #8

    Thanks again. That's what I was looking for and the explanation for calculating seems relatively easy to understand.

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