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Cultured stone veneer with interior XPS

2bablend | Posted in General Questions on

Sorry if this has been covered, but I have searched all over and cannot find definitive answer.

I am in Climate 4, but within few miles of 5 on East Coast, averaging 47” of rainfall.  I have a house with cultured stone veneer about 2 ft above the rim board on the front of the house.  The builder used D paper with another layer of JumboTex, lath, then cultured stone over the OSB and rim board. I know this is not the best solution, but it is what I have. 

The walkout basement is conditioned. Currently, the interior rim is insulated with unfaced fiberglass batt.  I would think this would allow warm moist air during winter to contact the rim board and condense, so I wanted to remove and apply 2” of XPS.  So here is my hang up…..If I apply XPS to the interior of the rim, wouldn’t this reduce the vapor permeance, not allowing the rim board (covered in cultured stone on the exterior) to dry to the interior efficiency?  I realize XPS is still permeable and depending on thickness can be semi-impermeable or semi-permeable.  With its reduced permeance over unfaced batt, would I be creating a worse scenario by using XPS?  And would 2” of XPS be worse than 1” with the given situation?

Thanks in advance!  Appreciate any feedback.

Craig

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Replies

  1. Expert Member
    Peter Engle | | #1

    Adhered stone veneer itself is the biggest risk in this scenario. Depending on the style of cap flashing above the veneer and drainage at the base, bulk water entrapment will be the biggest threat. If there is no cap flashing, and if the veneer continues down below grade without a drainage detail, it is installed wrong and it is likely to cause water damage regardless of your rim board insulation treatment.

    Also remember that there is no drying to the interior in winter. Moisture moves from warm to cold. The better you insulate the rim board, the less drying you will get. XPS or other closed-cell foam will reduce vapor flow from the interior to the exterior and therefore reduce the potential for condensation on the rim board in winter. But as you have realized, this will also slow drying of the rim board in summer.

    The bottom line with all insulation schemes is that you need to control bulk water first, before you worry about vapor migration. Adhered stone veneers are notorious for bulk water leakage problems. I generally do not recommend stone veneers over energy-efficient wall systems without a true drainage gap and aggressive drainage and flashing details.

    1. Jon_R | | #5

      > there is no drying to the interior in winter. Moisture moves from warm to cold.

      I'd be cautious with such statements. It's closer to:

      a) from high to low (gravity moving bulk water)
      b) from high air pressure to lower air pressure (with airflow)
      c) from high vapor pressure to low (which can be from cold to somewhat warmer)

      1. 2bablend | | #8

        Thanks Jon.

  2. 2bablend | | #2

    Thanks Peter!

    Yes, there is cap flashing and the veneer ends at grade. Weep flashing is also installed at the rim board/concrete transition. Would EPS be a better solution?

    1. GBA Editor
      Martin Holladay | | #6

      Craig,
      You wrote, "the veneer ends at grade." Ideally, the stone veneer should terminate at least 8 inches above grade.

      1. 2bablend | | #9

        Thanks Martin. I will fix that.

  3. Jon_R | | #3

    Yes, XPS could make things worse. Are there any signs that you currently have a problem that is worth the risk to fix?

  4. 2bablend | | #4

    Jon,

    No, there is no signs of water problems. Just trying to minimize warm air contacting rim board.

    1. Expert Member
      Dana Dorsett | | #7

      Sounds like you're trying to snatch a defeat from the jaws of victory here. Don't do it.

      An air barrier on the conditioned space side of the fiber insulation can help, and it helps even more if that air barrier is somewhat vapor retardent or a "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain), which is far cheaper and better than any foam board solution. A foam solution works best if it's fully adhered and guaranteed to be pretty air tight forever, such as sprayed polyurethane foam, but even then it can't be so thick/low-perm as to unduly limit the drying path.

      Using XPS for almost anything is the opposite of a "green building" solution, due to it's HFC blowing agents an less-friendly polymer. Most of the time it can (and should) be designed out.

  5. 2bablend | | #10

    Thanks Dana! I’ll look at the MemBrain as an alternative.

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