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Stucco Wall Retrofit

keithhoffman22 | Posted in Green Products and Materials on

Hi,

I’m investigating refurbishing our 1970s home that was one coat stucco’d on XPS in the 80s. The stucco is deteriorating due to a lack of expanse joints, horizontal stucco window sills, XPS shrinkage, and a zero overhang roof line.

I’d like to use the re-stuccoing to increase the outsulation R-value. I’d like to do something like install Roxul Comfortboard IS (in either 2 3/8″ or 3″ for R-10 or R-12), and reinstall one-coat stucco over the comfortboard.

There are some great articles on GBA that I’ve read regarding using Roxul in place of foam with the exterior strapping technique. I’ve also read the Building Science report on deflection and am comfortable with their findings. I think I’m comfortable with the sheathing and insulating parts of the job, though the innie window details will be tedious.

However, one thing baffles me (and maybe this is a stupid question), if you have 1/2″ or 3/4″ straps proud of the stucco, how would you install a lathed one coat stucco system? Is there a simple answer I’ve been missing?

Obviously, I could cut the roxul to flush install the strapping since it ‘shapes’ well but that sounds like a labor intensively solution that just reduces the R-value of the Roxul. I could strap with 2x4s and fill the space between straps with additional 1.5″ pieces of Roxul but that would increase the materials cost a decent bit. Roxul doesn’t offer a comfortboard thinner than 1.5″ and my quantities are insufficient for a custom run.

I’d ask my stucco contractor but I’m struggling to order ComfortBoard let alone find a contractor familiar with comfortboard. They all look at me like I’m crazy to want to do something other than 1″ of XPS (which is what the house currently has).

BTW I’m looking for at least a semi vapor open solution so a <1 perm wrap like Delta Dry probably isn’t for me. (Holler if you know of a vapor open product that is similar). One other related question: why does everyone (several Q&A comments here and other blogs) recommend against a WRB/air barrier over the sheathing instead of over the roxul? Thanks in advance. Keith

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Replies

  1. keithhoffman22 | | #1

    I think I may have found an answer to one of my questions in another post here on Gba.

    https://www.greenbuildingadvisor.com/blogs/dept/musings/install-stucco-right-include-air-gap

    "If you’re installing stucco over foam, here’s how to create a ventilated rainscreen gap:
    Install 1x3 or 1x4 vertical strapping on top of the foam, screwed through the foam into the studs.
    Fasten paper-backed metal lath to the strapping, and proceed with a standard installation of three-coat stucco."

    Should work for mineral wool as well I guess.

  2. GBA Editor
    Martin Holladay | | #2

    Keith,
    The last time I checked with John Straube (formerly a principal at the Building Science Corporation), he wasn't ready to recommend stucco over mineral wool. (To read what he had to say on the topic, see Installing Mineral Wool Insulation Over Exterior Wall Sheathing.)

    Admittedly, however, we had that conversation a few years ago, and you may have more up-to-date information. Your plan may work fine -- but you are blazing a new trail, so don't be surprised if you have to figure out details (or if there are a few surprises) as you go along.

  3. user-2537291 | | #3

    Hi Keith,

    Just curious. Did you consider synthetic stucco (i.e. EIFS)? I seen EIFS have systems that work with roxul.

  4. Richard Beyer | | #4

    Here's the original BSC document on Martins subject matter.

    http://www.roxul.com/files/RX-NA_EN/pdf/Technical%20Bulletins_Guides/13A08%20ROXUL%20COMFORTBOARD%20IS%20Application%20Guide%20-%2004-11-2014.pdf

    Stucco is listed in a couple illustrations in this link...

    http://www.buildingscienceconsulting.com/presentations/documents/120830%20Smegal-Roxul%20Cladding%20Attachment.pdf

    Then you may consider contacting the Portland Cement Association for further advice @

    5420 Old Orchard Road
    Skokie, Illinois 60077-1083
    847.966.6200 Fax 847.966.9781

    500 New Jersey Ave NW, 7th floor
    Washington, DC 20001-2066
    202.408.9494 Fax 202.408.0877

    http://www.cement.org/think-harder-concrete-/homes/products/stucco

  5. GBA Editor
    Martin Holladay | | #5

    Richard,
    Thanks for the link to John Straube's document on the Roxul web site.

  6. keithhoffman22 | | #6

    Wesley,

    I didn't consider EIFS. If you mean a foam system, I'm not interested. If you mean, a synthetic system like EIFS over Roxul, it's an item I'll consider if I trip over the right contractor but I prefer a rain screen style one-coat system.

    Is there a specific advantage you that makes you recommend it?

    One local consideration:
    I could be wrong but the local birds have left my existing one coat stucco alone and have a field day with my neighbors houses ... So I think EIFS would suffer and one coat on Roxul will be fine. Of course, my existing system has the air gap on the inside of the foam so it is possible stucco on paper and lath will appeal to them more ...

  7. keithhoffman22 | | #7

    Martin,

    1. Yeah, I read that.
    2. 3 years ago
    3. The deflection study doesn't actually have cause for concern; I think Straube is just being conservative given they don't have a field.
    4. This is one personal retrofit instead of say a subdivision full of them. I think that makes for a good situation for trying something that is safe (see deflection study) but could have unknown durability issues.

    But I appreciate the caveat emptor warning.

  8. keithhoffman22 | | #8

    Martin,

    I've been told by a local building material supplier that Sto and Roxul will warranty a rainscreen style application. I haven't got the details yet (which sto, stucco details, etc) but will follow up if I can nail them down and confirm that this info is correct.

  9. keithhoffman22 | | #9

    Cladding weights per this document:
    http://www.ronblank.com/courses/noc09a/noc09a.pdf

    3/8" to 1/2" one coat: 4.4 to 5.9 psf
    3/4" to 1" three coat: 8.8 to 11.8 psf

    Per other documents online
    Fiber cement lap siding 3 psf

    The difference between cement siding and one coat is significant but so is the different between one coat and three coat.

  10. omede-user-1113272282 | | #10

    Reawakening this thread as I am looking to do stucco over comfortboard and would love to cost engineer by carefully choosing rain screen and lath components.. and if there is a direct application option I am very intrigued

    It looks like in Europe there are synthetic direct application mineral wool systems akin to EFIS but with comfortboard + single coat...

  11. omede-user-1113272282 | | #11

    One similar option may be the Western 1-Kote W1K System. They have specified it with Unbacked Foam Plastic (https://www.westernblended.com/_files/ugd/15ffa3_6b657ff463fc4703b95566dbe41f9cf4.pdf, https://www.westernblended.com/_files/ugd/15ffa3_1a7a86bba3aa4354a8b10fcfb9dd658b.pdf) - but similar to the Sto system, it advises an embedded mesh coat. Makes sense. But the fact they specify with unbacked foam is reassuring..

    "A continuous Insulation Foam Board (optional). (EPS) foam plastic
    insulation boards shall be Type II as set forth in ASTM C578, with a
    minimum nominal density of 1.5 pcf (24 kg/m3). Extruded polystyrene
    (XPS) foam plastic insulation boards shall be Type IV or Type V as set
    forth in ASTM C578, with a minimum nominal density of 1.5 pcf (24
    kg/m3). Polyisocyanurate foam plastic insulation boards shall be Type II
    as set forth in ASTM C1289, with a minimum nominal density of 2.0 pcf
    (32 kg/m3)"

    "An optional EPS, XPS or Polyisocyanurate Foam Plastic
    Insulation Board is then attached over the previously installed water
    resistive barrier. Woven Wire or Metal Lath is fastened to the optional
    continuous insulation board. A basecoat of Western 1-Kote is then applied
    over the wire or lath, at a minimum thickness of 3/8”. Western 1-Kote
    products are available in Concentrated and Sanded presentations. An
    optional Skim Coat of BPS Kote with embedded fibermesh may be applied
    over the Western 1-Kote basecoat as part of a Crack Resistant System
    (CRS). A variety of finish coats may be applied over the Western 1-Kote
    basecoat or Crack Resistant System." (https://www.westernblended.com/_files/ugd/15ffa3_855a3330cc1248f8be636903484e78ad.pdf)

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