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Best practices for fastening lath / 3-coat stucco over rigid foam

Zane Bridgers | Posted in Green Building Techniques on

There seems to be some debate over how to anchor 3-coat stucco over rigid foam while maintaining <1/64 deflection to avoid cracking. It seems that for: <=1″ Foam, Standard Wide Crown 2″ Staples into Studs are Adequate 2″ Foam, Lathe & Plaster Washers and 4″ #10 Screws every 6″ 4″ Foam, Furring strips or add additional OSB/CDX Sheathing over Foam and staple lath My question is, 1. What would you all recommend for 1-1/2″ Foam (R-Max Polyiso)? 2. Would there be any advantage to a hybridized system of staples and screws/washers – i.e. screws every 12″, staples every 4″ 3. Would staples anchored only in the OSB provide any benefit or is it simply a waste of time, money and resources?

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Replies

  1. Peter L | | #1

    Zane,

    To be honest, conventional 3 coat stucco systems will crack over time. No way to stop conventional concrete cement systems like that from cracking as they are put on thin, they absorb moisture and freeze/thaw cycles will take their toll, heat/cold expansion, house movement, etc.

    Go to a synthetic stucco system like StuccoMax and you don't have to deal with stupid staples, lathe, chicken wire, cracks, water issues, etc. StuccoMax is a single coat system (2 pass) that uses fiberglass. So no staples, lathe, waiting for coats to dry, etc. It's rated at 3,000 psi and is impervious to freeze/thaw since it has ZERO Portland Cement in it.

    The product is a little more $ but the labor is much less since it's a one coat system. So the crew can knock it out fast without having to do 3 coats, which is much more labor intensive and requires more days at the job site.

    1. Merlin M | | #3

      Zane, could the StuccoMax be applied to fiberglass mesh or metal lath attached to furring strips?

  2. GBA Editor
    Martin Holladay | | #2

    Zane,
    Your question is addressed in an article titled "To Install Stucco Right, Include an Air Gap." In that article, I wrote:

    "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.

    "For further information on the use of paper-backed metal lath over furring strips, see the statement by Kohta Ueno, quoted in a comment on this Q&A thread: ‘Stucco over exterior rigid foam - yea or nay?’

    "If your local building inspector raises questions about the number and size of the screws you plan to use to fasten your 1x4 strapping, you may need to get an engineer to review your details. As long as you have an engineer’s stamp, your local building department should be happy."

    Note that I don't recommend using a method that lacks a rainscreen gap behind the stucco. The rainscreen gap is essential.

  3. AR T | | #4

    Zane,

    I’m not sure if this would help, but I have viewed Matt Risinger’s video on Boral drain and dry lath waterproofing and drainage layer, which is synthetic and or fiberglass he’s used over mineral wool without an additional rain screen, followed by a three coat stucco product. Because you’re using a stucco product as your exterior, a bit of deflection caused by the cap screws could be leveled out with the stucco application. Just a thought.

    1. Merlin M | | #5

      Robert, I have seen that video and it's interesting.

      We often used foil-faced foam for a radiant a barrier. That barrier needs a 0.75" air space to perform well, which the furring strips would provide. The Dorken product Matt used in the video is a narrower air gap.

  4. viapapillon | | #6

    What if you cannot find a stucco contractor who is familiar with paper-backed metal lathe and want to install it the traditional way. I want to add 2" of GPS, WRB, Rainscreen, and then lathe... They want to add plywood over that assembly to have something to attach to. What fasteners should be used to attach the metal lathe to the structure through the rainscreen, WRB, and GPS?

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