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Stucco over exterior rigid foam – yea or nay?

Reanna Alder | Posted in Green Building Techniques on

I already posted this question under the title “accidental deep energy retrofit” but am hoping a stray stucco professional or engineer might weigh in with a more specific question in the title.

Proposed wall assembly: 2×4 framing (furred to 2×6 on the inside with polyiso foam and 1x, because we didn’t know this siding project was in our future), plywood sheathing, 1.5″ polyiso foam boards (taped) with the windows furred out on 2x2s, tar paper, lathe nailed to studs through foam with spacers, stucco.

Climate: high desert, annual rainfall averages 4″, 50+ year old stucco we are replacing was over tar paper only, with no signs of water damage. (This is to justify that we aren’t planning to include a rainscreen gap, though if you tell me the stucco will be sturdier with furring strips, OK)

Concerns: Contractor, and the stucco rep he talked to, say the most they can do is western 1 coat over 1″ of foam. They are concerned about the weight 3 coat stucco hanging 1.5″ away from the studs. They were visualizing lathe staples. Contractor is open to trying nails, if I am confident.

So, should I be confident?

I don’t want to use 1-coat (or EIFS) because, as my contractor will admit, it’s not that hard to put a hole in it. All the exterior foam retrofits I can find documentation on use other kinds of siding. If necessary, we can use hardie siding or plywood, but for aesthetic reasons I prefer stucco (we are accenting other parts of the house with corrugated metal and reclaimed redwood b&b).

Experience? Pictures? Links? Thanks.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Reanna,
    I am going to copy and paste a comment posted by building scientist Kohta Ueno on this topic. Ueno's comment originally appeared here:
    https://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housewrap-go

    Kohta Ueno wrote:

    I have attached a shot of an installation of stucco over furring strips from BSC/Gauvin Construction's Coquitlam Vancouver test hut.

    A 3/4" air space is a code requirement in Vancouver, so this installation of stucco is pretty much their standard solution now. They use a product called Hal-Tex Rainscreen Breatherboard.

    However, I don't think that the installation here was over foam--I believe it was structural sheathing, building paper, 3/4" ventilated airspace, and BreatherBoard. Some of the folks who are closer to the experiment, like Joe Lstiburek, John Straube, or Chris Schumacher, would be able to confirm that.

    But there's no physical reason why you couldn't do it over foam with furring strips; there's a boatload of compressive strength available.

    Kohta

  2. Nate G | | #2

    I recommend reading and/or pointing your contractor at http://buildingscience.com/documents/bareports/ba-1204-external-insulation-masonry-walls-wood-framed-walls/view

    For stucco cladding systems, the amount of deflection anticipated would still be small (less than 1/32 in. for up to 4 in. of insulation and less than 1/16 in. for up to 8 in. of insulation). Note that this initial deflection would happen before the stucco mortar was hydrated and would not cause cracking of the solid stucco systems later. For cracking concerns, long-term movement of the system would need to be reviewed.
    [...]
    Examination of the data shows that at 0.015-in. deflection (1/64 in.), the capacities of the systems are around 10 psf (23 lb per fastener) for EPS and XPS and around 13 psf (29 lb per fastener) for PIC and mineral fiber insulation at a thickness of 4 in. As determined by the protocol in the TR-12 (AF&PA 1999), the predicted value for #10 wood screws with a 4-in. gap is 27 lb. The results correlate reasonably well with the findings of the FSC and NYSERDA/SFA research (Crandell 2010).

    You'll be totally fine.

  3. Eric Habegger | | #3

    Maybe I'm writing out of my sphere of expertise here. I would listen to Nate here about no need for a rain screen (from the previous thread) in a southwest climate with 4" of annual rainfall. In other words, installation of furring strips in that climate may be an unnecessary expense. New Mexico is not Vancouver. Despite rumors to the contrary, advisors here are often very tied to concepts that are required in their own climate even though they aren't always needed elsewhere. The reverse is also true here when an individual might say some building technique is not needed anywhere because it doesn't happen to be needed in their climate.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Eric,
    The point of my comment was not to insist on the need for a rainscreen gap. My point is that stucco doesn't need to be installed directly on plywood or OSB. Rigid foam will work, assuming that the metal lath is fastened to the studs every 16 inches.

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