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Community and Q&A

Exterior Foam & Stucco /.EIFS

Steve Mackay | Posted in Green Products and Materials on

I have a good handle on how to install siding over exterior foam but I’m not sure about the section of my house that will have a stucco look?

I assume I want to go with an EIFS rather than pure stucco or am I making a bad assumption here?

If I go with EIFS should I plan to install my 2″ or 3″ of polyiso in the EIFS locations assuming that the EIFS installer will utilize my polyiso or does the EIFS system use its own rigid insulation?

If EIFS uses it’s own rigid insulation (from internet searches it looks like it uses EPS) then to get the same R-value in my EIFS wall as 3″ of polyiso I’ll need to have a much thicker EPS foam than the thickness of polyiso I’m using elsewhere. Is there a limit to how thick EIFS foam can be and what are the other implications of this approach?

Steve

BTW snowy mountainous climate 6a.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Steve,

    I will provide a response so we can bump your question back to the top of the list. With EIFS, proper installation is critical. If you decide to use the system, you should discuss the installation process with your contractor to determine which approach will be most cost-effective and appropriate for your particular build.

  2. Jon R | | #2

    Don't know, but investigate two options: 1) put Stuccowrap over polyiso and then a thin layer of EPS over that. 2) put Stuccowrap against the sheathing and apply the synthetic stucco directly to the polyiso.

    https://buildingscience.com/documents/reports/rr-0406-face-sealed-drainable-eifs/view

  3. Mitchell Costa | | #3

    Limited exposure to anything colder than zone 5, but have seen stucco with chicken wire stapled directly to 16" studs last very long term in CA climates. Planning to stucco directly to the 1X4 furring strips over the external polyiso in the house I'm building. Wouldn't recommend for 24" stud spacing, but should be fine for the 12.5" inches between furring strips with 16" centered studs. The rain screen behind the stucco, or any exterior cladding that absorbs moisture, is good insurance for both the cladding and structure to be able to dry appropriately.

  4. Steve Mackay | | #4

    I think for structural reasons we end up with a 16" OC 2x6 wall here. I hope that my contractors "Stucco guy" has some experience with this because there doesn't seem to be vast amounts of information out there.

    I'm sure I'm not tree only one ding exterior foam with a mix of stucco rock veneer and siding on the exterior.

  5. Expert Member
    Michael Maines | | #5

    Steve, there have been a lot of EIFS failures but that approach can work fine as long as the insulation ratio is in the safe zone and water management is correctly detailed. For insulation ratio, this covers the basics: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing.

    I would not trust the average stucco installer to have a comprehensive understanding of building science. I would hope that EIFS dealers are better prepared, but this is an "owner beware" situation.

  6. Roger Berry | | #6

    Steve,

    I am writing from snowy, mountainous, 6a to 7 cz - that is 8000' in SW Colorado. Actually, not much snow this year, only 45% of snowpackso far. Anyway, my house is what I call faux-dobe Santa Fe style with flat roofs and some pitched metal. My outsulation depth totals about 6 1/2" of EPS under a hybrid of two EIFs company's materials. The top coat is Dryvit synthetic coating which in all honesty looks like pasty latex paint. The scratch coat, such as it is, is Parex 121 (I think, can't find left over bag) Hi Dri. They have many forms, but the critical concern for me and perhaps for you as well is the impact resistance of the base coat/mesh.

    In our area flickers and woodpeckers are notorious for drilling holes in Dryvit over foam. I have heard that they like the drumming sound for territorial reasons and also have heard they think it sound like a punky tree trunk perfect for nesting. Either way, they are bigger problems than the squirrels here. I had planned for a half inch base coat of browncoat cement in mesh to be top coated with Dryvit color to seal out most of the moisture. Despite having been quite clear about the bird problem and the mesh I found myself having to argue with the contractor about their unannounced plan to do straight up Dryvit based on total weight concerns that had not been brought to me. I had designed in anchor points deliberately for the purpose of attaching the mesh.

    The base coat with mesh on Dryvit around here is barely over 1/8th inch thick total and frankly the penetration resistance is very low. The stucco guy liked it because you could just stuff a knife through and cut whatever openings you needed. He was quite right about how easy it was and thats why I got so cross. The flickers are like flying can openers. Initially the answer to make the coating tougher was to upgrade the mesh one notch -somewhat expensive- or upgrade to heaviest mesh in critical areas - very expensive and worse it would make a level change in the coating.

    The solution was to use a base coat of the Parex with the same grade of mesh scheduled in the job and coat closer to 3/16. This fortunately has proven successful for two years. One flicker tried to attack the west patio wall but gave up. He did ding it, but did not puncture it. The birds seem to prefer working on the walls about 12-18" below the soffits, so if the pricey mesh idea appeals to you, it may not be required over the whole wall.

    My limited understanding of EIFs rules and techniques suggests that they will want to apply a veneer of 3/4 -1 1/2" of low density EPS to the house which they can rub at with scrubby tools to level, or in my case to make the round corner features. I don't think the base coat will adhere properly to polyiso sheets whether fiberglass faced or not. How they afix the EPS on most jobs I don't know, I do know that in my case it was cemented to my existing EPS outsulation layer with what looked like sticky tile thinset. They applied it with 3/8 notch trowels with all lines vertical. This forms my first layer of water control. My main one is located at the sheathing plane. Whatever they used, the foam boards were/are stuck quite tightly.

    Getting my rounded corners the way I liked was the difficult part aside from having to put my foot down about the method of embedding the mesh. The instructions are quite clear that a skim coat goes on first, mesh into that immediately, followed with a skim coat to cover full all mesh. Many stucco crews like to just trowel on a little thicker coat, toss up the mesh and push it into the surface. This does not wet the mesh properly or locate the mesh in the center of the cement layer. DO NOT accept the tired old "we do this all the time". Read the sheets for whatever manufacturers materials you choose and don't be afraid to contact the reps. The Parex guy was great.

    One thing I should mention is the product I used is bagged powder which became another point of contention. Measuring anything seems to be an unknown skill and try as I might I could not get them to do more than make their guesses less generous when adding water. The finish still looks good though. You mentioned "the EIFS areas" which suggests mixed finishes. I would be blindly suggesting any details to look at without a plan to view, but I can urge you to start now reviewing trim around openings no matter how small. My electricians gave me some plastic boxes made to set into foam before the stucco process and the result is nice clean exterior covered boxes that don't look like after thoughts.

    Real stucco or at least 3 coat cement based stucco can weigh several pounds per square foot, so anchor points are a real issue. I had them, they went un-used and the relatively light weight of what is termed synthetic stucco seems just fine with the base layer glued to my rather well anchored primary insulation layer. If you are going to do any kind of stone work, real or veneer, be sure to have gone over the anchor details with someone you trust. I won't get into all the water detailing here, as you can find far better info than I have on many websites.

    Do be prepared for some hairline cracking especially around windows subject to strong sunlight or adjacent to warming surfaces like metal roofs. The foam underneath the stucco will shrink and dance a little bit more there and by end of first season you will probably need to touch up some cracks. Try to have them leave full information on the materials, colors and caulks used for maintenance continuity. I just pray the tubs of color I kept will still be viable when I inevitably hit the house with car or tractor. I have patched with no visible sign twice, but I was told that if I dinged something the whole wall would need a re-coat to avoid the dalmation effect.

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