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Opinions on this Wall Assembly with Stucco Finish

TractorSteve C | Posted in GBA Pro Help on

My house in NE Oklahoma has vinyl siding over 3/4″ Dow foam directly on 2×6 studs 24 o. c.
I wish to pull vinyl, add 2nd layer 3/4″ Dow, 1 or 2 layers 30 lb. felt, 1×4 strapping with Cor-A-Vent SV-5 at top and bottom, then add 3 coat stucco with paper backed metal lath.  At foundation (12″- 18″ top to footing) weather proof stem wall add 1″ Dow covered with Durock concrete board and 1 coat stucco.
My questions-
-What should I do differently than I’m planning?
-Should I tape Dow seams and if so with what?  It’s 15 psi is that ok?
-Some Dow comes with plastic on each side and it’s stronger? ok to use?
-Should/ can I vent Cor-A- Vent into soffit?
-We have mud dobbers (wasps) everywhere and they love going behind vinyl siding, will they get thru Cor-A- Vent, if so do I screen it or use something else?
-Type and how many screws to use in each 8′ 1×4?  4 1/2″ long?

Different topic- I’m going to pour a monolithic slab on top of Dow foam. I want to sit footing on 1″ or 2″ foam. I can get 25 psi foam and special order 40 psi. my soil is clay and sandstone.  25 psi ok?
Thank you for time and advice, TractorSteve

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  1. Kyle Bentley | | #1


    The Building Science Corp. published a paper on this very subject back in 2014. Give it a read, and I think it will answer many of your questions.

    A traditional three-coat stucco system might weigh as much as 15 lb/ft^2, for reference. I would skip the inner layer of felt, the xps is already water resistant, and any water that gets past the stucco would likely not make it there anyways.

    I'm not certain of what a 1 coat system looks like, but I doubt it'll look 'beautiful' down at the bottom, at least plan on a finish coat.

    I would tape both layers of the insulation. As for the tapes, I'll refer you to Martin's post:

    To know whether 25 psi is good enough we'd need to know what kind of building it is, to get an idea of the weight. Same goes for the soil composition. If it's more clay than sand, it can get pretty soft with just a little bit of water. The design loads for the worst case of wet clay soils is around 1 Ton/ft^2, or about 15 psi. That should still be fine for a range of non-masonry residential construction, 25 psi would be fine in that case. Buy yourself a pocket penetrometer and measure the soil at your site under wet conditions to get an idea of its wet bearing capacity. They're pretty inexpensive.

  2. TractorSteve C | | #2

    Thank you Kyle for your response. I didn't design how i was going to do that on my own, i used articles by Martin and others to figure it out. I think i read some where that Martin said tar paper on foam under 1x4 was over kill but not a bad idea. It will take me some time to get the exterior ready for stucco, moving/ replacing doors and windows, etc., and I was worried about foam degrading from sun and weather.
    On foam under footing , not much weight 8' stud wall with shed roof. soil- sand and clay mix with lots of sandstone rocks everywhere all sizes. I do have a pocket penetrometer if I can find it.
    I agree on foundation one coat stucco and would discuss how to do it with stucco sub. I'm open to other options to protect foundation foam. i did not want run house stucco much below framing line for moisture and vent reasons.
    Still have the questions about Cor-A-Vent and screws.
    If i stagger wall foam would you still tape both sets of foam? Is it ok foam has plastic on both sides? Existing layer has plastic, new layer can be with or without. Tape- I will probably use is 3M All Weather based on Martins article (easier to find).
    Thanks to all who respond.

  3. Kyle Bentley | | #3


    I thought that may have been the case, I just wanted to post the links for more info as people come along and find the question.

    The additional layer of paper makes sense if the foam is going o be exposed for an extended period. I hadn't considered that case. I would still tape the seams in each layer. That's probably one more day of work and a few hundred bucks in tape, and you only get one chance.

    With that load you'll be fine with the 25 psi, just make sure you give the subgrade a firm compacting.

    I think the stucco over the foam will work fine, I would just ask them to ensure that it looks good and matches the top. It might depend on your location, but I think its a good idea to have a break /flashing between the top and bottom stucco layers. That way the one on bottom can be redone later without disturbing the top.

    As far as plastic on the foam goes, are you talking about xps or EPs? I had assumed xps, but I re read your post and I might have misinterpreted it. If it is xps the plastic likely won't matter as it is already impermeable.

    The spacing depends on the screw diameter. A standard #10 screw through 4" of foam (in that article) was good at 12" for quite a bit of weight. I would go with a 12-" spacing for the peace of mind. With a heaftier screw, like the fastenmaster headlok or spax powerlag " you might get away with more.

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