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Wall assembly with XPS foam and stucco

I am looking for help with my exterior wall assembly. This is for a house with Stucco exterior and XPS foam. We are building a house in Climate Zone 3 (SF Bay Area). Required wall insulation here is R13, but I would like to do better and add 1 inch XPS foam to increase R value and eliminate thermal bridging.

I am confused about the appropriate wall assembly after reading several articles on GBA and other sites. Code requires "2 layers of Grade D building paper" over the OSB/Plywood.

Here are two wall assembly options I have in mind:

Option A:
1. Drywall
2. Blown-in cellulose insulation between 2x4 studs
3. OSB or Plywood sheathing
4. Grade D building paper
5. Tyvek Drainwrap
6. 1 inch unfaced XPS foam
7. Stucco lath nailed through foam into studs
8. 3-coat stucco installed directly over foam

Option B:
1 + 2. Same as above
3. Zip-system wall sheathing
4. Tyvek Drainwrap
5. 1 inch unfaced XPS foam
6. Stucco lath nailed through foam into studs
7. 3-coat stucco installed directly over foam

Here are my questions:
a. Which wall assembly Option is better?

b. Is there a better way to do the wall assembly? Did I miss something?

c. Is installing Tyvek Drainwrap over Grade D building paper ok? Code requires "two layers of grade D paper" and Tyvek counts as a substitute for one layer, so I need one additional layer. However, are there any issue installing Tyvek Drainwrap over a layer of Grade D paper?

d. What if I use regular Tyvek Building Paper instead of Grade D paper (so, Tyvek Drainwrap over Tyvek regular building paper)? Is it better? Any issues?

e. Should I use Plywood instead of OSB? It will cost about 60% more. Not sure if it makes sense in my case.

f. Can Tyvek Draiwrap be installed over Zipsystem sheathing, or will moisture get trapped between the two WRB layers?

g. Which is better from a moisture control perspective: Zipsystem or plywood+tyvek

h. What foam brand is recommended? I am considering the unfaced version of Foamular 150 board. Does that sound right? What other brands are out there?

i. Should the foam boards be installed with spacings to allow for contraction/expansion? Or can they be installed flush next to each other, possibly using the T&G flavor?

Appreciate any advice the community has.

Please note that this house is located in a Climate Zone 3 area with low humidity and low rainfall (16 inch precipitation/year). I did read the "install stucco right" article but feel that using furring strips over the foam and between the stucco is overkill for my region. The stucco will rarely get wet . On most other houses stucco is simply install over 2 layers of Grade D paper without any problems.

Asked by Thomas Schulz
Posted Mon, 12/16/2013 - 00:17
Edited Mon, 12/16/2013 - 09:40

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2 Answers

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Thomas,
In my view, installing stucco on a wood-framed house is risky unless you include an air gap behind the stucco. If you decide to proceed with your approach in spite of this advice, suffice it to say that you were forewarned. (For GBA readers who haven't seen it, here is a link to the article explaining my reasoning: To Install Stucco Right, Include an Air Gap.)

As you point out, the code requires that stucco be installed over two layers of Grade D paper. You are suggesting two alternate approaches: one that uses Grade D plus Tyvek, and one that uses Zip System sheathing plus Tyvek. To get these alternate systems approved, you have to talk with your local building inspector. It doesn't matter what GBA readers think about this kind of code interpretation; the only ruling that matters is the one you get from your local building inspector.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/16/2013 - 09:37

2.
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Thomas, I have been researching a similar wall assembly for a stucco exteror house, albeit in a very different climate, Zone 5. Have read as much as I could on the issue and got good info here at GBA as well as buildingscience.com, among others. My 2 cents and personal bias, stay away from OSB. Many will say if the wall is designed right it should work. I strongly feel OSB under stucco is a bad idea. Too many opportunities for error and failure potential. Ideally I had sought to avoid all natural products in the assembly altogether. Keeping to a failure resistant wall, ie ICF, CMU, metal framing (but thats another issue). Plywood, WRB, insulation and an air gap would be the most conservative approach. I like the Korax system previously pointed out by Martin; http://www.koraxtech.com/pages/panel.html but have yet to price it.

Answered by Sal Lombardo
Posted Mon, 12/16/2013 - 18:44

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