Wall construction questions — stucco
We are building a house in Asheville NC, climate zone 4A, 2,600’ elevation. I’ve read many GBA articles on stucco and wall construction – all good stuff. We will have stucco exterior walls on our house with at least 3’ overhangs, and we are considering the following wall construction:
• Drywall
• Intelligent membrane (do we need one)?
• 2” x 6” stud bays filled with either Mineral Wool, or blow in Cellulose
• Huber 1-1/2” Zip R Sheathing
• Benjamin Obdyke 10 mm Rain Slicker
• Water resistive vapor permeable barrier (which is best)?
• Stucco Lathe
• Hard coat stucco
We have three questions on the wall details which I hope you can answer for us:
1) Do we need an intelligent membrane beneath the drywall on the inside of this wall structure?
2) What should we use for the Water resistive vapor permeable barrier?
3) Is it wise to apply a self-adhering membrane under the weep screed, bridging the gap between the Zip R Sheathing and the concrete foundation?
Thanks in advance!
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Replies
With the ZIP-R sheathing properly installed you won't need the "Water resistive vapor permeable barrier", since ZIP uses a factory installed high quality spray on weather barrier.
It's not clear that Rain Slicker is needed or even desirable under the stucco lath. An air space would be good enough. There are stucco-specific permeable underlayments used in commercial construction that may be more appropriate, but all will wick more moisture and impede drying air flows more than a decent air gap. Installing 1x furring and mounting metal lath for the stucco on the furring is probably the best bet. That's a 19mm nominal gap, but with the variable flex of the lathe and the scratch coat squeezed through it'll be on the order of 10-12 mm air over much of it. Air is a much better capillary break than any mesh product!
A zone 4A climate doesn't need a smart vapor retarder on the interior, just reasonably air tight drywall with standard latex paint is fine.
My stand-by wall assembly is:
• Drywall
• 2x6 studs @ 24” o.c. filled with dense packed cellulose (You don’t need intelligent membrane)
• Regular taped Plwd/OSB + 1” taped polyiso (cheaper than Zip-R with 1” foam). Your foam if taped with approved tape is a moisture barrier.
• (Good) StuccoWrap and 60 JumboTex or (Better) HomeSlicker and 60 JumboTex
• 3-coat Stucco system
Here is my opinion on Zip-R panels I expressed on the previous question:
When it comes to Zip-R panels, not only is more expensive than regular OSB + foam, but I’m of the opinion that the foam on the Zip-R panel is on the wrong side of the sheathing, which places a much greater emphasis on quality control on the Zip tape installation. The reality on jobsites, in all states I work, is that it is very common for installers (framers) not to use the roller or to use it incorrectly. Installers miss-align, twist and miss-joint the tape constantly. Tape is installed weeks after sheathing installation. Sheathing installation was deliver in the mud or soil, not covered and weathered before installation.
Now, exactly the same can happen with regular taped OSB, but you get a second chance to cover the panel joints with taped rigid foam on top and with staggered seams. You can choose 1” outsulation boards with R-values from 5-6.7. Check on recycled rigid foam, it’s common and cheap.
Nancy,
Q. "Do we need an intelligent membrane beneath the drywall on the inside of this wall structure?"
A. No.
Q. "What should we use for the water-resistive vapor-permeable barrier?"
A. Ether asphalt felt or housewrap (or both) are options.
Q. "Is it wise to apply a self-adhering membrane under the weep screed, bridging the gap between the Zip R Sheathing and the concrete foundation?"
A. Yes.
Benjamin Obdyke recommends Slicker MAX 6mm for Stucco. There's also Delta Dry (Austin builder Matt Risinger is a fan of the product for stucco applications). I think either is a good idea for Western NC with the amount of rain you get (I grew up not far from Asheville).
Whether you go with ZIP-R or OSB/Tyvek/Foam it's only as good as the install.
10mm is probably overkill in your situation, but do what you feel comfortable with. MTI manufactures a product called Sure Cavity which has rigid vertical drainage channels and mortar blocking fabric. It lays flatter than entangled mesh type products. With the mortar blocking fabric you don't need the "water resistive vapor permeable barrier" between the lath and the drainage mat to keep the air gap clear, and the taped Zip sheathing functions as your WRB.
Recommend ventilating your wall system (vent at top, weep screed with large slots for draining and ventilation at the bottom) to make the most of the air gap. Here's an example: Thin Veneer to Top Vented Wall