Is stucco finish practical on advanced framing?
Is there a way to make 1″ stucco over 1/2″ plywood a good assembly over 24″ c-c 2×6 framing? I have designed a 2 level house with the idea that advanced framing (24″ c-c, single top plate, minimal studs) will improve energy efficiency (the energy calculations show that to be true), but my conversation with the lath and plastering industry rep indicates a plaster sub might refuse the job because of concern over excessive deflection from wind (>1/360 of span), and also I find that UBC generally only fire-rates plaster on 16″ c-c spaced studs (this project does not require a rating, but it could be an indicator of reasonable limits). I am having second thoughts about advanced framing, even after having argued long and hard with the GC.
We are using engineered lumber LVL’s for the studs because of wind load on some walls roughly 10′ high. The wind is not extreme; we are in a high seismic risk zone in California.
Is this an instance when advanced framing is a bad idea?
Brian
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Replies
If all that concerns you about using the advanced framing is the stud spacing I'd go ahead. For decades the standard detail for stucco walls here in BC was 2"x6" at 24" oc with 7/16" OSB. Many of those wall subsequently had problems, but they were unrelated to the wall framing.
Malcolm,
Thanks for the comment. My main reason for advanced framing is to improve thermal performance--the contractor states the extra studs are not enough to make a difference in cost and the carpenters would be happier and more efficient with traditional 16 c-c. and double plates. I sure don't want to follow something different and then have a client unhappy with cracking. As for "other problems" I intend to seal the insulated cavities against air leakage, am using plywood not OSB, and will pay attention to WRB - bldg paper & flashing workmanship details, which I think would be the main causes of mold issues. Is there a bugaboo other than that?
The plastering industry rep suggests a "buffer' of high density foam over the plywood (and under the paper) would somewhat protect the stucco from wind-caused deflection and cracking.
Brian
While I have had a fair amount of experience using stucco on both residential or commercial projects I can't say I've ever had wind loading brought up as a common cause of cracks. Much more common are improper curing and lack of control joints. Perhaps wind can be a problem, but I simply haven't heard of it being one that generally infuences wall design.
If I were specifying stucco I would use a proprietorial rain screen product, something like Delta Dry, to create a capillary break and keep my sheathing safe.