I am Building an addition for a client with a stucco house. The architect requested that I cover all of the 5/8 OSB with 1 inch foam taped at the seems. The layers will be as follows. 2×6 , 5/8s OSB, Tyvek, 1 inch t&g foam panels, durock panels, stucco.
Does this seem like a solid system or some bad recipe for moisture problems?
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Replies
I want to believe that there should be some type of barrier to prevent the insulation from wicking moisture.
What you are describing is not true stucco, it is a form of eifs. Or maybe difs. That got a real bad name in NC 10-15 yrs ago because of poor flashing and drainage.
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EIFS would put the stucco directly on foam, not on durock.
This is some hybrid of regular stucco over foamboard, using durock instead of a scratch coat.
I think the foam is meant to be a drainage plane in this case, which is weird. My questions would be: how is the durock structurally attached to the sheathing, and why not put a layer of felt behind the durock. Or use denseshield instead of durock?
Or a more accepted overall detail?
k
Edited 1/14/2009 11:16 pm ET by KFC
The architect wants the foam for the thermal value. I am just trying to work some sort of matching stucco over the requested foam. Maybe I don't even need any tyvek. I was just worried about trapped moisture.
I would think you could have moisture problems, depending on how the outside of the stucco is detailed. Typically, avoid shelving, where the stucco has an opportunity to (eventually) crack at a 90 degree turn. This often occurs when foam trim popouts are placed around windows and doors, or a horizontal band is called out. Best case, the walls are 'clean', with no shelving, and can drain quickly.
The OSB (or ply, for that matter) is an effective vapor barrier all on it's own, so what you are creating is a foam-filled cavity between two imperfect vapor barriers. Tricky. I've always thought the ideal was to have a single barrier in any wall, so the moisture has a direction to move in or out. In your case, though you probably have no control over it, I would think the foam should be placed on the warm (interior) side of the wall and the stucco should be applied directly over the OSB.
The Durock is an interesting variation; my stucco guy has always used a 3/8" wire reinforced brown coat followed by the color coat (thick paint). The latter is amazing stuff and I would hate to be drinking water anywhere within 50 miles of the plant that makes it.
Of course, I have no idea where you are or what climate you are in, and that makes a difference, as well.
Best -
I've heard of stucco over cement-board, but have never seen it done. Someone once pitched it to me as a cost-saving method of stucco. How many coats of stucco over the board? I'm guessing two. But then why the board - we do two-coat stucco over foam all the time. I would inquire of the architect if he has used this method before with good success?
It sounds like if there are any moisture issues, it will be moisture from within trapped by the tyvek. Although, you have to wonder why he's taping the foam seams if its over Tyvek?
Is he asking the cement board be taped? Is he spec'ing cement-based stucco, or synthetic stucco?
That is a thick skinned wall! I'm guessing at least 2 1/2" by the time you're finished? Probably require some extra thought on detailing door and window trim and flashing.
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I used the Durock system on my own house 10-12 years ago. It's held up fine. I applied a coat to the taped joints like drywall and then a finish texture coat. I was working by myself and there was quite a labor savings vs. real stucco. Mine was installed over felt over osb, so I've got no input on the foam board.http://www.quittintime.com/ View Image