Dow blue board under OSB sheathing before stucco??


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Old 05-11-14, 09:39 PM
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Dow blue board under OSB sheathing before stucco??

Well just as the title says, can I install the 3/4 dow blue board under/first to studs then install osb for the use of stucco?? Will I have to have it re engineered?? Will the building inspector be ok with it??
 
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Old 05-12-14, 05:05 AM
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Is this new construction or re-model? One concern that comes to mind is your windows & doors. How will you accommodate the extra wall thickness?
 
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Old 05-12-14, 10:41 AM
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New construction. Doubt 3/4 blue board would cause that much of an issue. Videos show windows being installed over foam.
 
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Old 05-12-14, 11:06 AM
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Depends on what windows are being installed, how the interior will be finished, and if jambs will be made onsite. If you increase the wall thickness, doors will need thicker frames also.

The foam under the OSB isn't an issue since longer fasteners can be used for the windows...but I wonder about the effect of trapping the OSB between 2 waterproof materials (depending on how the stucco will be applied). The only type of stucco I'm familiar with is tarpaper over the sheathing, then wire mesh, then stucco. That would seem to be 2 barriers?
 
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Old 05-12-14, 12:03 PM
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Inspectors usually want to see wood sheathing nailed directly to the studs for shear value. This is often at the corners of a building, and every so often on long buildings or where load paths or shear strength requires it.

Also, many professionals will not use OSB under something like stucco because OSB tends to swell with changes in humidity... and that can crack your stucco finish. They pay the extra $$$ and use an exterior rated plywood instead.

Because of that, I'd say that you'd want to do it the other way around. Plywood sheathing on the studs, then foam sheathing and stucco on top of that. Some codes still require 2 layers of 30 minute paper, some will make an exception for one layer of 60 minute paper.

Having the foam on the outside will place a vapor barrier on the outside, which is fine in a hot humid climate. Generally in Texas, that's only along the coast. It's done in many other climates without ill effects.

Your new construction windows should have a nailing fin (there is also flush fin, often used in stucco installation) that will get installed on top of the outer layer no matter how you look at it, and like Vic said, moving the window out farther just requires a wider jamb (or drywall return).
 
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Old 05-12-14, 12:09 PM
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What I've seen on new construction is like X described, OSB/plywood nailed directly to the studs and then the foam board with the lath/stucco over it.
 

Last edited by marksr; 05-12-14 at 01:51 PM. Reason: fix typo
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Old 05-12-14, 01:22 PM
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I wanted the foam to break the thermal bridge. So what if I did something like a home made sip panel, osb, foam, zip panel?? I was talking to a guy just 5 minutes ago that said the zip panels were better than osb for stucco application.
 
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Old 05-12-14, 02:23 PM
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The foam provides a thermal break no matter where you put it. Since you can apply the stucco right to the foam, adding another layer of sheathing on top of the foam would be pretty pointless.

Zip system sheathing is like OSB but with a WRB coating sprayed on. Most code still requires the 2 layers of 30 min felt under stucco no matter what you use.
 
 

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