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Old 09-28-2011, 09:06 PM   #16
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Synthetic vs felt over wall sheathing


ill handle this...don't you have a bar to tend?

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loneframer (09-28-2011)
Old 10-02-2011, 04:35 PM   #17
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Synthetic vs felt over wall sheathing


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Originally Posted by Tom Struble View Post
all hypothetical...show me damage not caused by improper flashing details

osb can handle occasional wetting[as long as it has a chance to dry]much better than ply
I agree you need to properly flash. And I agree with OSB handling occasional light wetting, but not of any magnitude. And not repeated wetting. It has it's place.

OSB is slow to wet and even slower to dry. You really should use a rainscreen or at least a “draining” housewrap to get dispersing of the inward bound water past certain sidings: http://www.buildingscience.com/docum...d%20in%20walls OSB acts like a vapor barrier, stopping most of the inward/outward moisture drive, unlike more permeable plywood sheathing. “Back-vent your cladding and be happy. How much of an air-gap? We know from experience that 3/8 of an inch is a pretty safe dimension4 with stucco, manufactured stone veneers, wood claddings or other claddings like fiber-cement that lie flat against the housewrap and OSB (Photograph 3 and Photograph 4). Typical vinyl and aluminum siding have plenty of air-gaps behind them without having to do anything.”

With any SPF inside or rigid foam outside the OSB, be certain to include the draining housewrap or an air space, not to stop the moisture at the OSB face.
Look at the permeance of OSB and plywood at 100% humidity—plywood is 4 times more vapor open to dry out in ½ the time as OSB- pp.3, fig.#2: http://www.eima.com/pdfs/The%20Perfe...r%20Stucco.pdf


Despite OSB's readiness to absorb liquid water, the National Research Council Canada team determined that it is about 100 times less permeable to water vapor than plywood. When laboratory dishes partially filled with water were set upright and sealed at the mouth with samples of the two panel materials, the water evaporated from the dishes capped with plywood.
The OSB-covered dishes, by contrast, still held water eight months later. Interestingly, the undersides of all of the OSB samples in the vapor permeability test developed mold growth, while fewer of the plywood samples were affected.
Nofal speculates that OSB's impermeability and apparent attractiveness to mold may be a factor in building envelope failures associated with EIFS. Although such failures have also occurred with plywood, Nofal observed that OSB failures tend to be more severe. "If you get liquid water inside the wall," he says, "plywood will buy you some time as it dries out. OSB grows mold within a week.” From: http://docs.google.com/viewer?a=v&q=...LNhzTCKBYStvcg


Take into account the risk factors of using OSB over plywood for each individual project; http://www.mastercodeprofessional.co...vs_Plywood.pdf

When using OSB, add a high perm housewrap or felt (varies according to wetness), rather than a low perm one, pp.13: http://repository.tamu.edu/bitstream...pdf?sequence=4


Sorry if this is all over the board, from something I'm working on. Most doesn't pertain to the Zip system, just OSB in general, for those interested.



Gary

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