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title=http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing
style='href: "http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing"'><FONT
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face=Calibri>http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing</FONT></A></DIV></DIV>
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<DIV style="TEXT-DECORATION: ; FONT-FAMILY: ; COLOR: ; DISPLAY: inline"><FONT
style="size: 4" size=4 face="Times New Roman">I am always on the lookout for
failures with the new methods insulating buildings that have not been time
tested. The link above has an article that describes buildings in South
Carolina where they found rotting osb sheathing where open cell foam was
installed on the bottom. I understand that open cell foam is an air
barrier, but vapor permeable. However, generally, I had thought, although
now reconsidering, that vapor attached to air was prevented from going
through. This article sites a study that believes the vapor is penetrating
and eventually getting to dew point and condensing. Can vapor condense
inside the foam? </FONT></DIV></DIV>
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<DIV style="TEXT-DECORATION: ; FONT-FAMILY: ; COLOR: ; DISPLAY: inline"><FONT
style="size: 4" size=4 face="Times New Roman">My bet for the cause of these
problems is that the roof shingles and flashing could have delivered bulk
moisture (liquid water) to the surface of the osb. In a typical air
permeable insulation assembly such as fiberglass, warm air is still passing to
dry out such delivered liquid, which protects the structure such that there is
no observed problems. This is one of the problems with truly air tight thermal
barriers. This problem has been observed on SIPs that have siding
installed without a drain-dry space to allow for drainage or evaporation where
wind and rain pushed liquid into the cracks of the siding and no air ventilation
or leaking heat dried it out, causing rotting. </FONT></DIV></DIV>
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<DIV style="TEXT-DECORATION: ; FONT-FAMILY: ; COLOR: ; DISPLAY: inline"><FONT
style="size: 4" size=4 face="Times New Roman">Whether the source of humidity is
vapor migrating through open cell foam, air and vapor migrating through flaws in
the installation, or liquid delivered by flaws in the flashing and roofing,
developing a drain-dry space above the assembly would allow that moisture to
dry. If the source of moisture is determined to vapor passing through the
foam, a closed cell foam vapor barrier would be a superior, though more
expensive choice. I am an advocate for putting the lower priced eps foam,
also air permeable, on top of the roof deck allowing it to stay warm (above dew
point) and putting purlins on top that corrugated metal roofing is installed
on. </FONT></DIV></DIV>
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style="size: 4" size=4
face="Times New Roman">El</FONT></DIV></DIV></DIV></DIV></DIV></BODY></HTML>