[Greenbuilding] Open Cell Failure

[conservationarchitect at rockbridge.net]

http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing
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?  
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.  
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.   
El
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