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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'>I like the term predictive failure that was used to describe roofing technology. Mostly in reference to commercial flat roof construction but it makes sense. What it basically means is that we can assume that each material in an assembly will be compromised to some degree. The degree of failure depends on the materials ability to cope. Moisture studies in plywood and osb have shown basically that plywood can withstand repeated wettings and dry significantly enough to cope. OSB can be more moisture resistant but once saturated has a limited ability to transport the moisture out. This basically means that OSB has to be fully protected from moisture – meaning a fully adhered waterproofing membrane over (one of the conclusions or recommendations from a Building Science study) and a thick enough open cell foam barrier inside to act as a vb. (if relying on open cell foam – minimum 4” usually). Still a shaky assembly if the water barrier or vapour barrier is compromised you have a material that will fail in the middle. I’ve started liking engineered wood product but don’t think osb should be used on a roof.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'>Plywood is different in that it transports moisture to exterior faces. My thought is that in such an assembly it would be a benefit to have an open cell foam – but that is just a thought as I’ve not seen any moisture studies on this.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Greenbuilding [mailto:greenbuilding-bounces@lists.bioenergylists.org] <b>On Behalf Of </b>conservationarchitect@rockbridge.net<br><b>Sent:</b> January-22-14 8:01 AM<br><b>To:</b> Green Building<br><b>Subject:</b> [Greenbuilding] Open Cell Failure<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><div><div><div><p class=MsoNormal><span style='font-size:13.5pt;font-family:"Calibri","sans-serif";color:black'><a href="http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing">http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing</a></span><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'><o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'> <o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:13.5pt;color:black'>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? </span><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'><o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'> <o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:13.5pt;color:black'>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. </span><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'><o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'> <o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:13.5pt;color:black'>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. </span><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'><o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'> <o:p></o:p></span></p></div></div><div><div><p class=MsoNormal><span style='font-size:13.5pt;color:black'>El</span><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:black'><o:p></o:p></span></p></div></div></div></div></div></div></body></html>