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Old 02-23-2015, 06:31 AM   #16
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Gary,

Obviously I misunderstood your recommendation.

We have, especially in this homes that will be going through a re-roof at some point in the next 3-5 years, have skinned the underside of the decking with a layer of XPS. The two most recent examples I am thinking of were homes that were equip with decking boards and felt. I have seen spray foam jobs that, even with 2lb foam, allowed the foam to seep out from behind the shingles.

Couple that with the fact that the the removal of the roof (especially in the decking board case) can disturb the adhesion of the foam even without any decking board removal. Walking the roof, you can clearly see where the foam has become loose in certain locations by pushing your fingers through the gaps in the sheathing.

It is a PITB as compared to just spraying the roof but we push a layer of foam tight to the underside of the decking and spray over the top of that. Seems to allow foam to lock tight to the foam and the rafter bays without getting messed up if we have to remove the roof or some sheathing.
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Old 02-23-2015, 12:56 PM   #17
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So you install board insulation into the rafter cavities as tightly as possible then spray foam over that?
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Old 02-24-2015, 06:49 AM   #18
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That is what we have done on previous projects.
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Old 02-24-2015, 04:06 PM   #19
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WoW, yes, I see that now. One of the biggest perks of ccSPF is the air-sealing ability of the product. That, plus the moisture barrier if 2" thick. Using rigid board we get similar moisture perm ratings but not the air sealing/water vapor qualities because it is not fully adhered to the roof sheathing. Using a sill sealer or fabric/poly, etc. between the boards and SPF gives air a chance to leak and condense moisture there- because it is not bonded (as SPF) directly to the boards but is only in somewhat contact with them. I could see where canned foam would be beneficial if applied in a closed-grid pattern to stop air-flow; similar to basement foam board on concrete walls. The failure of SPF in that link was wet OSB; anything that gives air a chance- same as too fast an application leaving gaps or de-bonding/shrinking from the roof sheathing in bad applications. When your insulation sub SPF's and doesn't have the bond required (as per link)- with fanfold/rigid board/board replacement later- does he just leave it that way? How does he handle that, I don't see a way around it- unless he figures the side foam on rafters is enough, is why I'm asking. The article didn't mention wet rafters also- just the OSB.... I need more research.

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Old 02-25-2015, 08:04 AM   #20
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With the exception of any solar vapor drive on the shingles in the summer months, I don't really see an opportunity for moisture to enter and/or condense on the back side of the sheathing in the winter months if you spray over a sacrificial layer as I mentioned before.

The foam board (depending on facing) could provide all the vapor perm rating that you require to keep moisture out of that assembly and would even allow you to spray OC SPF in that case. OC SPF is preferable if you ask me and is plenty air barrier to provide the proper seal. I see less pull back from OC foam as well when compared to closed cell and its application is far less critical than CC foam.

I have seen enough homes burned down with CC SPF that I will steer clear of it when I can.

For the record, I fully concede with BSC and the reports say. I also know that your recommendations are spot on. I just don't like CC SPF.
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