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Insulation issues

Oliver L | Posted in General Questions on

I have two questions and can’t seem to find a solid answer, just more confusion. First off, I have spent the last couple of years building a home my wife and I designed. We have made some mistakes, mainly in financing the project, so things are moving slow. Anyway, like so many people whose stories I have read I am having issues on my unvented, low slope roof ( sorry to beat a dead horse). I have one level home with a couple of wings. The first wing has been done to what I thought was the way to go…the gable ends and the underside of the sheathing were spray foamed with about 1.5″ closed cell followed by 12.5 inches of blowin fibreglass, and then covered with Membrain vapour barrier. I had different guys come in to spray the foam on my next wing, but they refused to do the underside of sheathing with foam, stating that they would not guarantee it. They said it needs to be packed with cellulose and covered with plastic. I thought the foam kept the humidity off the sheathing? …or is 14″ of cellulose enough to keep anything from reaching the sheathing? It would be way cheaper for me without the spray foam. 
My next problem is moisture on the inside of my wall sheathing during winter. We are living in part of the home as I work on it and I have noticed moisture and some ice on the upper parts of the sheathing. I seems to have dried now, but is this a normal occurence for a home( the walls are 2×6 with fibreglass batts and OSB on the exterior)? Should I be looking at adding something to the exterior?
I live in southern Quebec btw.
Thanks

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Replies

  1. Expert Member
    Akos | | #1

    Quebec is somewhere around Zone 5/6. I would check here for the correct amount of spray foam for your roof.

    https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

    Bottom line, an unveted cellulose only roof is a bad idea. 1.5" of ccSPF plus 12" of cellulose is also not enough. You need closer to 4" to 5" of SPF bellow your roof deck.

    The spray foam guy is wrong. There is nothing wrong with spraying the underside of your roof with sufficient amount of closed cell spray foam for condensation control (the link above) and filling the rest of the cavity with cellulose.

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