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Community and Q&A

What kind of foam, and where?

brentwilson | Posted in Energy Efficiency and Durability on

I am still trying to finalize what the make up of the 5:12 pitch cathedral roof should be for my parent’s retirement in climate zone 6B.

At this point, I am thinking to go with a vented roof. Since the rafters will be I-joist style, I am thinking I can use rigid foam to form the vent baffle areas by cutting it to go from I-joist web to I-joist web. This means the wider part of the I-joist will form the spacing for ventilation. With a 14 inch I-joist type rafter, I could do two inches of rigid foam to form the ventilation channel, then I would have a bit under 11 inches for dense packed cellulose. I think this would give an r-value of around r51, right? If I want to get to a pretty good house standard of R60, I think I would need about another 1.5” of rigid foam. Can I add that below the rafters? Any issues with sandwiching cellulose between two layers of rigid foam? Would it be better to use 2.5” of rigid to form the vent baffle and only 1” below the rafters? Would this make it easier to attach T&G ceiling below the foam? With rigid foam below the rafters, do I still need taped and mudded drywall between it and the T&G? What kind of rigid foam should I be using in these applications? The same type both above and below the cellulose? Or different types? Do I need an air and/or vapor barrier sheet in this assembly? If so, where?

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  1. Expert Member
    Akos | | #1

    I would check your local code about vapor barriers. Ours requires one on the warm side, this could be the foil faced insulation under your T&G. The foil faced insulation is also much easier to tape as your air barrier.

    For the baffles, you want something vapor permeable, there should be a way for any moisture that makes it through your ceiling to make it to the vent channel. This could be unfaced EPS, felt faced polyiso, OSB or fiberboard.

    If you are dense packing, I would talk to the installer as to what they prefer. Not all sheet goods will work as some will tend to bow. I think you can loose fill up to 6/12, might be easier/cheaper than dense pack.

    You'll have a hard time hanging T&G over 1.5" foam as the nails won't get much embed. You'll probably have to strap it out with 1x3 and nail the T&G to that.

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