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Community and Q&A

Unvented cathedral ceiling stackup

MTTim | Posted in Green Building Techniques on

Martin, Dana, Akos,
as a follow on to previous posts I would like to get your opinions on a new option for  finishing my roof.  First the stackup I used for one main dormer is shown in the attached jpeg – it was based on our previous discussions.
However, due to the difficulty and labor intensiveness of this  approach i was thinking on switching to spray in  closed cell polyurathane foam on top of my 2×6 T&G.  because of the constraint of using the T&G I already have and to meet the same stackup depth i have I would need to spray the poly from top down onto the T&G to a depth of 5 to 6 inches between 2×10 rafters.  Then either leaving a large air gap in the cavity or filling with fiberglass before installing my 5/8th OSB roof sheeting.
I know this is backwards but the spray foam installer didnt think it would matter since the interior air/vapor is taken care of by the spray foam alone.
What do you guys think?

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  1. Expert Member
    BILL WICHERS | | #1

    I wouldn’t trust doing it backwards the way you describe. Everything about an unvented ceiling assembly like this is critical to avoid moisture problems. You do NOT want to have an air gap between the foam and the sheathing. This gap is a cold place where moisture can accumulate, so any leaks anywhere and you’ll have problems. I wouldn’t want to trust everything to be perfect so I wouldn’t trust this assembly.

    Is your only option to work from above? If you can work from below, you can install spray foam in the normal way directly against the underside of the roof sheathing. Done this way you should be ok as long as you install a sufficient amount of spray foam. I’d recommend filling the remaining air space with fluffy insulation if you do this.


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