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More metal roof over unvented cathedral ceiling

Rich Feldman | Posted in Green Products and Materials on

This is a follow-up to my own question, posted earlier on 9/30. For some technical reason, the Q&A tab and its content, and the Answer button, became inaccessible when I logged in again. Even registering with a new username, and signing up for 10 day Pro trial, did not help.

So here’s my response as a brand new question. The worry is about unvented cathedral ceiling under a permeable shake roof for 50 years, about to be replaced with new OSB + felt + standing-seam metal roof.
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Thank you, Martin. Here’s some more information.

Our concern has been about ventilation and condensation, much more than about R-value. It never snows here, and we get only a few freezing nights in a typical winter.

I am feeling less desperate after more physical investigation. The foil-faced foam blocks are placed between the rafters, and are only 4 1/2″ thick. There is a whole inch of space between them and the roof deck. The challenge seems to be a few bits of wood that stand in the way of a continuous soffit-to-ridge plenum.

There is evidence that the roof was originally built with planking proudly exposed to the interior. That could explain the high grade 2×10 T&G, over heavy rafters 48″ o.c. and spanning about 16 feet along the slope. Drywall and insulation were a later addition, that includes a 2×6 joist halfway between each pair of rafters. The joists extend all the way up to the ridge, with perpendicular 2×6 blocks a few inches below for drywall nailing. The bottom end of each joist hangs from a 2×6 that goes from rafter to rafter, a couple inches inward from the exterior wall.

Sure wish those blocks were 2×4’s. They go all the way up to the T&G, but can be accessed for notching or drilling through. At the ridge, we remove the topmost T&G board on each side. At the bottom, we first have to saw or drill through painted blocks between exposed rafter ends, at the top of the exterior wall.. If those holes don’t let us see from soffit to ridge, because of extra blocks midway, we’ll get creative.

Does that all make sense? I’d welcome guidance about the cross-sectional vent area that’s needed to be useful. Numbers like 1/300 of the permeable drywall area have popped up.

Thanks again, you all.
Richard & Stefanie

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rich,
    Here's a link to your previous Q&A thread:
    http://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/28749/new-metal-roof-over-unventable-old-cathedral-ceil

    Unless the foam blocks were very carefully air-sealed, they probably aren't very effective as insulation. The best way to install rigid foam insulation is as a continuous (uninterrupted) layer.

    Considering the fact that your existing roof is poorly insulated, using a method that is not recommended, and considering the fact that you are about to install new roofing, the best way to proceed would be to install a new layer of rigid foam above the existing roof sheathing. That way you'll get a real insulation layer -- and you won't have to worry about ventilation, because you will be building an unvented roof assembly.

    For more information, see How to Build an Insulated Cathedral Ceiling.

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