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Air gap between cathedral roof insulation and inner wall

donarntz | Posted in Energy Efficiency and Durability on

Hello, thanks for entertaining my question. I need to obtain r-38 in my cathedral ceiling (really an A-frame), and the rafters are 11 1/4 deep. There will be 2″ of closed cell foam against the inner roof deck. Can I leave a gap between the foam and 6″ of fiberglass and drywall?  If so would I use something like simpson strong tie insulation support?

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Replies

  1. user-2310254 | | #1

    You don't need a gap and really should fill the bay to maximize the r-value of the roof (wall?) assembly. See Mike Maines article for more info: https://www.greenbuildingadvisor.com/app/uploads/sites/default/files/Why%20Flash%20and%20Batt%20Makes%20Sense.pdf.

    If you leave a gap, one of the experts may be able to advise on whether this will create a bypass and whether that might be an issue.

    1. donarntz | | #2

      I should clarify the gap would be from the fiberglass to the drywall, not between the closed cell foam and the fiberglass. Otherwise I need to put 11" of insulation over the foam to make it flush. Since I have 3200sf of roof/wall, that's a lot of insulation.

      1. Expert Member
        AKOS TOTH | | #3

        Gap between drywall and insulation is not an issue. With a tall wall like that you might still get a fair bit of convection, so spending a bit extra on high density batts would be not hurt.

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