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existing cathedral ceiling insulation

Jerry_Lammers | Posted in Energy Efficiency and Durability on

I am in climate zone 2a,(at the boarder with 2b) and am working on an existing 1930’s house that has a cathedral ceiling with what once was a “vented” roof system. A second floor was added 10 years ago that extended the roof plane above the ridge and blocked the ridge vents. The roof construction is white composition shingles, ice and water shield underlayment, 1×6 t&g roof deck, 2×6 framing, r-19 batt insulation, 1×3 strapping and a 1/2″ finish plywood ceiling. The roof is less than a year old, so I am trying to avoid adding insulation on top of the deck.  The renovation calls for a new gyp bd ceiling.  Here’s my question: Can I convert the system to a unvented system by leaving everything in place, blocking the eave vents and adding a Polyiso insulation board to the interior of the ceiling before the gyp bd goes up? or do I need to rip it all out and add spray foam.  Thanks,

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  1. Expert Member
    BILL WICHERS | | #1

    No, you can't trust polyiso for this. You're also likely to have plenty of connections between that ceiling area and other parts of the home, which means lots of air leaks between them. You're likely to have moisture problems.

    You'll probably need to use closed cell spray foam applied to the underside of the roof sheathing here. That's likely to be your only safe option. I have a very similar cathedral ceiling in my own home that couldn't be vented properly (it's very low slope and ends low down into a vertical wall on the upper level), and ccSPF was my only practical option and has worked well.

    This is one of the two big niche areas where spray foam is really the best option, the other niche being irregular walls like those with stone foundations.


    1. Jerry_Lammers | | #2

      Thanks Bill, we'll go with the spray foam

  2. Jerry_Lammers | | #3

    Thanks Bill, we'll go with the spray foam

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