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Cathedral Ceiling with Tongue-and-Groove Finish

jwalker1511 | Posted in Building Code Questions on


I haven’t been able to find an answer to this question. I am building a new house, zone 6 southern Maine, it will have cathedral ceilings, and I was considering doing tongue and groove instead of drywall ceilings. Roof assembly from outside in is: asphalt shingles, underlayment, 1/2″ plywood, 5″ closed cell spray foam against sheathing, remainder of 2×12 (and some areas 2×10) cavity filled with dense packed cellulose, Siga Majrex, 1×4 strapping, then finish layer. Can I lay the tongue and groove right against the strapping over the Siga Majrex? Or do I need sheetrock for a fire barrier?

Thank you for any help!!

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  1. Expert Member
    PETER Engle | | #1

    For single family residential construction built to IRC requirements, there should not be any requirement for drywall as a fire barrier. For this to be a durable system, it is important that the Majrex be properly taped and sealed to make a complete air barrier.

    1. PBP1 | | #2

      Is that correct given "5″ closed cell spray foam against sheathing". In zone 6, have flash-n-batt (4" of closed cell spray foam) with tng and nothing between the batts and the tng. The only issue that arose between tng and drywall was the size of the beams, the engineer wanted larger beams (or more closely spaced) if I went with finished drywall on the ceiling to be more stable under snow load, as drywall was deemed to be more "fragile" to any movements. With 5" of closed cell spray foam, a cold condensing surface may be unavailable. My energy consultant said no air barrier between the batt and tng.

      1. Expert Member
        PETER Engle | | #3

        You are correct. I was focusing on the fire code issues and missed the 5" spray foam part. I thought this was a "flash and batt" type installation. With a ratio of 50-50 or so, there should be no condensation on the face of the spray foam. The spray foam, if properly applied, provides control of both air leakage and water vapor. In this installation, the Majrex really only serves to hold in the cellulose. You could probably find a cheaper alternative to the Majrex. No drywall is required for fire protection in most cases. The prescriptive code requirement for spray foam in a roof assembly allows for the use of T&G planking as the thermal barrier.

        1. jwolfe1 | | #4

          Peter, what would be some of your suggested cheaper alternatives to Majrex in a tongue and groove cathedral ceiling application? Thanks.

          1. Expert Member
            PETER Engle | | #6

            Any of the standard cellulose netting products would do just fine. Insulweb, Propac, etc. A quick search shows Siga Pajrex costing $.55/sf to $.60/sf. Propac is closer to $.45/sf. Not a big cost driver for the entire house, but every little bit helps.

  2. jwalker1511 | | #5

    Thank you Peter for your reply, very helpful!!

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