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Cathedral ceiling insulation

John_Hunt | Posted in General Questions on

A quick clarification needed to settle my mind, I am hoping I solved my condensation problem. I am redoing my leaking cathedral ceiling which had R30 Roxul, no vents and tongue and groove pine. I am in northern VT and as you can imagine I was getting condensation on the underside of my roof deck.

I removed my tongue and groove (easier than expected), pulled the insulation down flush to the rafters and vented behind my Roxul (2×12’s so I had room). I then added 2″ of felt faced Polyiso flush across my rafters, taped and foamed for a tight air seal. Next put up new strapping and reinstalled the tongue and groove. Soffit and ridge vent installed.

Should I be all set with this fix?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should I be all set with this fix?"

    A. Well, since you are asking after the fact... you had better hope so.

    You now have a ceiling that is much better than the one you had before. It still doesn't meet minimum code requirements for R-value, but at about R-42 or R-43, it's closer to code minimum (R-49) than you were before.

    I hope you used a quality tape. Depending on the texture of the felt facing on the polyiso, the polyiso seams might be hard to tape. If you used a good tape like Siga Wigluv or Zip Sheathing tape, you'll probably be OK with your air barrier. (Next time you do it, use foil-faced polyiso -- foil facing is the easiest type of facing to tape.)

  2. John_Hunt | | #2

    Thank you, Martin

    I have half of my ceiling done and had a second guessing moment, that's the reason for the late question.

    I am confident in my air seal, I am using Dow Weathermate tape and it has stuck very well. Is there any reason to believe this tape wont hold up long term?

  3. GBA Editor
    Martin Holladay | | #3

    Dow Weathermate was a disappointing performer in my backyard tape test.

    That said, I have not tested Dow Weathermate on felt-faced polyiso.

    For more information on tape testing, see:

    Backyard Tape Test

    Return to the Backyard Tape Test

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