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Community and Q&A

Air gap in unvented cathedral ceiling

Nilsk | Posted in Energy Efficiency and Durability on

I’m in the midst of framing a small cabin in climate zone 6. The cabin will have an unvented cathedral ceiling that I am trying to keep to a minimum thickness while still hitting R-49. Because of snow loads I need to use 2×8 rafters or larger. I’m planning on using enough rigid exterior foam to meet at least 51% of the roofs R-value (ideally not much more than R25). Because I am trying to keep the roof as thin as is responsible, I was wondering if it is acceptable to not fully fill the rafter bays with insulation. I had planned on using rock wool and was wondering if I could use 5.5inch rock wool with a small air gap either above the sheetrock or below the roof sheathing. It seems as though installation would be a pain but would this air gap be problematic? ┬áIf there are more elegant solutions to this I would love to hear them as well.
Thanks in advance,
Nils

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nils,
    If you are planning to insulate your roof assembly with a combination of rigid foam above the roof sheathing and fluffy insulation (like mineral wool) between the rafters, the building codes require (and building scientists recommend) that the fluffy insulation be in direct contact with the roof sheathing. So you can't have "a small air gap below the roof sheathing."

    That said, "a small air gap above the Sheetrock" is acceptable.

    More information here: "How to Install Rigid Foam On Top of Roof Sheathing."

  2. Nilsk | | #2

    Martin,
    Thanks for your quick reply. So while it may be a pain to install it is acceptable to have an air gap above the sheetrock.
    Any recommendations for an easy way to achieve such a gap?
    Cheers,
    Nils

  3. Expert Member
    Dana Dorsett | | #3

    An R25 fiberglass batt is nominally manufactured with 8" of loft, and will be a compression fit into a 2x8 cavity with no gaps or void, and for that reason it is preferable to an R23 rock wool solution with potential gaps/slop/sag.

    Compressed into a 7.25" cavity of a 2x8 rafter bay it's performance is R24, so with R25 above the roof deck you'd meet code on an R-value basis.

    That would beat code minimum performance by a bit on a U-factor basis.

    1. Nilsk | | #4

      Dana,

      Thanks for this suggestion, using 8" fiberglass batts sounds like an easier and more elegant solution.

      Cheers,
      Nils

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