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Down South cathedral ceiling insulation confusion

Gerald Briggs | Posted in GBA Pro Help on

A little bit confused most everything I find is for northern climates and the little info I have found for my climate seems to contradict one another. Need some help to figure this out.

I’m in zone 2a, central Florida. The house is an octagon in an octagon with the main floor roof about halfway up the 2nd floor/loft walls, sitting on top of the garage/basement. Except for the 3 bedrooms and baths the entire house has cathedral ceilings covered in cedar planks.

I had to pull some of these planks down and did not like what I found, so I pulled the ceiling down in the entire house. I found bunched up insulation, a few places with no insulation and a line of dust on the insulation everywhere there was a seam between the planks, plenty of air penetration. Explains the electric bill. On a good note everything looks good no mold no water damage.

Wife wants drywall put up “tired of living in a ski resort in Florida” so how do I insulate this. It has soffit vents, no top vents but the way its framed only four of the eight bays in each section go to the top, not really any good way to vent out the top.

The rafters are 2×8 24oc and I’m going to strap the ceiling with 1×3 16oc for the drywall. I could go up to 2×3 strapping without problems.

Also most of the a/c duct work is in the crawlspace above the bedrooms so I want to semi-condition this space. Same basic framing but also 2×6 run horizontal from outer walls to central walls for ceiling, going to strap this also as the old sheetrock was sagging.

Spray foam is out and so is anything above the roof deck just had it re-shingled about 3yrs ago.
Any and all help appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    It's an unvented roof assembly, because there are no ridge vents. This type of roof assembly can be insulated either (a) with rigid foam above the roof sheathing, or (b) with spray polyurethane foam on the underside of the roof sheathing.

    The limitations you have listed -- "spray foam is out" and "so is anything above the roof deck" -- mean that you can install drywall if you want, but you can't insulate this roof assembly.

    For more information on insulating this type of roof, see How to Build an Insulated Cathedral Ceiling.

    Your predicament is fairly common. For some unexplained reason, this sequence happens a lot:

    1. Homeowners decide to install brand-new roofing.

    2. One or two years later, homeowners want to improve the insulation in their cathedral ceilings.

    The solution is obvious: homeowners need to reverse the order of (1) and (2).

  2. Gerald Briggs | | #2

    Thank you Martin for the response and the link, I have read that before, but like I said it seems geared more toward northern cold climates.
    "When fiberglass batts are installed in unvented rafter bays, the batts allow moist indoor air to reach the cold roof sheathing"

    Cold roof sheathing and moist indoor air are opposite of what we have here. I'm not saying doing it that way is a good idea just pointing out that most everything is biased for northern climates.

    Two things in that article:

    "First of all, you can’t use air-permeable insulation (for example, fiberglass batts, mineral wool batts, dense-packed cellulose, or blown-in fiberglass) to insulate an unvented roof assembly unless the roof assembly also includes a layer of air-impermeable insulation (either spray polyurethane foam or rigid foam panels) directly above or directly below the roof sheathing."

    "If you want to install a combination of closed-cell spray-foam on the underside of the roof sheathing and air-permeable insulation between your rafters — an approach sometimes called “flash and batt” — the building code requires that spray foam (or, arguably, rigid foam insulation) be “applied in direct contact with the underside of the structural roof sheathing”

    So is there any reason I can't put rigid foam insulation below the the roof sheathing then mineral wool?
    And yes I wish I had known some of this 3 years ago.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Is there any reason I can't put rigid foam insulation below the the roof sheathing then mineral wool?"

    A. That's called the cut-and-cobble technique. For more information, see Cut-and-Cobble Insulation.

    There have been several reports of failures (damp roof sheathing) when the cut-and-cobble approach is used in an unvented roof assembly. Admittedly, however, these reports come from cold climates. If you really want to use the cut-and-cobble approach, you can, as long as: (1) your local code official approves, and (2) you realize that this approach may be riskier than the use of spray foam.

    Note that even in central Florida, the code does not permit you to use only air-permeable (fluffy) insulation in an unvented roof assembly. If you want to combine foam insulation and fluffy insulation in an unvented roof assembly in your climate zone, the foam insulation layer must have a minimum R-value of R-5.

  4. Dana1 | | #4

    Attic venting in the gulf-coast states brings more moisture into the attic of an air conditioned house than it removes. Attic venting is really designed for dealing with moisture problems in cold climates.

    Installing cellulose (but not fiberglass) against the roof deck in an asphalt shingled roof with NO foam is still very safe in a zone 2A climate, even if it's technically a code violation. See the WUFI simulation results in Table 3, which in for R30 cellulose in Houston on p.12 of this document:

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