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Insulating a Non-typical Cathedral Ceiling

Steve Robertson | Posted in Energy Efficiency and Durability on

Insulating a normal cathedral ceiling has been covered by GBA in Martin Holladay’s How To Build an Insulated Cathedral Ceiling .

This covers insulating vented and unvented typical cathedral ceilings. A “typical” cathedral ceiling refers to a symmetrical cathedral ceiling with a ridge beam/board at the top. What about a cathedral ceiling that doesn’t end at a ridge beam/board? For example a bonus room with a lean-to roof connected to a normal vented attic. (This might be similar to the salt-box houses in the northeast but I don’t know.) In my case the cathedral ceiling continues into a vented attic. I could insulate this bonus room ceiling using the vented cathedral ceiling method in Martin’s article. But wanting to save every precious inch of head height in this bonus room I would rather insulate using the unvented method? Are there studies on this? The home is in Zone 3 (mixed-humid).

The rest of the story is that I really want to use the cut and cobble method to insulate this ceiling with used poly-iso sourced locally. But after reading countless articles on GBA and BuildingScience.com, it appears to me that unvented cathedral ceilings insulated with ccSPF are the only ones that do not leak air. For me ccSPF is not an option. Also, installing insulation on top of the roof sheathing is not a viable option.

Other unvented cathedral ceiling types such as SIP roofs and cut and cobble roofs apparently leak or are going to leak air. Building Science’s paper “Insight-036 Complex Three Dimensional Airflow Networks” describes the issues with SIP roofs. Comment 13 by Kohta Ueno in GBA’s article “Cut and Cobble” illustrates the problem with cut and cobbled insulation. This example appears to be a first rate effort by friends of Kohta Ueno of Building Science Co. I’m assuming his friends would have access to his knowledge and if they can’t make it work, what are my chances.

The problem with leaking insulated unvented cathedral ceilings seems to be ridge rot. In my case there is no ridge to rot. Will the inevitable air leakage around the cut and cobbled poly-iso insulation in my bonus room’s ceiling find its way into the vented attic and do no harm?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Steve, I believe you are essentially asking how to insulate a lean-to style or shed-style roof, one where the ridge terminates a sidewall, or in your case another roof?

    In general, you should choose either a vented or unvented approach. Either one can work with a lean-to roof, but venting them can be difficult. In your case, with access to a vented attic, it would not be hard to do, so you're lucky.

    Your other options are to create a hot roof by insulating above the roof deck. The examples you found are problem-prone but there are ways to do it safely. Or you can use spray foam on the interior.

    There is no way to know whether there would be enough air leakage in a cut-and-cobbled system for it to remain rot free. In general, they are not a good way to get a roof that performs well.

  2. GBA Editor
    Martin Holladay | | #2

    Steve,
    I'm not sure what your question is.

    You wrote, "But wanting to save every precious inch of head height in this bonus room I would rather insulate using the unvented method?"

    I'm not sure why you put a question mark at the end. Is this a statement or a question?

    Unvented assemblies work fine. The two basic choices are installing rigid foam above the roof sheathing -- concerning which you wrote, "installing insulation on top of the roof sheathing is not a viable option" -- or using closed-cell spray foam on the underside of the roof sheathing -- concerning which your wrote, "For me ccSPF is not an option."

    I'm not sure what you want, Steve. If you want an unvented roof assembly, there are two options, and you just rejected both. I guess that means that you intend to install a vented roof assembly.

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