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Community and Q&A

Open-cell foam in an unvented cathedral ceiling

sjnick | Posted in Green Building Techniques on

I’m getting quotes from contractors right now to insulate a small cottage with an unvented cathedral ceiling using 2x10s. I’m in zone 3C.

One contractor has suggested spraying 5.5″ of open-cell foam between the rafters and calling it a day. I asked if we could just fill the entire 9.5″ cavity and he said, “You achieve maximum efficiency @5.5. Any more is overkill. It will expand a couple more inches as we will not cut it but leave the excess.”

Code requires R-30 for ceilings around here.

I may be wrong but doesn’t more insulation mean higher R-value?

Is there a maximum efficiency for certain insulation materials?

Another contractor has proposed 1 inch of closed-cell foam and filling the rest with blown dense pack fiberglass. No one that I’ve talked to wants anything to do with cellulose. Maybe it’s a west coast thing.

Your opinions are greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Code requires R-30 for ceilings around here."

    A. To achieve R-30 with open-cell spray foam, you need at least 8 inches of foam.

    Q. "Doesn't more insulation mean higher R-value?"

    A. Yes.

    Q. "Is there a maximum efficiency for certain insulation materials?"

    A. Every time you double the thickness of an insulation material, you double the R-value. Of course, at some point the cost of the increased insulation is more than is justified by the amount of energy that is saved.

    I don't recommend that you install less than the minimum required R-value under the building code.

    One issue with all of the methods you have described is that they don't address thermal bridging through the rafters.

    For more information on insulating a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

  2. sjnick | | #2

    Thanks, Martin. I've read your article about building cathedral ceilings. It's a great resource. Regarding thermal bridging via the rafters, do you suggest that I install 1x strips across the rafters and have the installer fill the insulation flush with the strips? That would cover a majority of the rafters with a bit of insulation.

  3. GBA Editor
    Martin Holladay | | #3

    One inch is something -- certainly better than nothing. How about 2 inches? That would be better.

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