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Ceiling insulation for a room addition to my house

peaceonearth | Posted in Energy Efficiency and Durability on

I’m planning a room addition, 16 x 16 with a 6/12 gable roof and cathedral ceiling, zone 6. I’m not yet committed to anything specific so would like to run this scenario by folks for advice and opinion. I’m wanting to avoid the 5-6 inches of exterior foam (R-25) that would be needed if I went that route. And, I would like to go unvented. Would the following scenario work?

A 12″ fiberglass batt in between the rafters. I can arrange for adequate depth by using 2×12 rafters (11.5″ actual so slight compression of the batt), or use a narrower rafter and scab down. Below the rafter I’m thinking 2″ polyiso, (either one layer or two 1″ layers) meticulously taped for air sealing and reduction of thermal bridging thru the rafters. Sheetrock below. I could use strapping over the polyiso, which would be particularly appropriate with foil facing on the polyiso (I also need advice on the facing I should use). Above the rafters could be OSB or plywood decking, or 2×4 purlins and no decking since I will be using metal roofing. This would provide a cold air space but is not “official” venting I don’t think, but might help with drying to the exterior. Or, would a permeable facing on the polyiso allow for drying to the interior and be preferable?

In theory this would have an R value of something around 50, which would satisfy me if other aspects of this arrangement are not in conflict.

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  1. GBA Editor
    Martin Holladay | | #1

    The assembly is risky (and violates the building code) unless you include a ventilation channel between the top of the fiberglass insulation and the underside of the roof sheathing.

    Lots of insulated cathedral ceilings have moisture problems, so I'm conservative -- especially for roof assemblies with fiberglass batts (the riskiest type of insulation to use). Follow the rules. Include a ventilation channel under the roof sheathing.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  2. peaceonearth | | #2

    Thanks, Martin. I was just trying to get off easy, but if I had problems that would not be easy. What you suggest sounds like conventional venting, correct? I assume I would need soffit vents and a ridge vent, stuff I was obviously seeking to avoid.

    Any advice on polyiso facing if I do my scenario with the addition of a vent channel above the fiberglass, as you suggest? I'm thinking it matters less now because drying is established to the outside.

  3. GBA Editor
    Martin Holladay | | #3

    Read the article I linked to. It discusses the use of interior polyiso as a component of vented roof assemblies.

  4. peaceonearth | | #4

    Martin; I've read your excellent article numerous times, and perhaps haven't extrapolated things that weren't specifically mentioned. I didn't know my idea was counter to code (no enforcement up here), and thought maybe real careful air sealing at the ceiling level could allow for the unvented design. But, I value your "better safe than sorry" perspective. Am I correct that venting above the fiberglass is a drying to the outside arrangement, and so the polyiso facing below the rafters matters less, -and could be impermeable foil, well taped for a good air sealing solution?

  5. GBA Editor
    Martin Holladay | | #5

    If you plan to install fiberglass batts between your rafters, the building code requires either:

    (a) a ventilation air space between the top of the fiberglass batts and the underside of the roof sheathing, with connections to soffit vents and a ridge vent, or

    (b) an adequately thick layer of rigid foam on the exterior side of your roof sheathing, or

    (c) an adequately thick layer of closed-cell spray foam on the underside of the roof sheathing before the batts are installed (the so-called "flash-and-batt" approach).

    Q. "Am I correct that venting above the fiberglass is a drying-to-the-outside arrangement?"

    A. Yes.

    Q. "Am I correct that the polyiso facing below the rafters matters less?"

    A. Less than what? Interior polyiso isn't required by code, if that's what you are asking.

    Q. "Am I correct that the facing below the rafters could be impermeable foil, well taped for a good air sealing solution?"

    A. I don't recommend that approach. For your interior air barrier, the usual solution is to use the drywall as your air barrier. In Zone 6, the building code requires an interior vapor retarder, but that can be satisfied with either the kraft facing on your fiberglass batts, or vapor-retarder paint.

  6. peaceonearth | | #6

    Martin: rereading "How to Build an Insulated Cathedral Ceiling", under the section "are my rafters deep enough", you seem to suggest what I mentioned above (3rd paragraph), My intent for this design was to 1)add more R value, 2)provide a good air ceiling plane, and 3)reduce thermal bridging. Am I missing something? Your comment above says you don't recommend that approach.

  7. GBA Editor
    Martin Holladay | | #7

    Yes, you're right. As I pointed out in Comment #3, my article on cathedral ceiling discusses the use of an interior layer of polyiso. It's one approach to cutting down on thermal bridging through the rafters, and raising the total R-value of the roof assembly.

    You are mistaken, however, when you wrote that I don't recommend that approach. It's one of several ways to build an insulated cathedral ceiling. In Comment #5, I was just pointing out that interior polyiso isn't code-required. It's entirely optional.

    In Comment #2, you were asking whether polyiso "matters less" in vented assemblies than unvented assemblies. I'm still confused by the question. It's certainly true that your original plan for an unvented assembly with fiberglass batts was a code violation. I guess I would say that complying with the code "matters more" than whether or not you choose an assembly with interior polyiso.

  8. peaceonearth | | #8

    Martin: thanks for the clarification. I'm glad to know this scenario is not a "no-no", and I am still considering it. Perhaps I was unclear in my polyiso question. I was asking really about the facing in particular, and guessing that the facing (with a vented assembly) might not matter much since drying is exterior. It probably could be foil but could also be any of the other typical facings. I get where you say I only need the sheetrock for air sealing, but if I wanted to do the polyiso for the other reasons that would be an option.

  9. GBA Editor
    Martin Holladay | | #9

    You're right that the type of facing on the polyiso is irrelevant.

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