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Where should I put the foam in this roof?

davidmeiland | Posted in General Questions on

Zone 4 marine. I’m pondering insulation strategies for cathedral ceilings.

In the past we have used typical cardboard baffles and either batts or blown in FG. Recently we did a small project where we cut and fit 2″ polyiso for the baffles, with the usual 1″ vent space above and foamed the edges, my theory being that this will reduce wind-washing of the fiber insulation and improve air-leakage performance. We also did one with 6″ of solid polyiso where there was only 2×8 rafter space available.

We are building a 400sf accessory building with a 10:12 roof/cathedral ceiling: snap-lock metal over felt over 5/8″ CDX over 2×12 @ 24″ OC over drywall. My first thought was to repeat the polyiso baffle approach, and dense-pack the balance of the cavities. That would give me a vented assembly with about R41. My next thought was, why bother with the vent space? Maybe we can just push the polyiso tight to the roof deck and skip the 1″ space (which requires some labor and material to create). That way I have an unvented assembly at about R44, and I have a low-perm layer at the top to prevent dewpoint issues (assuming R13 is enough for that purpose).

I am aware of the options for running the foam continuous over the deck, and haven’t ruled that out, although it requires a lot of work and more material. I am not much interested in installing it under the rafters, although I suppose I could.


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  1. GBA Editor
    Martin Holladay | | #1

    Many people combine rigid foam insulation and air-permeable insulation in cathedral ceilings. This approach is allowed in most building codes. However, you need to be sure that the rigid foam insulation has a high enough R-value to meet code requirements and keep your ceiling assembly free of moisture issues.

    The code requirements and my recommendations can be found here: How to Build an Insulated Cathedral Ceiling. In your climate zone, you need at least R-15 of rigid foam to make your approach work. Your R-13 of polyiso isn't quite there. My guess is that a dewpoint analysis might show that you can get away with R-13, but that approach won't meet code requirements.

    Finally, cut-and-cobble foam between the rafters is nowhere near as good as a continuous layer of rigid foam above the roof sheathing. I would put the rigid foam where it belongs if I were you -- on top, not between the rafters.

  2. Expert Member
    Dana Dorsett | | #2

    It takes more foam-R than the IRC prescriptive R15 if you're insulating to above code min R38, which it appears you are. It's the foam/fiber RATIO that matters- take that to R80 with only R13 on the exterior and the top portion of fiber will be soaked by the end of the winter.

    But if you leave the 1" gap and end up at R41, you'd probably going to be OK if you're using dense-packed cellulose rather than fiberglass or mineral wool, since the cellulose will redistribute & buffer the moisture without damage or loss of performance, as long as you don't ask TOO much of it.

    I'm with Martin- the rigid foam really belongs above the roof deck, where it thermally breaks the rafters, and keeps the roof deck warmer (= drier), and doesn't create a moisture trap. Putting ANY foil faced goods directly against the roof deck its a positively BAD idea (at any R value) , since it creates a severe moisture trap between the roofing above and foil facer below. If you're going to cut'n'cobble foam tight to the roof deck use either fiber-faced iso, which is typically ~0.5 perms, give or take, or UNfaced EPS, which would still be about 1 perm at R15, which is a reasonable compromise permeance value for unvented assemblies that MUST dry toward the interior.

    Otherwise, you'd be well advised to put a vented nailer deck above the structural roof deck, with nothing more vapor-retardent than #15 felt on the structural roof (with a more robust underlayment for the shingles on the nailer), which would allow the structural roof deck to dry toward the exterior.

  3. davidmeiland | | #3

    Thanks for the feedback. Sounds like I need 3" in the roof. I've thought long about putting it over the deck, but ultimately I have to match another building nearby on the property, in terms of rake board, fascia, soffit, and edge metal details, and can't incorporate the extra thickness easily. That, and the added complexity of working our way up a steep roof twice (or maybe more) instead of once

    Dana, your advice on the fiber faced iso against the roof deck seems counter to what we usually hear, which is to put a very low-perm layer there to avoid moisture buildup in/under the deck. Virtually all the foam-roof-problem threads seem to involve open-cell sprayed into rafter bays and allowing condensation, where closed-cell would not. My thought was to used foil faced in the roof, and fiber faced over the wall .

  4. Shakennotstirred | | #4

    I hope one of the experts here reads this response.
    A few months back I asked a similar question about foam vent shoots and the potential for traped moisture in the bay. The expert responses were, paraphrased, "no problem if the foam is EPS not XPS and if it were not more than 1 inch thick (or 1 1/2"?)" Now the answer seems to be, you must go with thick foam. What gives?

  5. GBA Editor
    Martin Holladay | | #5

    The answer is simple: If you have a vented air space between the top of the insulation and the underside of the roof sheathing, you can use foam of almost any thickness to create the baffle.

    However, if you choose to build an unvented roof assembly, you have to follow some fairly strict guidelines to be sure that the roof sheathing doesn't accumulate moisture during the winter. The details are all explained in my article, How to Build an Insulated Cathedral Ceiling.

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