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Vent channel size in cathedral ceiling?

Thaddeus Cox | Posted in General Questions on

Greetings all – I’m getting ready to tear the roof off my 1944 Cape Cod in zone 4C. The house has a finished attic and I would prefer to not disturb the interior, so (since the roof is coming off anyway) I plan to do any work from the outside.

This may all sound familiar – I just learned, quite accidentally, that my house was the star of a GBA blog post last month!
https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/adding-insulation-1944-roof

Presently the plan is to add 2x4s to the existing 2×4 rafters in order to provide more space for insulation. My contractor is proposing adding 4.5″ of rigid foam (to meet code req of R30), but assuming a 7″ cavity, this will leave 2.5″ of space.

I assume I’d be better off adding more rigid foam, but I don’t want to compromise the ventilation in any way as minimizing heat gain in the attic room is one of my primary concerns.

2.5″ seems like more than enough (too much?) for the ventilation channel. I’ve seen different numbers – on a roof with a fairly steep pitch, how much space is considered ideal? Is there such thing as too much? Is there a formua for vent channel size to soffit/ridge vent area?

Thanks as always.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Thaddeus,
    Your contractor seems to misunderstand a basic point: when adding rigid foam insulation to the exterior of an existing roof, the rigid foam isn't cut up into narrow strips and inserted between new framing members (new rafters -- in your case, 2x4s). Instead, the rigid foam is installed as a continuous layer that covers the existing roof sheathing. In most cases, two layers of rigid foam with staggered seams is preferable to a single layer.

    If you want roof ventilation, you can create ventilation channels above the top layer of foam. The code requires roof ventilation channels to have a minimum depth of 1 inch, but 1.5 inch is better. That's why a lot of builders create such channels using 2x4s laid flat on top of the continuous layer of foam.

    These 2x4s should be installed from soffit to ridge. Each 2x4 goes on top of an existing rafter. The 2x4s are attached with long screws that extend through the foam to the rafter below.

    Once these 2x4s are installed on top of the foam, you can cover everything with a layer of plywood or OSB.

  2. Thaddeus Cox | | #2

    Thanks for the reply Martin. I will bring up your suggestions with my contractor and see what he thinks.

    I understand how your solution would greatly improve the thermal boundary, but I have a few questions/concerns.

    First about the structural integrity of this construction - it seems like the roof would be "floating" in a sense on the foam, and the natural movements/expansion/etc over time would tend to loosen the fasteners and/or open up the penetrations, reducing the overall integrity.

    Of course this is coming from a layman's perspective. Would you be able to point me to any resources that would help put my mind (and potentially that of my contractor) at ease?

    Second, I'm concerned that code may require me to extend the height of the rafters (presently 2x4) anyway, and combined with several inches of foam/sheathing/vent/roof, I'll be adding a pretty large amount of height.

    Third - I assume this foam layer should cover the roof from soffit to ridge, which would effectively be bringing my wing attics inside the conditioned space?

    We're also planning to add a front porch, which will involve adding a new gable at right-angles to the existing roofline. I assume (since it is not conditioned space) that the foam layer should go "under" this new gable, following the original roofline?

    Finally - my existing roof sheathing is 1x skip sheathing. is this suitable as a base for applying the foam or would it need to be resheeted with OSB/ply?

    Thanks much for your advice.

  3. GBA Editor
    Martin Holladay | | #3

    Thaddeus,
    First of all, if you have concerns about the structural sufficiency of your existing rafters, you should consult an engineer. All subsequent decisions about the best way to insulate your roof will depend on your engineer's advice on how to beef up your rafters -- assuming that is necessary.

    Concerning your other questions:
    Q. "It seems like the roof would be 'floating' in a sense on the foam ... any resources that would help put my mind (and potentially that of my contractor) at ease?"

    A. All I can say is that the installation of continuous foam above roof sheathing is routine and has been done for years, especially in commercial construction. Perhaps your engineer can explain this to your contractor.

    Q. "This foam layer should cover the roof from soffit to ridge, which would effectively be bringing my wing attics inside the conditioned space?"

    A. Yes, unless you wanted to omit the foam in certain areas and substitute furring strips or purlins over these areas to lower your foam costs.

    Q. "We're also planning to add a front porch, which will involve adding a new gable at right-angles to the existing roofline. I assume (since it is not conditioned space) that the foam layer should go under this new gable, following the original roofline?"

    A. Yes.

  4. Thaddeus Cox | | #4

    Thanks once again Martin.

    Trying to fully understand the impact of thermal bridging on the overall R value of the roof... figuring 24 OC rafters, 1-5/8" rafter width and R1/inch for the fir, I'm calculating that the rafters account for about 8% of the roof area and would reduce a nominal R30 to about R28.

    Clearly I'm oversimplifying somewhat but does it sound like I'm in the right ballpark, or are there factors I'm overlooking?

    Thanks again.

  5. GBA Editor
    Martin Holladay | | #5

    Thaddeus,
    Here are two links that discuss whole-wall R-value calculations:

    http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/Whole_Wall_Therm/content.html

    http://www.inergyhomes.com/WholeWallClearWall.pdf

    The principles for insulated roofs are similar to the principles for walls.

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