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Creative ways to make vaulted ceiling rafters deeper

Joe Norm | Posted in General Questions on

Going back and forth on roof construction.

The cheapest vaulted roof structure is 2×12 lumber. But it’s only 11.5 inches deep. 

What about furring down the rafter to the interior to get a little more insulation depth? Is this a common method?

Also wondering if its a good idea to put a layer of foam on the underside of rafters in a vented assembly prior to sheetrock? If this is a bad idea, Why?

thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    Here is a link to an article with lots of suggestions for solutions to your problem: "How to Build an Insulated Cathedral Ceiling."

    Look for the section under the heading, "Are my rafters deep enough?"

  2. Joe Norm | | #2

    Thank you,

    So there are no problems associated with installing 1-2 inches of foam on the underside of the rafter(with drywall planned to be applied to it with paint). With double top plates, the detailing might not even be that complicated since the drywallers would still have plenty of room to fasten. If the idea is to have as air tight a ceiling as possible, I can only imagine this method actually helps with that.

    I could see electrical boxes being a bit of a pain, but they would be limited of course.

  3. Joel Cheely | | #3

    For deeper rafters, I have added to 2x4 in line with the rafter above and attached with scraps of plywood. The void between the 2x4 and the rafter bothered me (using fiberglass batts) so I ripped some 1-1/2" foam to fill it up.

  4. Matt F | | #4

    Are you planning a vented roof? Are you doing a lot of the work yourself or is someone else doing it?

    For an unvented roof, put the foam on the outside if you don't have roofing yet.

    What are you planning on using for cavity insulation? What are you planning for baffles?

    You can run continuous interior foam board. Or alternatively you can fur up to 2" pretty easily with foam strips similar to this:
    https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge

    I am in the middle of installing foam 1" foam furring on my flash and batt roof. This is 3" CC (R21) spray foam, 7.25" R30 Rockwool, 1" polyiso strips. You can install drywall directly to the foam with long screws. I am concerned about screw pops though (probably irrational) and being based in Mass., my ceiling is getting perpendicular strapping nailed up prior to drywall.

    How deep do you want to go?

    1. Joe Norm | | #5

      Currently I have 18 inch Parallel chord trusses spec'd out. They are quite expensive so I was looking into other ways. It would be vented with the parallel chord. If I go unvented the foam and strapping adds up to about the same cost.

      Unvented certainly leads to a better performing roof. But I hesitate a little with it (mainly because it is not the norm around here and people are unfamiliar with them. I have read all the articles

      1. Expert Member
        Akos | | #6

        Joe,

        If your spans are not too large, you can go with 14" I joists. With 2 layers of R23 mineral wool plus 3/4" foam vent baffle, you are pretty much there with a "standard" installation.

        The good thing with I joists is you can use the bottom side of the top chord to hold and space the vent baffles. Quick to install and insulate.

        1. Joe Norm | | #8

          That's a good idea. And I could do the same with 2x12 if I added 2 inches to the bottom, but that is a lot of extra labor. So that would give an R-46 assembly?

          Will the rock wool fit snuggly into the bays of I-joists?

          1. Expert Member
            Akos | | #9

            Joe,

            With the foam vent panel that will get you a bit over R49.

            For I joists get batts for metal studs, these are wider and fit snug between the joist webbs. You will still have to notch the bottom batt by the bottom chord so a bit of extra labor.

  5. Expert Member
    Peter Yost | | #7

    HI Joe -

    Interior rigid foam insulation makes everything to the exterior colder, of course, and depending on your roof cladding, means that you may not have particularly good drying potential in either direction (if your roof assembly is unvented). This does not mean it can't work, it just means that even more care must be taken to get your water and air control layers continuous.

    Peter

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