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Community and Q&A

Cut and cobble insulation on the ceiling

Iulian | Posted in Green Building Techniques on

Hi, i am currently renovating a 1965 ranch home in Charlotte NC, i am planning on insulating my walls with 3.5 inch reclaimed polyiso, i am getting a great deal and i will insulate my crawl space and basement walls with it also. I would also like to do the ceiling because i am buying in bulk so it going to be cheaper, i have 6 inch ceiling joists and am planning to set it between the ceiling joists with a continous layer of 2 inch polyiso over the 3.5 foam and joists. Also in the next year or 2 i will have to replace my roof and i was thinking of adding another layer of 2 or 3.5 on the roof then. Do you guys think this is a good idea or are there any drawbacks except labor. The house has a hip roof, ranch and about 2.4k sq ft. I have looked and looked and did not find any answer to this question. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    There have been a few failures in unvented ceilings insulated with the cut-and-cobble approach -- mostly in climates colder than yours, but you should still educate yourself about these problems. Here is a link to a relevant article: Cut-and-Cobble Insulation.

    The best way to use rigid foam insulation -- whether for walls or roofs -- is to install the rigid foam as a continuous layer on the exterior side of the sheathing. That way, the rigid insulation addresses thermal bridging through the framing.

    If you are planning to replace your roofing in a year or two, you should hold off on your plan to insulate your ceiling. Install the polyiso as a continuous exterior layer in a year or two, as part of your re-roofing project.

  2. Iulian | | #2

    Martin, thanks for the answer, yes, I have seen the issues you are talking about, i want to install the polyiso in place of the fluffy stuff, between ceiling joists and over them, not beneath the rafters, basically on top if the ceiling drywall, I did not find any information about an install like that, and how it is going to work with foam on roof later. Not planning to make the roof unvented

  3. GBA Editor
    Martin Holladay | | #3

    While you tell us that "I did not find any information about an install like that," in fact the article I linked to (Cut-and-Cobble Insulation) discusses what you are talking about.

    You have some misconceptions. I'll try to state things clearly.

    1. If you want to install insulation between your rafters, I don't think that rigid foam has any advantages over well-installed fluffy insulation. In other words, I don't recommend the cut-and-cobble approach for roof assemblies.

    2. If you disregard my advice and install rigid foam between your rafters, a vented assembly is much safer than an unvented assembly. So make sure you include a ventilation channel between the top of your insulation and the underside of your roof sheathing.

    3. If you decide to install rigid foam above your roof sheathing in two years in connection with a re-roofing job, that means that you are deciding to create an unvented assembly. You can't have rigid foam on the exterior side of your roof sheathing if there is a ventilation channel under the roof sheathing. These two approaches are mutually exclusive. So if you follow your plan, you will need to seal up the ventilation openings at the soffit and ridge when you install the continuous rigid foam on the exterior of your roof sheathing.

  4. Iulian | | #4

    Martin, please see the post above, I am not talking about the rafters, I have seen the issues about that, I am talking about ceiling joists, I have a regular ceiling, not cathedral, I am sorry for the miscommunication, if I install between the ceiling joists and on top of them, is it possible to install insulation on the roof sheathing after that or this is also mutually exclusive

  5. GBA Editor
    Martin Holladay | | #5

    I'm sorry that I misunderstood your question.

    If you currently have a vented unconditioned attic, the best way to insulate the ceiling below the attic is with a deep layer of cellulose on your attic floor. That's easier than the cut-and-cobble approach, and the cellulose will do a better job filling in all the odd spaces and cracks.

    If you lack enough room near the eaves for full insulation depth, you may want to convert your vented unconditioned attic into an unvented conditioned attic. If you want to do that, you could install the rigid foam on the exterior side of the roof sheathing as you propose (when it's time to install new roofing).

    In general, I don't advise installing insulation in both locations -- on your attic floor as well as along the sloped roofline.

    For more on this issue, see Creating a Conditioned Attic.

  6. Iulian | | #6

    Thanks Martin, the reason I wanted to install polyiso on the ceiling is because I have a lot of it, u would rather not spend extra money on cellulose. I don't have any hvac equipment in the attic so it's not mandatory to have a conditioned attic. Could you please explain why a vented attic and 2-3 inch of insulation on the roof is mutually exclusive, won't it just lower the whole temperature of the roof attic?

  7. GBA Editor
    Martin Holladay | | #7

    There are two reasons why you probably don't want to have two different layers of insulation in two locations:

    (1) Building codes require ceilings or roofs to be insulated to certain minimum standards, and codes don't allow the type of separation you describe. If your ceiling insulation doesn't meet code standards, that might be a problem when you sell your house.

    (2) Some unvented conditioned attics develop moisture problems, and the key to avoiding these problems is to condition the attic so that the attic is close to the temperature of the house. Two layers of insulation complicate the issue and may, under certain circumstances, introduce moisture problems.

  8. Iulian | | #8

    But if I install the polyiso on the attic floor and on the roof then the roof sheathing will not be covered from underneath, and the attic will still be vented, would that just make the insulation on the roof useless?

  9. GBA Editor
    Martin Holladay | | #9

    Q. "If the attic will still be vented, would that just make the insulation on the roof useless?"

    A. Yes. You don't want to invite outdoor air into an insulated space.

  10. Jon_R | | #10

    Overhead insulation in a vented attic will block some heat and the attic will be cooler. But you will block more heat from entering the house if you put that same optional insulation on the attic floor.

  11. Expert Member
    Dana Dorsett | | #11

    What Jon R said. Even if the ceiling joists are only 2x4s, stack the additional polyiso at the attic floor, which is where it will do the most good.

    With 5.5" of reclaimed roofing foam (nominally R5.7/inch) you're at R31+ at center cavity. Adding another layer of 3.5" would push it into the R49 (code min), range. But with at least 2" of it being continuous it will measurably outperform a code minimum construction.

  12. Iulian | | #12

    Thank you guys for the help, the ceiling joists are 6 inches actually, so i could fit 3.5 and 2 1/4 and another 3.5 or 2/14 on top , will see how much i have from the rest of the house, my attic is huge, close to 2400 sq ft ) Also i think code minimum over here is R38 so yeah, i will have plenty over.
    The reason i was asking about the roof is that i thought the roof layer will stop the attic from getting as hot then the lower layer will work better. I guess i will just resheath with techshield whenever i do my roof, i can find a lot of leftover on the production builders lots since i'm there every day with my job

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