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Insulating sloped ceilings from inside

FrankFulton | Posted in General Questions on

I am planning to insulate the sloped ceilings above a Cape-style kneewall (between the kneewall area and top attic). Unfortunately our best access is from inside the second floor.

First, here is my proposed setup – how does this look?
Air channel under roof deck>2″ polyiso baffle>fiberglass batt>drywall
(We will use canned foam to foam in the baffles)

Second, is there any reason to rip smaller or larger cuts in the existing drywall – ie, should we cut 3′ or 4′? We plan to hire drywallers to hang/paint the new drywall.

I can post photos for anyone interested.

Thank you.

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  1. Steve Knapp CZ 3A Georgia | | #1


    Where are you located? Do you know how deep the rafters are in this area? Do you have a clear ventilation path from the soffits to the ridge?

  2. FrankFulton | | #2

    Thanks Steve, we are climate zone 4. The rafters are 2x8. We do not have vents from soffit to ridge (gable at top only), but I was advised that 1" pseudo-vent clearance would be better than nothing, and we could add venting if needed when we reroof.

    So the total setup would be:
    1" air pseudo-vent
    2" polyiso baffle, foamed in (R13)
    4.25" compressed R21 batt (R18)

  3. GBA Editor
    Martin Holladay | | #3

    It's your house, and you can make your own decision. There is the right answer to this question, and then there is a second answer. The second answer is something along the lines of, "What can I get away with in terms of rule breaking without destroying my house?"

    Here's the right answer:

    1. You need soffit vents and a ridge vent if you take this approach.

    2. You need ventilation channels in every single rafter bay, from soffit to ridge, with a minimum depth of 1 inch. Two inches is better.

    3. You need enough room for code-minimum insulation levels (R-49 in Climate Zone 4).

    4. You need to remove the drywall completely on the sloped section of your ceiling to do this work properly.

    All of these facts are explained in this useful article: "How to Build an Insulated Cathedral Ceiling."

    I'm reluctant to answer the second question, which entails issues like, "What if I don't install soffit vents? What if I don't install a ridge vent? What if I can only fit in R-35 insulation? What if I don't want to remove all of the drywall on my ceiling?"

    Briefly, people do what you propose and get away with it. Installing less than code-minimum insulation happens all the time. Not having soffit and ridge vents is a real problem, however -- I wouldn't go more than three years with your system without retrofitting those essential features.

    Here's a link to one more article you may want to read: "Insulating a Cape Cod House."

  4. FrankFulton | | #4

    Martin thank you. Your articles are great. There are gable vents in the top attic, and it is a slate roof.

  5. GBA Editor
    Martin Holladay | | #5

    What kind of roof sheathing do you have under the slate roofing? If you have skip sheathing -- that is, 1x3s or 1x4s with spaces between the boards -- your roof has lots of air leaks, and will dry very quickly to the outdoors. If you roof has no soffit vents, that's good news, because it means that your ventilation channel will stay dry.

    If you have plywood or OSB roof sheathing, you still need the soffit vents.

  6. FrankFulton | | #6

    Thanks Martin
    From the underside, the roof seems to be 2x6 boards, not 1x3 or 1x4. But it is definitely not plywood or osb. Do these details modify your original assessment of my plan:
    1” pseudo-vent
    2” polyiso baffle
    4.25” R21

  7. GBA Editor
    Martin Holladay | | #7

    How well your roof sheathing can dry outward depends in part on the thickness of the asphalt felt, if any, installed between the roof sheathing and the slate. If your roof has #30 asphalt felt, drying will be slow.

    If there is no asphalt felt, or just rosin paper, the rate of outward drying will be high.

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