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Community and Q&A

Best plan for knee walls and cathedral ceiling on an ICF house

interplexr | Posted in General Questions on

I am going to be starting construction soon on our home in Virginia (4A). It will be built with 8″ ICF walls for the basement and 6″ ICF walls for the remainder of the house. The ICF blocks have 5″ total of foam insulation. Since the house will be very well insulated, I want to make sure my gable and knee walls plus cathedral ceiling are insulated nearly as good. The center of the home has a 12:12 pitch roof and cathedral ceiling cutting the home into 3 sections. The roof slope drops on either side to a lower pitch. Its a similar shape to a barn with a gull wing roof. We’ll take the ICF up to the ceiling in the flat ceiling portions of the house. The gables will be stick framed at some point once the sides start angling in. There will be knee walls running down the house where the cathedral ceiling is as well. I was hoping to get some opinions on the best way to address these sections to make sure they perform similar to the ICF walls.

I’ve scoured this page and other sources looking for details and have cobbled this together.

For the gable walls, I was looking at 3″ exterior foam board, OSB sheathing (for strength), 2×6 framing with unfaced R19. All seams and joints would be sealed.

For the knee walls, the attic side would have foil faced foam board of at least an inch thick with 2×10 framing and R30 insulation.

We are looking at a metal roof and synthetic underlayment and everything I seem to find says we should not make that a vented roof. I haven’t talked with any insulators that seem comfortable with an unvented roof either. With that in mind, the flat ceilings would be flashed with foam and blown to R60. The cathedral part would be 2×12’s with baffles R38 unfaced and then at least 1″ foil faced foam on the interior with furring strips and then drywall.

Does this seem like a good approach? What am I missing?


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it sounds like your ICF walls will have an R-value of about R-20. That's OK, but not particularly high.

    Second, you can't install synthetic roofing underlayment over an unvented roof assembly; most manufacturers of synthetic roofing underlayment won't allow it. So if you like unvented roof assemblies, use asphalt felt as your roofing underlayment. (It's unclear to me whether your want your cathedral ceiling to be vented or unvented.)

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