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Attics with complicated rooflines: Bringing the attic into the home’s thermal envelope?

hertzr | Posted in General Questions on

Based on Martin’s advice, am starting to investigate possibly bringing my attic within the home’s thermal envelope. My question is – what’s the best approach for attic areas with merging rooflines?

I have an area where a typical Cape Cod triangular knee wall (yellow area in the attached diagram) meets a cross gable roof (blue). There’s also a connecting porch overhang (orange). What is the typical approach for establishing a thermal/air boundary at attic areas with two or more merging roof lines?

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  1. user-2310254 | | #2

    Were you planning on a new roof at any point in the near future? If so, installing rigid foam on top of the sheathing would be worth considering.

  2. GBA Editor
    Martin Holladay | | #3

    You need to create walls (partitions) in your attic to divide the area of your attic that is above the heated portion of your house from the area of your attic that is above your porch.

    So go up into your attic and figure out the location of the top plates of your insulated walls. You want to build partitions between those top plates and the roof sheathing. The required work is ordinary framing, requiring use of a plumb bob or a level.

    Once these partitions are built, insulate the partitions (following ordinary advice to keep things as airtight as possible).

    Then you can follow the advice in this article: Creating a Conditioned Attic.

  3. hertzr | | #4

    Partitioning around the portion of the porch overhang that's beyond the top plate should be fairly easy. Where I'm getting confused is the corner area where framing members for the knee wall (yellow) merges with the cross gable's framing (blue) and the porch overhang. What's the best approach for sealing this transition are with perpendicular framing members?

    The previous homeowner installed a new roof 10 years ago, so re-roofing and installing exterior foam isn't really an option any time soon.

  4. GBA Editor
    Martin Holladay | | #5

    Your question is unclear. Presumably, you want to figure out a way to insulate the underside of the roof sheathing with closed-cell spray foam, or to install rigid foam on the exterior side of the roof sheathing.

    If there is roofing on the exterior side of the visible sheathing, the sheathing needs to be insulated -- unless the sheathing is above an outdoor area like a porch.

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