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Insulation question

Tom Sloss | Posted in Energy Efficiency and Durability on

If I have a gambrel style roof, with 10/12 pitch on the lower, and 4/12 on the upper. Should I consider the lower part walls, since they are the sides of the building? If so, what do I need for R value?

On the upper part, is it OK to foam the roof rafters (with soffit to ridge venting), and then use fiberglass or foam board on the top of the ceiling below? Or is it better to apply the foam board right on the rafters?

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  1. GBA Editor
    Martin Holladay | | #1

    User-7381287,
    First of all, can you tell us your name? (I'm Martin.)

    Q. "I have a gambrel style roof, with 10/12 pitch on the lower, and 4/12 on the upper. Should I consider the lower part walls, since they are the sides of the building?"

    A. No. The 10/12 section (the lower part of the gambrel roof) is a roof.

    Q. "What do I need for R-value?"

    A. The answer depends on your climate zone. Here is a link to the climate zone map.

    If this is a new construction situation, the prescriptive code calls for a minimum of R-38 for roofs in Zones 2 and 3, and a minimum of R-49 for roofs in Zones 4, 5, 6, 7, and 8.

    If this is a retrofit situation in an older house, talk to your local code authority to find out what codes apply in your situation.

    Q. "On the upper part, is it OK to foam the roof rafters (with soffit to ridge venting), and then use fiberglass or foam board on the top of the ceiling below?"

    A. This question is unclear. First of all, it's hard to get soffit-to-ridge venting with the upper half of a gambrel roof, because most such roofs don't have soffits where the slope changes. But perhaps your gambrel roof has a soffit in that location.

    Second, I'm not sure what you mean by "foam the roof rafters" -- does that mean "install spray foam between the rafters"? Or does that mean something else?

    I'm also unsure of what you mean by "use foam board on the top of the ceiling below." Is it your intention to create a vented unconditioned attic with insulation at the ceiling level? Or is it your intention to create an unvented conditioned attic with insulation that follows the sloped roofline?

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