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Community and Q&A

How feasible would it be to use iso board to insulate the rafter bays on an unvented hipped roof?

ASpAr6ToWh | Posted in Energy Efficiency and Durability on

My main roof is hipped, with cornice all sides. Can i use the iso board with paper face to insulate the rafter bays from inside?

I would lay it against the sheeting, leaving 1” of air space, and seal with 2″ of foil faced foam board sealed with foil tape. Strap with 1x and finish with 5/8 drywall, creating habitable space. Roof has new light colored dimensional shingle.

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  1. GBA Editor
    Martin Holladay | | #1

    It's doable, but it's fussy work -- much fussier than spray foam.

    1. The air space, if any, belongs directly under the roof sheathing, not elsewhere.

    2. Whatever you do, add up the layers to figure out your total R-value. You don't mention your location, but many areas of the country need R-38, which means 6 inches of polyiso.

    3. The most effective layer of polyiso is the continuous one on the underside of the rafters, because that's the one the interrupts the thermal bridging. Make that layer at least 2 inches thick if possible.

  2. ASpAr6ToWh | | #2

    sorry for the lack of scope. the rafters are 2x6, bay widths vary. i want to apply the foam board (2" thickness) in lapped layers, snug to each other. fill any gaps with low expansion spray foam. Should the foil faced board be snug to the rafters, with no airspace?

  3. GBA Editor
    Martin Holladay | | #3

    It depends on how many layers of foam you are installing, and it depends on the size of the rafters.

    If the 2x6s are planed, they are probably 5 1/2 inches deep. If you are using 2-inch polyiso, that probably means you want to leave a 1.5 inch air space directly under the roof sheathing. Install 1.5"x1.5" sticks to establish the air space. Then install two layers of polyiso between each rafter, and one continuous layer to interrupt the thermal bridging.

  4. ASpAr6ToWh | | #4

    do i need an air space next to the sheeting, or is the paperfaced iso board able to be applied directlly to the underside of the sheeting? i would like to build this up into the 40-50 R level.

  5. GBA Editor
    Martin Holladay | | #5

    If you don't want the air space, you can omit it -- as long as you have a high R-value, and as long as you pay close attention to air sealing details.

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