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Community and Q&A

Installation of polyiso roofing insulation panels

nwintfeld | Posted in Energy Efficiency and Durability on

I am planning to install 4 inch polyiso insulation panels on the roof deck of my Cape Cod style home located in Cape Cod Massachusetts. There is currently 6 inches of fiberglass insulation installed between the 12 inch rafters up to the collar ties. The insulation is then located between the collar ties just above the ceiling of the upper floor. There is a vapor barrier between this insulation and the ceiling sheet rock. There is a small vented area beneath the roof above the collar ties.

My questions:

1. Can I install the polyiso panels and leave the current insulation in place?
2. Can a use a ‘rubber’ underlayment on top of the new roof deck beneath the new shingles? Would this create a problematic double vapor barrier?

thanks for your advice.

Neil

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Neil,
    Q. "Can I install the polyiso panels and leave the current insulation in place?"

    A. Yes. Remember, two layers of 2-inch insulation (installed with staggered seams) are better than one layer of 4-inch insulation.

    Q. "Can a use a 'rubber' underlayment on top of the new roof deck beneath the new shingles? Would this create a problematic double vapor barrier?"

    A. Yes, you can do it, but I don't recommend it except at eaves and valleys. If you are worried about ice dams, I think a better approach is to install 2x4 vent channels (eaves to ridge) on top of the top layer of plywood or OSB.

  2. user-939142 | | #2

    "Yes. Remember, two layers of 2-inch insulation (installed with staggered seams) are better than one layer of 4-inch insulation."

    why is introducing more failure points better?
    i understand you should stagger if you have seams, like say using 6 or 10" of XPS and using 2" sheets, but why stagger if you don't need to?

  3. GBA Editor
    Martin Holladay | | #3

    Bob,
    If you use a single layer of foam, you will get convective air currents through the seams, even if you foam and tape the seams. (Rigid foam shrinks.) The heat loss at the seams will telegraph through the roofing -- it will be visible on cold days when the roofing has a layer of frost.

  4. user-939142 | | #4

    that makes sense

    while you double the number of seams and weak points, the additional layer(s) remediate that

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