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Using polyiso and XPS rigid insulation in a vented vaulted ceiling

user-6053782 | Posted in General Questions on

We are considering using cut-and-cobble rigid insulation in a small 4’x15′ vaulted ceiling. This is a small part of a larger roof replacement. The contractor who originally finished this bonus room did not insulate it well.

I have a large amount of polyiso board from other flat roofing projects on hand. I also know that in colder climates it loses its R-Value as the temp drops. Would it be advisable to install 4-5″ of polyiso and then another 2″ of XPS insulation on top. I’m guessing there are 2×8 rafters max. It’s such a small job that it did not make sense to use the spray foam contractor we work with due to cost and logisitc issues. We are going to be also adding ventilation to the knee walls and to the flat cieling. We really do not how much we will have to add until we open the roof. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    As long as the roof is vented, cut-and-cobble isn't too risky. (I would never use the cut-and-cobble approach in an unvented cathedral ceiling.) Of course, the best way to use your rigid foam insulation would be to install a continuous layer of rigid foam above the roof sheathing -- but I guess it's too late for that.

    If you have two types of rigid foam, it's a good idea to put the XPS on the exterior side of the polyiso, as you suggest. I feel duty-bound to remind you, however, that green builders try to avoid the use of XPS, because XPS is manufactured with a blowing agent that has a high global warming potential. From an environmental perspective, EPS is preferable to XPS.

    For more information, see Choosing Rigid Foam.

  2. user-6053782 | | #2

    Thank you for the response. I have no problem using EPS instead of XPS. It is relatively a small area of the roof that is vaulted, it really wouldn't make sense financialy to install exterior rigid foam. Since we are also insulating the knee wall would you suggest using 1" of ISO as an air barrier on the outside of the R-19 fiberglass batts we plan on installing? The interior is finished with drywall and painted.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Since we are also insulating the kneewall would you suggest using 1 inch of polyiso as an air barrier on the outside of the R-19 fiberglass batts we plan on installing?"

    A. Yes. For more information on detailing kneewalls, see “Two Ways to Insulate Attic Kneewalls.”

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