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    Combining Foam and Fiberglass Insulation Layers?

    Asked Jan 21, 2009, 10:27 AM 3 Answers
    I am insulating the upstairs of a cape cod and am trying to maximize the R-value without creating any moisture problems. The picture below illustrates my plan (the multiple layers of R-13 are due to framing issues which aren't shown).

    My question concerns the 2" Owens Corning Foamular insulation on the sloped ceiling. My thinking is that the Foamular can be used to create the ventilation gap below the roof sheathing, as well as provide additional R-value. Although this theoretically creates a double vapor barrier, the ends of the fiberglass batts are open which should provide a path for moisture to leave this cavity. (I don't want to put the Foamular inside the framing because I don't want to loose any more space.)

    Should I be concerned about moisture here, or should I be OK?

    Thanks in advance for any help and thoughts!


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    21boat's Avatar
    21boat Posts: 2,441, Reputation: 212
    Ultra Member

    Jan 21, 2009, 10:50 PM

    You have to have the foam board against the 6ml poly to get away from the double vapor barrier And the non faced insul R13 and R15 on top of the foam board.
    If just 5% of moisture gets into the insul it looses 95% of its insul quality's
    If this takes some cutting down the foam board then that's what it takes. Again it has to be correct no matter what we feel or what's the sense in losing R value in wet fiberglass insul and possible mold.

    If there are framing issues here and vapor situations I would call a spray insulation co and have them look at is and price it. Then all of your issues can be address and spray foam is the ticket

    Signed 21 Boat

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    LinfieldPA's Avatar
    LinfieldPA Posts: 32, Reputation: 3
    Junior Member

    Jan 22, 2009, 09:23 AM
    Thanks very much 21boat. That's what I was afraid of. I was trying to come up with something similar to the attached (from the Owens Corning Foamular Product Data Sheet). Is there some variation of this that I may be able to use and not run into moisture problems? Obviously, the last thing I want is mold and ineffective insulation!

    Thanks again.
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    21boat's Avatar
    21boat Posts: 2,441, Reputation: 212
    Ultra Member

    Jan 22, 2009, 11:36 PM

    As a builder I don't quite agree with the pic. Of course there are many different ways to build. For me the pic above is a way to sell foam board insul The pic borderlines a double vapor barrier. There's a big difference of a side wall in this application than on a ceiling. Think of the heat rising and our moisture out of our mouths. Kitchen cooking etc. Most all of that moisture rises up in a heated room. The ceiling is exposed too much more moisture rising in the room from heat. Look at the pick and ask yourself ( how come there are not 2" channels against the exterior side wall to breath like we need on the inside of the roof sheeting. Answer is basic earth science.
    I would personally use spray foam in the areas that you are having troubles with. I know its about 3 times the cost to insul, But it is 3 times better in insul with NO air flow inside the spray foam insulation.
    Just check out the R values per inch of spray foam compared to fiberglass insul

    Compare Spray Foam | McGlaughlin Spray Foam Insulation

    Signed 21 Boat

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