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Thread: Attic Insulation - vapor barrier

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    Engineering Technician The old college try's Avatar
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    Default Attic Insulation - vapor barrier

    I'm looking for advice on how to deal with an existing vapor barrier attic situation. I went up to the attic to evaluate the situation on existing insulation and was not surprised to find two layers of 3-1/2 inch fiberglass both having a foil vapor barrier facing up (the previous owners did alot of strange things). I want to add about 12 more inches (cold Michigan winters), but I'm extremely nervous about adding over the vapor barrier layers. Any advice? I think I may have to rip out the second layer and flip over the first, but that's alot of work.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Is there any vapor barrier above the ceiling? You're right, you should only have one, and it should keep any trapped moisture in an area that won't get cold enough to condense. You could slit the backing every 6" or so (have to think about how much might be needed. More insulation in today's economy usually pays for itself (and it would keep the condensation point closer to the ceiling.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Engineering Technician The old college try's Avatar
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    Quote Originally Posted by jadnashua View Post
    Is there any vapor barrier above the ceiling? You're right, you should only have one, and it should keep any trapped moisture in an area that won't get cold enough to condense. You could slit the backing every 6" or so (have to think about how much might be needed. More insulation in today's economy usually pays for itself (and it would keep the condensation point closer to the ceiling.
    There isn't an existing barrier beneath the exist. insulation. My wife just had the idea of peeling off the existing barrier from the second layer after flipping over the first layer to place a barrier in the proper location. Following that I would lay 12" over the first two layers. Any thoughts? Is it difficult to remove the foil? I imagine it's easier that hauling an existing layer out of the attic and through the house.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    That would probably work, but it may be easier to just use a razor knife boxcutter, to slice the insulation a bit. Try both methods, removing it and slicing it. Ideally, the vapor barrier would be a continuous sheet. The flaps on the insulation normaly would overlap if stapled to th ebottom of the joists before th eceiling was installed. You can aid things by putting a barrier paint on the ceiling.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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