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Community and Q&A

Large structure with sealed attic / fiberglass

D Badger | Posted in Green Products and Materials on

I have a 48×75 post frame structure with a metal ceiling. Prior to putting up the metal ceiling I installed a 6mil plastic vapor barrier. The entire interior is encapsulated with the 6mil plastic under corrugated barn/ag panel metal. I want to blow R60 into the ceiling, but my metal spans 8.5′. I am worried about the weight of cellulose. I used R-panel on the ceiling for a higher load rating and span, but I still think R60 with fiberglass would be much lighter.

My main question is, since I have totally encapsulated the interior and I do not have to worry about air movement, would I lose much benifit by going with blown fiberglass instead of cellulose?

Thank you

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    D. Badger,
    First of all, you didn't mention your climate zone. If you are in a cold climate, and you have no plans to use air conditioning, the use of an interior polyethylene vapor barrier probably won't cause any problems. However, it you plan to install air conditioning, the polyethylene wasn't a great idea.

    In any case, the main advantage of cellulose over blown-in fiberglass is that cellulose resists convection currents better than cellulose. These convection currents (or convective loops) can occur in very cold weather, even when there is no air leakage through the ceiling. (Here's what happens: the layer of cold air between the fibers at the top of the insulation layer sink down through the insulation, while the warm air above the ceiling rises upward through the insulation fibers.)

    Manufacturers of blown-in fiberglass insulation have changed the way they manufacture fibers to reduce the problem of convective looping. These changes have reduced but not eliminated the phenomenon.

    I wouldn't worry too much about it, however. The best way to address the problem is to just blow the insulation a little thicker than you were planning at first.

    For more information on these issues, see these two articles:

    GBA Encyclopedia: Blown-In or Loose-Fill Insulation

    Fine Homebuilding: Blown Insulation for Attics: Fiberglass vs. Cellulose

  2. D Badger | | #2

    My climate zone is number 4. I was thinking of going R60 with the blown fiberglass. Is that overkill? Do you think I should go with cellulose? I assume the convection looping is because the air can move inside of the fiberglass where it has more difficulty moving through the cellulose? Thanks for the help!

    I am going to air condition someday. I sure hope that plastic does not cause me issues. The walls are R19.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    D. Badger,
    There's nothing wrong with installing R-60 ceiling insulation. While I think that cellulose is a better choice, blown-in fiberglass will work OK too.

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