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Fiberglass and Cellulose performing differently in attic

Reid Baldwin | Posted in General Questions on

In our vented attic, we specified 18″ of blown-in cellulose. In one particular location, the insulation contractors were called back to redo an area that was not covered. When it got cold outside, I noticed that the ceiling in that area was noticeably colder than in other areas. Suspecting that the insulation contractor didn’t actually fix it, the builder and I went to the attic today to check. They had attempted a fix. Instead of putting cellulose in the problem area, they had installed fiberglass batts. The batts looked to be about the same thickness as the cellulose. We are going to have the contractor return and do what we specified. However, I am wondering why the fiberglass wouldn’t have performed about the same as the cellulose. I would have expected them to be similar but, based on the ceiling temperature, the fiberglass is noticeably inferior.

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  1. Nate G | | #1

    There are a variety of reasons, but the phenomenon you're observing is real and under-appreciated. Compared to fiberglass, cellulose covers better and more consistently, and also performs better even at equivalent on-paper R-value because it's denser and doesn't let air through it as easily. In typical attic installations, there is an air barrier on only one side of the insulation (the attic floor), if even that. This makes attic floor installations more sensitive to perfect air sealing of the floor as well as the air-permeability of the insulation. Fiberglass batt manufacturers recommend having an air barrier on all six sides of the batt--obviously impossible with a floor installation.

    Cellulose is also better in the summer because it's more opaque to infrared radiation. Fiberglass is slightly IR-transparent, lowering the effective R-value of the top inch or two.

  2. Bill Dietze | | #2

    Check out this article: ites/default/files/Blown%20Insulation%20for%20Attics.pdf
    In which you can read this: "Convection is a worry only in the coldest climates.
    In 1991, researchers at Oak Ridge National Laboratory reported that when attic temperatures drop below about 0°F, convection currents passing through air-permeable loose-fill fiberglass cut the effectiveness of the insulation by 30%." Loose fill cellulose insulation is not nearly as bad in this regard.

  3. Reid Baldwin | | #3

    The article you linked to is mostly comparing blown-in fiberglass to blown-in cellulose. It does note that both are superior to fiberglass batts, especially poorly installed fiberglass batts. When I was in the attic yesterday, I wasn't in a position to directly observe how well the batts were installed and I wouldn't necessarily know what to look for. Based on the observed poor performance, I suspect that they were not installed carefully.

  4. Reid Baldwin | | #4

    Here is the rest of the story behind this area not being covered the first time:
    Conventional practice around here is to have an attic access in the ceiling of a closet. To reduce air leakage, we provided attic access through an exterior gable wall instead. When the contractors arrived to do the attic insulation, they could not find an attic access in the usual places. Instead of calling the builder, they decided to make an access by punching a big hole in the guest bedroom closet ceiling. Obviously, they were unable to put insulation over the hole. After the drywall contractor came back to repair the hole, the insulation contractors were called back to insulate that spot, with explicit instruction about how to reach it.

  5. Joe Suhrada | | #5

    Reid, you seem like a nice fellow. Patience of a saint. I think I would be a person of interest in a missing persons case if I were in your shoes at the moment. There would be a few insulation guys seemingly disappeared from the earth at this stage.

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