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Can blown-in fiberglass/cellulose be too dense?

jbmoyer | Posted in Energy Efficiency and Durability on

I’ve been through a couple of homes where the fiberglass within the wall cavities was extremely dense. Is there such a thing as too dense with blown-in fiberglass? If so, how do you measure the appropriate density in the field.
I know the R-value of fiberglass batts decrease if compacted in a cavity due to the lack of air between the fibers, and I’m wondering if the same principle applies to blown-in fiberglass and cellulose insulation?

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  1. GBA Editor
    Martin Holladay | | #1

    You have it backwards. Within the ranges that can be achieved with available blowing equipment, the R-value per inch of blown-in fiberglass increases with density. Dense is good.

    The situation with cellulose is slightly more complicated. The R-value per inch of dense-packed cellulose is sometimes slightly less than the R-value per inch of cellulose that is less dense. But the thermal performance of dense-packed walls is still better due to reductions in air flow.

  2. Riversong | | #2

    As Martin suggests, there is a prevalent mythology about the effect of compressing fiberglass batts in a cavity. The R-value per inch increases with increasing density, but the thickness decreases at a faster rate so the installed R-value is less.

    For instance, compressing an R-19 batt (actual R/18 in 2x6 wall cavity, or R-3.3/inch) into a 2x4 wall will yield only R-13, though the R/inch will increase to 3.7. When the fiberglass manufacturers introduced an R-13 batt for a 2x4 wall, it was simply a compressed R-19 batt.

    Semi-rigid fiberglass insulation board can have as much as R-4.2/inch because of its exceptionally high density. Obviously, there's a point of diminishing return for any fibrous insulation. At some point, fiberglass becomes glass, which has only R-0.24/inch. It is the trapped air, not the fibers, which insulates.

    Fine-fiber dense-pack fiberglass, like BIBs material, at 1.8-2.2 pcf installed density, has a claimed R-value of 4.2/inch. But, because fiberglass is so light, loose-fill in an attic can have as little as R-2.2/inch.

    Cellulose has a different thermal response curve. Because it is heavier, it's settled attic density is higher and its settled R-value is from 3 to 3.4/inch, depending on thickness. At the recommended 3 pcf dense-pack density, it has R-3.8/inch. So there is less discrepancy in R per inch over its installed density range.

    Beyond about 3 pcf, most fibrous insulating materials lose R-value per inch. But, as Martin also stated, internal convection may decrease which will improve overall thermal performance and increase durability by restricting air-born moisture migration.

  3. Riversong | | #3

    Another way to answer the question - "Can blown-in insulation be too dense?" - is this:

    If it's a closed-wall blow and it either pops or bows the drywall, it's too dense.

    If it's an open-wall blow behind insulweb and you can't easily install the drywall tight to the framing, then it's too dense.

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