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Compare merits of dense pack fiberglass vs dense pack cellulose in walls + loose pack in ceilings

Phil Lawson | Posted in Energy Efficiency and Durability on

Dense pack fiberglass is now easily available from reliable contractors and carries a higher R value than cellulose. Cellulose is more environmentally friendly. Some assessments suggest cellulose does better at higher depths and is able to absorb moisture better. What other things should be taken into consideration when choosing cellulose vs fiberglass dense pack for a wall design that includes 2×6 walls with 4″ polyiso (could reach R45 which is “high performance” in VT with fiberglass and not cellulose) and planned 2 ft loose pack in ceilings? Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Phil,
    Dense-packed cellulose does a better job of limiting air leakage than blown-in fiberglass.

    Dense-packed cellulose provides more of a moisture buffer than blown-in fiberglass. The ability of cellulose to hold and redistribute moisture lessens the risk of mold or moisture problems in wall assemblies compared to blown-in fiberglass.

  2. Michael Scannell | | #2

    Martin. I've been researching this question for a while now. I like the aspects of cellulose over Fiberglas that you've mentioned above, but my insulator says he hasn't found one manufacturer of cellulose that can guarantee an installation configuration that won't settle from too loose a pack or won't bulge with too much. We are in the framing stages of a net zero house with 8" walls and we are still wondering which to use .

  3. David Meiland | | #3

    In my experience if your installer gets the density right, you can use cellulose or fiberglass in new construction walls and it will not settle. I have inspected a fair number of blown-in jobs using IR, at various periods after they're done and occupied, and the experienced installers don't seem to be having a problem with this. You can tell by the bag count whether enough material has gone into a given area.

    It looks a little more challenging to blow closed walls, partly because they don't always know where the blocking and other obstructions are, and it is probably harder for the air to move and take the insulation everywhere it needs to go.

    Cellulose in particular can be a headache to hang drywall over, because the correct density means a fair amount of bulging. You need the right insulation roller (have seen guys use a laminate roller that does not bridge the studs) and you need to get it flat. I would specify in your contract that the insulator is to roll everything flat.

  4. Phil Lawson | | #4

    Considering a good air barrier with 4" rigid foam and a Zip system wall, the air leakage differences should not be significant in fiberglass vs cellulose wall systems. If air leakage is not an issue- ie the ACH is well < 1; then moisture transfer should really not be a problem- therefore it would make sense to get the better insulation rating from a good blown fiberglass job than cellulose (not accounting for environmental concerns of fiberglass/ benefits of cellulose). Am I missing something in this conclusion? And why don't more people use fiberglass?

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