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Advantages of fiberglass for loose-fill attic insulation?

cascadian76 | Posted in Energy Efficiency and Durability on

The comments around here are almost unanimous in favor of cellulose to insulate an attic floor, and I can understand why. But a lot of builders are using loose-fill fiberglass instead. There must be a reason.

So I ask you, what advantages are there for loose-fill fiberglass? Does it cost less? Is it easier to install? Is it lighter on the ceiling below? Does it dry out faster if the roof leaks? Is it just easier to buy your attic insulation from the same place you buy your fiberglass wall insulation?

Are there any circumstances in which R-60 of loose fill fiberglass would be superior to R-60 of cellulose? What about R-75 fiberglass versus R-50 cellulose?

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  1. GBA Editor
    Martin Holladay | | #1

    I think the main reason the people install loose-fill fiberglass in attics is because the local installers are familiar with the product and would rather push a product they are familiar with.

    I think it's true, though, that blown-in fiberglass probably dries out faster if it gets wet than cellulose.

    The problem with loose-fill fiberglass is that is it so fluffy that you end up with convection problems and a drop-off in performance at cold temperatures. To solve these problems, many homeowners with poorly performing fiberglass insulation jobs end up capping the loose-fill fiberglass with 3 or 4 inches of cellulose, just to cut down on the convection and air leakage.

    The R-value of loose-fill fiberglass is very low -- about R-2.5 or R-2.6 per inch -- so you need a huge pile to do any good. You asked a hypothetical question about R-75 of loose-fill fiberglass. Such a layer would be 29 inches deep, and most attics don't have 29 inches available, especially at the perimeter under the eaves.

    More information here: Blown Insulation for Attics: Fiberglass vs. Cellulose

  2. cascadian76 | | #2

    Thanks for the response and the link. A good article, and a good reason to upgrade my membership.

    Asking around more locally, it sounds like fiberglass is generally a bit cheaper than cellulose (because cellulose is bulkier), so that is probably part of it. And it has a higher R-value blown into walls, so it's preferred for that application, and that may skew the attic insulation choice.

  3. casadia | | #3

    I have used blown-in cellulose and fibreglass in ceiling insulation. I went overboard with the cellulose because it was less expensive than fibreglass. I rented the blower from the supplier and I purchased more than sufficient to insulate 2 x 1400 sq ft ceilings. Who knew. However, the cellulose settled more than a few inches in the following year.
    I am alergic to fibreglass. I itch just looking at it, so I hired a contractor to provide and install 12" of blown-in fibreglass. Firest, I installed 3" batts between the ceilling roof joist and then used a black felt pen to mark the 12" mark above the 3" batts. The contractor blew in approximately 9" of insulation claiming the directions on the bag indicated the numbers required for a 900 sq ft ceiling. I was able to confirm the 12" requirement by pointing out the felt pen marks to the contractor.
    Not everyone is able to mark out the required depth. I advise anyone who is able to do so should do so. There is no point paying for 12" when 9" has been installed.

  4. user-723121 | | #4

    When you get to very high ceiling R-values the weight of cellulose could be an issue. Rafters 24" oc and 5/8" drywall will only support so much weight before bowing. I have R-100 fiberglass in my attic in MN and would hesitate to use cellulose because of the weight.

    I have been using cellulose blown insulation for R-values up to 60 and have been very pleased with the performance. Cellulose also has far better accoustical properties if you live in a high noise area.

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